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Default Quad II question: High B+ a problem?

Hello all, long time lurker/sporadic poster here with a question concerning my current project.

I recently rebuilt a couple of Quad II's from what was pretty bad shape with parts missing etc. Everything now works and sounds great and even looks mostly original... except for the highish B+ Im getting. This is the new configuration:

Rectifier tube (5AR4) - 33uF/450v - the original choke - 47uF/450v

With 5AR4/GZ34 I am getting around 380-385v at the rectifier. I would like to use this tube since it is more modern than the GZ32/5V4. With GZ32 there is a bit of voltage drop to 372v or so but its still much higher than the original schematic.

I raised the cathode R to 220 ohms to go a bit more easy on the few vintage KT66's that I have, and also to be able to use 6L6GC. KT's are showing 29,5v on the cathodes, and 6L6 (old RCA) are at 27v in the same configuration.

Do you think the transformers and chokes are up to such a high B+ in prolonged use? How about tube life? Could I try, say, a 5R4 instead of the indirectly heated ones (for the bigger voltage drop) or is this asking for trouble.
I would like this rebuild to last long and be trouble free and run as cool as possible within its design limitations.
Thanks for reading!
Martin
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Phil Allison[_3_] Phil Allison[_3_] is offline
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Default Quad II question: High B+ a problem?





I recently rebuilt a couple of Quad II's from what was
pretty bad shape with parts missing etc. Everything
now works and sounds great and even looks mostly
original... except for the highish B+ Im getting.
This is the new configuration:

Rectifier tube (5AR4) - 33uF/450v - the original choke
- 47uF/450v

With 5AR4/GZ34 I am getting around 380-385v at the
rectifier.


** What is your local AC supply ( measured , not nominal) and what voltage
tap is the Quad II set to ??



..... Phil







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Default Quad II question: High B+ a problem?

Right now the wall outlet shows 235.5 vac, but in the evening it can reach at least 237 vac. The Quad Im measuring now is set to 240v, and the HT winding is producing 312-0-312vac. 5v heater is 5.0v and 6.3v is 6.5v (with RCA 6L6GC, 0,9A heaters). At the first (33uF) cap I now have exactly 380v, and at the screen supply (at the 47uF cap) there is +375v. Plate voltage 370-372 and shared cathode R sits at 27,5v with these tubes. Rectifier is Philips Miniwatt GZ34.

Using the KT66 makes the bias voltage go up and the 6.3v go down a bit, since these tubes draw a bit different currents. But the HT stays around the same high value.

I did an experiment: connected a 4W 100 ohm wirewound in series with the 5AR4. Presto, 340v B+ like in the original schematics! But the R got _very_ hot and where does one put it (since I dont want to drill the chassis)....

Any ideas?

Martin
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Default Quad II question: High B+ a problem?

I can also add that now trying 5R4GYA's for rectifiers in the Quad II's brings the B+ voltage down to 337v (yay! within specs!) and everything runs comfortably halfwarm instead of hot. The 6L6GC heat up more quickly than KT66, so the voltage peak (440v) at cold startup lasts for only 10 seconds or so. If only there was a place to add a standby switch and this would now be solved. The trouble of restoring to original appearance...
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Default Quad II question: High B+ a problem?




I can also add that now trying 5R4GYA's for rectifiers in the Quad II's
brings the B+ voltage down to 337v (yay! within specs!) and everything runs
comfortably halfwarm instead of hot. The 6L6GC heat up more quickly than
KT66, so the voltage peak (440v) at cold startup lasts for only 10 seconds
or so. If only there was a place to add a standby switch and this would now
be solved. The trouble of restoring to original appearance...


** Huh ?

The input and AC supply connections used on a Quad II are horrible and
impractical. So I long ago fitted a standard IEC inlet to mine and also an
aluminium cover plate with a 6.3mm jack behind the input socket hole. Looks
very neat and is very practical.

The output valves are Sylvania 6L6GCs - made in the 1980s and used mainly
in guitar amps. They out perform and outlast all other 6L6s or KT66s. My
Quad II tests 0.03% THD at 10 watts output.

The PSU was rebuilt decades ago, as the original 120V PT was burned out by
connection to 240V mains - so I have a bridge rectifier, 100uF 450V
electro, original choke and 56uF 450V electro now. The DC supply is 362V at
the first electro. The electros are compact, single ended types held to the
chassis with Silastic.

All the small components, valve sockets, tag board and wiring are new since
I stripped and replaced the lot about 6 months ago. Cathode bias is by two,
390ohm, 5 watt cement resistors in parallel (= 195ohms) .

The new 9 pin sockets have snap on COVERS - so the poor EF86s are no
longer exposed to accidental damage.

Oh, and I have a dummy valve (5AS4) in the rectifier socket for appearances
....



..... Phil









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Default Quad II question: High B+ a problem?

Good to hear that 6L6GCs work well at 360v in your amps. Parallelled cathode resistors must result in less heat, nice solution.

My pair must be late production, serials in the 70xxx range, because they have no voltage selector beside the fuse and no Bulgin power connector. Instead there was just a hole for what I think had been an early IEC C10 plug. So I could fit modern grounded 3 prong IEC there without any drilling. It seems that Quad made some different chassis configurations for different markets?
Im in two minds about the Jones connector. On one hand it looks retro funky.. On the other hand, it is less practical to use adaptors so perhaps it would be best to put in phono sockets. I have already removed the internal HT connections from the Jones for safety.
The only giveaway that the amps have been modified is that I placed AC switches where the fuse holders were. But they are old British chromed switches and look contemporary. The fuses are now inside the enclosures.

Martin
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Default Quad II question: High B+ a problem?




Good to hear that 6L6GCs work well at 360v in your amps. Parallelled cathode
resistors must result in less heat, nice solution.

** Same amount of heat - just more spread around ..


My pair must be late production, serials in the 70xxx range,


** My one has s/n 8734

because they have no voltage selector beside the fuse and no Bulgin power
connector. Instead there was just a hole for what I think had been an early
IEC C10 plug. So I could fit modern grounded 3 prong IEC there without any
drilling. It seems that Quad made some different chassis configurations for
different markets?

** German products once used an odd IEC like connector with two pins and
ground via the casing.

Same panel cut out I think, see them on old "Dynacord" amps and " Echolette"
tape units from the 1960s.


Im in two minds about the Jones connector.

** Schizophrenia is hard to cure....


On one hand it looks retro funky.


** Huh ? Looks appalling to me.


The only giveaway that the amps have been modified is that I placed AC
switches where the fuse holders were. But they are old British chromed
switches and look contemporary. The fuses are now inside the enclosures.

** You have just given me an idea....


..... Phil


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Default Quad II question: High B+ a problem?

** German products once used an odd IEC like connector with two pins and

ground via the casing.


Interesting, could be that they were German imports since Im in Sweden. Or they picked up the same specs here. All I know is that they were at least not used in broadcasting studios since they would have had a special cutout part of the chassis then.

Same panel cut out I think, see them on old "Dynacord" amps and " Echolette"

tape units from the 1960s.


And Leitz Focomat photo enlargers from the 60's

Im in two minds about the Jones connector.
On one hand it looks retro funky.

** Huh ? Looks appalling to me.


The think is, the Jones plug does not look hifi at all, just sort of vintage industrial. I think it fits to the general design of the Quads. But I thought of changing it to a phono anyway to make it more usable for future users (and yes I hope these amps will last another 50 years)...

The only giveaway that the amps have been modified is that I placed AC

switches where the fuse holders were. But they are old British chromed

switches and look contemporary. The fuses are now inside the enclosures.

** You have just given me an idea....

.... Phil


Old Bulgin switches have nicely ribbed chrome rings. But I have seen different fuse hole sizes in Quads. Im mine they fit precisely.

To conclude, I did finally find some 5V4GA and those behave very much like GZ32. Dropped the B+ voltage back to around 360v and this seems to work well. Great sound. The transformers and choke get warm but not hot.
I am still trying to find out why Peter Walker's drawings say 340v. Perhaps this was measured with the original QCII preamp drawing current. I dont have one of those (yet).

For other readers coming by, I can try to sum up my Quad II rectifier experiences so far:

5AR4/GZ34 are great but can result in excessive voltage! Do measure your amps if you are running these. I had around 385v at the first cap, which could be bad in longtime use depending on ones tubes, caps and resistors. They need just 1.9A heater current which is good, though. I had installed new computer grade caps of 33uF and 47uF @ 450v which may have given different results than using the original TCC 16uF caps in the grey box.

GZ32 and 5V4G/GA seem to make it just right at 360v. Especially 5V4 has a 2..0A heater which is better than GZ32. Indirectly heated. The closest approach to the original specs

5R4GYA is a directly heated rectifier, and the B+ voltage comes on instantly and can peak at over 400V. Was not a problem for me, no arcing or anything, but I dont think the output tubes will like such treatment forever. Best would be to have standby switch. B+ after warmup became 340v just as in the original drawings. This tube also has a 2.0A heater and worked well otherwise. The coolest running Quad II rectifier combination: My amps did not get very hot even after hours of use.

I almost accidentally tried a 5U4G at first, until I checked my old RCA manual. 3A heater! Using this in a Quad would NOT be good for the transformer.

Also note that I was only using vintage tubes. I dont know how the new Chinese or Russian ones will behave.

I hope I got all the numbers right.
/Martin




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To conclude, I did finally find some 5V4GA and those behave very much like
GZ32. Dropped the B+ voltage back to around 360v and this seems to work
well. Great sound. The transformers and choke get warm but not hot.
I am still trying to find out why Peter Walker's drawings say 340v. Perhaps
this was measured with the original QCII preamp drawing current. I dont have
one of those (yet).


** The Quad 22 stereo control unit only draws 4.5mA from the HT of one Quad
II - plus 0.5A at 6.3V for heaters from both Quad IIs. The mono QCII takes
under half that.

So the effect on the HT is tiny.


I almost accidentally tried a 5U4G at first, until I checked my old RCA
manual. 3A heater! Using this in a Quad would NOT be good for the
transformer.

** Probably do no harm, the heater voltage might be a tad low.


..... Phil







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Default Quad II question: High B+ a problem?

On Friday, 11 April 2014 22:24:26 UTC+10, wrote:
Hello all, long time lurker/sporadic poster here with a question concerning my current project.



I recently rebuilt a couple of Quad II's from what was pretty bad shape with parts missing etc. Everything now works and sounds great and even looks mostly original... except for the highish B+ Im getting. This is the new configuration:



Rectifier tube (5AR4) - 33uF/450v - the original choke - 47uF/450v


I am not sure you know what you are doing. The original greay potted choke in Quad-II is to filter the screen supply. It does not filter the anode supply to the OPT CT.

If you wish to use the existing Quad-II choke to CLC filter the B+ at OPT CT then you might see a huge reduction in B+ because that choke has Rw = 600r.


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"Patrick Turner"



I recently rebuilt a couple of Quad II's from what was pretty bad shape
with parts missing etc. Everything now works and sounds great and even
looks mostly original... except for the highish B+ Im getting. This is the
new configuration:

Rectifier tube (5AR4) - 33uF/450v - the original choke - 47uF/450v


I am not sure you know what you are doing.

** The OP knows - his description is of the entire DC supply circuitry.

The changes are the rectifier and filter cap values.


If nothing else is changed in an original Quad-II amp, the The use of GZ32
or GZ34/5AR4 makes only a slight rise of B+ which is entirely OK and nothing
to worry about.

** The voltage rises by about 30-40V so is significant.

You don't need to do every single thing I usually do to old Quads to get
better music from them. The use of a single 180r common cathode Rk is
terrible.

** Hardly a fair comment seeing as it works so well with the specified
output valves ( and most 6L6GCs ) long as they are still in good condition.


..... Phil









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Patrick Turner Patrick Turner is offline
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Default Quad II question: High B+ a problem?



I recently rebuilt a couple of Quad II's from what was pretty bad shape
with parts missing etc. Everything now works and sounds great and even
looks mostly original... except for the highish B+ Im getting. This is the
new configuration:

Rectifier tube (5AR4) - 33uF/450v - the original choke - 47uF/450v


I am not sure you know what you are doing.

** The OP knows - his description is of the entire DC supply circuitry.

The changes are the rectifier and filter cap values.

Well, from what he was saying,it looked like he didn't know all that could be known, and for which we must allow for.

If nothing else is changed in an original Quad-II amp, the The use of GZ32
or GZ34/5AR4 makes only a slight rise of B+ which is entirely OK and nothing
to worry about.

** The voltage rises by about 30-40V so is significant.

Its about 10% B+ rise, and quite OK in itself. However, with a rise in B+ the Iadc rises a bit so the idle Pda rises, which is not so good, because tubes run hotter than they should. So 220r common cathode Rk would be OK.

You don't need to do every single thing I usually do to old Quads to get
better music from them. The use of a single 180r common cathode Rk is
terrible.

** Hardly a fair comment seeing as it works so well with the specified
output valves ( and most 6L6GCs ) long as they are still in good condition.

Yes but anyone can build a better amp than the Quad-II, its not a world beater. While KT66 or 6LCGC are new and in good condition and matched, the Idc in each tube will remain balanced but after some time they age differently and become unmatched and there is nothing to prevent the Idc imbalance. Its a major flaw in Quad-II design. If one 6L6GC begins to get a tiny bit gassy, the 680k grid biasing Rg begins to draw current and the grid goes slightly positive, and tube conducts more Iadc, which heats up the tube which causes more grid current and Ia, and more heating. The higher Ia of this tube raises Ek, which tends to turn off the other tube. So the Idc can become unbalanced sooner in Quad-II than in any other better designed amp which has individual cathode biasing networks.
The 680k bias R is way too high and should be no more than 120k, but then the **** weak EF86 have a load that's too low to get the wanted voltage gain..
Just why Walker used EF86 is unknown when better ideas with say a pair of 12AU7 would have been so much better. Money is the root of most amplifier crapitological practices used by so many minimalist amplifier makers of 1950s..

In Quad-II I always use a 470r plus 470uF from each 6L6 cathode to end of CFB winding which has its CT taken to 0V. This allows slightly worn old tubes to last a lot longer without one tube beginning to glow red while other is too cold, and sound has gone to mud.

The distortion measurements one is supposed to get with Quad-II are often a complete fiction and once output tubes become unbalanced the mild amount of GNFB fails to reduce it much. Quad-II benefit from being completely gutted, and starting all over again with a better schematic that ensures output tube Ia balance, has lower value grid biasing R, has a lower distortion input driver amp with at least 2 x 12AU7, or perhaps 12AU7 input plus 6CG7 LTP driver
with CCS in cathode circuit. There must be individual RC cathode biasing and PSU needs total revision. Then the amp works fairly well in class AB with recent made speakers of say 6r0 which have low sensitivity.
Quad-II have RLa-a when 16r load is used with OPT strapped for 16r. But 4k0 does not give pure class A. For that you must use 32r, so RLa-a becomes 8k0,
and each 6L6GC sees a class A load of about 4k0.
Many ppl strap Quad-II for 9r, then connect 4r0 speakers, and I have done it myself when testing amps I've modded. They cope because of my mods as long as high levels are not used. having 4r0 with OPT set for 9r, means the RLa-a = 1k8, and then the amount of class A is tiny before class AB threshold, so at high levels the Quad-II THD-IMD-noise is bloody atrocious.

I've never accepted that Quad or any other old fuddy-duddy brand was fabulous or even adequate when I could see so many shortcomings. I refuse to bow down to the past.

Patrick Turner


..... Phil

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"Patrick Turner"

I recently rebuilt a couple of Quad II's from what was pretty bad shape
with parts missing etc. Everything now works and sounds great and even
looks mostly original... except for the highish B+ Im getting. This is the
new configuration:

Rectifier tube (5AR4) - 33uF/450v - the original choke - 47uF/450v


I am not sure you know what you are doing.

** The OP knows - his description is of the entire DC supply circuitry.

The changes are the rectifier and filter cap values.

Well, from what he was saying,it looked like he didn't know all that could
be known,

** What a ****ing asinine, out of context, thing to say.


You don't need to do every single thing I usually do to old Quads to get
better music from them. The use of a single 180r common cathode Rk is
terrible.

** Hardly a fair comment seeing as it works so well with the specified
output valves ( and most 6L6GCs ) long as they are still in good condition.

Yes but anyone can build a better amp than the Quad-II,

** So ****ing what ?




..... Phil


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Default Quad II question: High B+ a problem?

First of all, Im glad to see some activity here on old Usenet. Most people seem to be in web based forums these days.

In my description of the DC supply, I meant that instead of using the existing 16+16uF cap, I placed two separate caps in the circuit - one 33uF before the choke, and a 47uF after. I did not add another choke or anything.

Phil, thanks for sharing your experiences while I was working on the amps. Since I do not own a dozen of them I could not check what would be the normal behaviour of the circuit.

Patrick, I have read your interesting site about tube electronics. I understand your angle, and I think it is how one should think about new production amps. But in these Quads I was more concerned with safety and longevity than altering the circuit.

I saw a Quad system in a museum recently, together with an early EMT turntable, SPU cartridge, a Revox G36 and so on. Special, quirky but high quality stuff that must to most people seem like steam engines these days. Call me sentimental but I personally dont want to alter such machines too far from how they were made in their time.

I understand the problem with the common cathode resistor, and I wonder if maybe this is one of the reasons many Quads have failed with melted tar all over. In my amps I swapped tubes around and measured voltage differences for a little bit better DC balance. But separated cathodes sounds like a good idea.(I built a Williamson inspired amp once, using huge wirewound pots between the two output tube cathodes and ground. Was easy to balance but maybe not the last word in highend audio

It would be interesting to hear more ideas about how to make Quads safe for future use and longevity. I understand it is not a good idea to put a fuse between the HT center tap to ground?

Martin

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Martin said......

First of all, Im glad to see some activity here on old Usenet. Most people seem to be in web based forums these days.

And the web based forums don't work as well as the Usenet forums. BUT, the Usenet forums are non moderated which means all the arsoles can terrorize everyone who does not agree with the arsoles' opinions, so most newbies and ppl who didn't like the slightest challenge to whatever they typed fled in terror to the smaller moderated groups where everyone is forced to be nice to each other. The result is like a lot of benign zombies attempting communication in short almost content free 1 sentence postings, and that's why I've stuck to this place. If people don't like the heat, get out of the kitchen. If a terrorist enters the kitchen, tip a pot of boiling oil on him, kick him out, and set fire to him then close the door. Nobody gets away with terror with me. I'm here to discuss technical issues, not to pose as smarter than anyone else, but to share my experiences regardless of whether I am in error or not.

In my description of the DC supply, I meant that instead of using the existing 16+16uF cap, I placed two separate caps in the circuit - one 33uF before the choke, and a 47uF after. I did not add another choke or anything.

In Quad-II, the original had C1 16uF + C2 16uF in one box cap, plus 20H choke with Rw about 600r. The B+ for OPT CT was taken from C1 16uF where the Vripple = approx 19Vrms, 100Hz, saw tooth wave. The L + C2 form a filter to the fixed screen supply for KT66, also for input tube B+ and any external tuner or Quad22 preamp. The 20H + 16 uF reduce the Vripple at Eg2 to be about 25mVrms, low enough to keep noise at output low enough.
Using GZ34 with 33uF will be OK but only halve the Vripple at OPT CT. The use of 47uF for C2 will further reduce Vripple at Eg2 to about 3mVrms, a considerable betterment.
BUT, if you use 8r0 speakers with amp strapped for 9 ohms, the RLa-a anode load is about 3k5, and above what is a low class A threshold, the amp works in class AB where the Vripple at OPT CT is in series with the "ON" half of the OPT load and its tube on every 1/2 wave cycle so the THD and noise increases hugely.
To force the Quad-II to work better with low load values, most especially when 4ro speakers are used, and, they often are, despite ppl saying oh no they don't,
the B+ supply at OPT CT should be far better filtered and be a low impedance to 0V, so I like a modern 470uF cap, rated for 450V, easy and cheap. But 470uF used for C1 in CLC would kill any tube rectifier because the peak charge current rises way above the rating which is often less than 0.2A.
SO, I get rid of the tube rectifier. Then I install a pair of 1N5408 in series on each 1/2 primary of HT winding and the B+ at C1 will rise to around +400Vdc if wanted, but better is to use say 150uF for C1, then replace the existing 20H choke with a 4H choke with Rw 50r, and have the 470uF after the L. additional R can be added to between HT winding ends and diodes, say 47r, 68r, 100r to reduce the working B+ to where you want it, or to about +375Vdc, ( +5%, -10%.)
The Vripple at OPT CT will be 3mVrms, and then an R&C filter with say 1k0 + 220uF for Eg2 will reduce Vripple to 0.021mV, and utterly negligible.
With individual R&C cathode biasing networks with 470r + 470uF, the amp will give better bass, and the amount of Vdc change during transients becomes neglible, and the noise injected in class AB is minimized.
A lot can be done to make Quad-II amps far better than the original designers intended.

Phil, thanks for sharing your experiences while I was working on the amps. Since I do not own a dozen of them I could not check what would be the normal behaviour of the circuit.

Patrick, I have read your interesting site about tube electronics. I understand your angle, and I think it is how one should think about new production amps. But in these Quads I was more concerned with safety and longevity than altering the circuit.

But the mods I routinely do to old original junk like Quad-II does make them last longer, ie, KT66 life is longer, and their safety is made better. Both issues require circuit changes that exceed what you are doing just by changing the two main electros in PSU.

I saw a Quad system in a museum recently, together with an early EMT turntable, SPU cartridge, a Revox G36 and so on. Special, quirky but high quality stuff that must to most people seem like steam engines these days. Call me sentimental but I personally dont want to alter such machines too far from how they were made in their time.

OK, you are in love with the crummy old quality of the past. Maybe you would not mind using hand operated shears to shear a sheep, but I'd prefer the powered wool cutter. I really like old steam engines, and the locals in Canberra have a Garratt articulated loco almost restored, and there are no mods. OK, quite a sight'n'sound to see running. 262 tonnes of power from 1960. Wow. But I'd hate to own the darn thing. Same goes for a Spitfire, or, rather like a Quad amp, a Morris Major. Now I just cannot think of a 1950s amplifier that excites me like say a D Type Jaguar, or a Vincent Rapide. Back in 1950s, amps were useful slaves, seen and not heard, tucked away out of sight, turned on by a switch on a preamp on a console.
The amps sold to the public and to BBC were pretty bloody awful, compared to what we can do now because of better diodes and caps, and because decent wire and iron for PTs and OPTs costs far less in real terms than in 1950, when most of the good materials had been sent to the bottom of oceans or blown all because men who ruled human kind are blind, vain, stubborn, vile, ruthless, greedy, and arsolic fuctards.

I understand the problem with the common cathode resistor, and I wonder if maybe this is one of the reasons many Quads have failed with melted tar all over. In my amps I swapped tubes around and measured voltage differences for a little bit better DC balance. But separated cathodes sounds like a good idea.(I built a Williamson inspired amp once, using huge wirewound pots between the two output tube cathodes and ground. Was easy to balance but maybe not the last word in highend audio

Leak only used individual biasing of the 2 output KT66. To get the most years of trouble free use with output tubes, there MUST be individual biasing, and the MUST lower value grid bias resistors used to prevent the inevitable positive grid current causing grids to go positive even in the idle condition. Quad used 680k, and its LUNACY. Quad should have used a 6CG7 or 12AU7 LTP which have LOW Ra and then Rg could be 150k max. Aging KT66 will develop "reverse grid current", ie, you'll see perhaps Eg1 at +5Vdc, indicating 7.4 micro amps which sounds like SFA, but that +5Vdc causes about 20 to 30mA of extra Ia to flow, and tube gets too hot, and other becomes cold with Ia too low because tubes don't age at the same speed. So the Idc balance in OPT becomes poisonous to the music because of unwanted dc magnetization of an un-gapped core. This imbalance is reduced with individual R&C bias networks, and the use of Rg = 150k would reduce
Eg1 from say 5Vdc to 1Vdc, and imbalance is then tolerable, and you get a few more years from KT66. I've watched this happen in many amps I have serviced and where owners would cry tears of blood before ever consenting to buy new tubes for their bloody horrid old junk amps!!!

It would be interesting to hear more ideas about how to make Quads safe for future use and longevity. I understand it is not a good idea to put a fuse between the HT center tap to ground?

Chassis MUST be grounded. So each power amp chassis must have IEC connector to take standard 3 wire leads with IEC connector at one end and the mains 3 pin plug at other for wall sockets.

The IEC chassis plug can have a mains fuse in it, so that the mains cable MUST be removed from amp to remove the mains fuse.

To minimize hum, the 0V rail must have 5r0 between it and 0V rail, and if there is a B+ fuse, best place is between HT CT and 0V, NOT to chassis "ground."
Original Quads do have a 5r0 between 0V and Chassis. Now have a real good look at your original Quads' earthing method. What do you find?

The trouble with fuses is that they may take time to blow, and PTs can heat up and potting mix will melt and run out before the fuse blows, and I've seen this a few times because of arcing ESL57 speakers. You can have one KT66 operating with red hot anode, the owner is oblivious, and amp limps along for awhile damaging itself, maybe towards a very expensive repair. SO, the ONLY remedy is to have some active circuit which detects excessive Idc flow in output tubes and this turns off the amp and a red LED lights up which tells an owner "I will NOT work until YOU fix a problem". The active circuit works far better than any HT fuse. I have installed such circuits many times into new and old amps and owners became so glad I did.

Tube amps can be hell to live with. I've repaired and rebuilt tube amps which have nearly burned houses to the ground. Get real, Old junk is always old junk unless proved otherwise, and much must be done to save them from themselves and from dopey owners so that THEN the past can be really enjoyed, with added benefit of better modern parts. Needless to say, all old Quad amps, especially the Quad 22, need total R&C replacement, and with a few wise other mods I mention at my website.
I'm preparing a page about how I rebuilt a pair of Quad-II-Forty. These Chinese made souped up versions of Quad-II have many of the same horrid features to be found in original Quad-II.
Patrick Turner.


Martin



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Default Quad II question: High B+ a problem?

Wow, that was a lot more info than I expected. Thanks!

Re Usenet: Yes it is not always polite, and sometimes people cannot cool down in time. But it comes with the "unmoderated" territory. It is what Internet was and is about for those of us who have been around here long. I might not personally like everything I see or hear but, hey, at least everybody can say what they want. So flame me, ha! (actually dont, Im not here for that purpose)

Back to Quad:

Lots of food for thought. Thanks for sharing the ripple voltage measurements under different PSU configurations. Just to clarify, I use both box type "studio" speakers, 8 ohms, and original ESL57 (rebuilt EHT blocks) with this pair of Quad IIs. Amps are strapped for 9 ohms. No fires yet. But I see why you would want to put warning and shutdown circuits in your amps. If this was something I built for someone else, which as a hobbyist I dont do, then one needs a lot of built-in failsafes. As a tinkerer, I open up my amps now and then and check for any health problems. So I dont worry about fires yet. My fake Williamson looks like a rat's nest inside (a "work in progress", over years) but actually had no issues whatsoever in 15 years of constant use. I caught and changed a dry PSU cap once, before it could make a mess. On the other hand, would I want my kids to inherit this? Uh, not before checking the safety arrangements, and maybe tucking the whole thing into a grounded cage. Which kind of proves your point Patrick, so I think I will look into at least separating the QII KT66 cathodes in the future. Also, these chassies were probably late production, with exactly fitting holes for new IEC power sockets already there.

But the mods I routinely do to old original junk like Quad-II does make them last longer, ie, KT66 life is longer, and their safety is made better. Both issues require circuit changes that exceed what you are doing just by changing the two main electros in PSU.


Well, actually, I had to rewire the whole thing (both of them) from scratch, using some original wire looms from two other junked chassies. The previous owner had gutted my amps to build something else but had given up. I put everything back in place except the dangerous HT wiring at the Jones plug. Nowadays there are standalone preamps that are nicer (but not as good looking!).


OK, you are in love with the crummy old quality of the past.


Yes! It has its charm. And the fact that the Quads use a strange circuit, cathode feedback, pentode inputs and too high grid R's, is interesting. It is very different from the other things I have seen or built for myself. I mostly like tubes like 5687 or ECC88 driving 6B4Gs or triode strapped 6L6 or 807 in SE or PP. But I know I am not in any way an inventor in this field, since I usually just start with a known good base circuit, steal it, and tweak it using whatever old iron and tubes I can find.

Slightly OT: Old Jags apparently had trouble with the Lucas electronic components. Some people gut them and rewire everything with modern wires, LEDs and electronic ignition timing. There are crazy forums for that too online

To minimize hum, the 0V rail must have 5r0 between it and 0V rail, and if there is a B+ fuse, best place is between HT CT and 0V, NOT to chassis "ground."

Original Quads do have a 5r0 between 0V and Chassis. Now have a real good look at your original Quads' earthing method. What do you find?


No, I missed this. There are, AFAIK, no 5R resistors anywhere, unless you are looking at the long wire from the "E" point.How does it work? Was it intentional, like the C to GND of the PIO caps? Or did I rewire the whole thing wrong....

You can have one KT66 operating with red hot anode, the owner is oblivious, and amp limps along for awhile damaging itself, maybe towards a very expensive repair. SO, the ONLY remedy is to have some active circuit which detects excessive Idc flow in output tubes and this turns off the amp and a red LED lights up which tells an owner "I will NOT work until YOU fix a problem".


Sure. But Id rather have a magic eye, neon or at least a funky Nixie lighting up, instead of those LEDs!

Tube amps can be hell to live with. I've repaired and rebuilt tube amps which have nearly burned houses to the ground. Get real, Old junk is always old junk unless proved otherwise, and much must be done to save them from themselves and from dopey owners so that THEN the past can be really enjoyed, with added benefit of better modern parts.


Ive worked on a few old guitar amps for musician friends, with varying results. Original AC30, Fender Twin and such. THAT is a can of worms. They are not at all babying their gear like audiophile guys do. Half broken wooden boxes, redplating tubes and loose wires everywhere from previous DIY attempts. And the owners dont want any new R or C because that would alter the sound :/

I'm preparing a page about how I rebuilt a pair of Quad-II-Forty. These Chinese made souped up versions of Quad-II have many of the same horrid features to be found in original Quad-II.

Patrick Turner.


I just saw that one! Nice writeup! Good to see you got rid of the ugly PCB. I am curious about the thermistor they used on the rectifier in the original version. What kind was it? Did it work? Did they REALLY not bother with a dedicated 5V winding? Crazy.

Martin

(not having nearly as much time as I would like to tinker with stuff or even read here - day job and family comes first)


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Default Quad II question: High B+ a problem?

Just caught an error: Of course I meant I can use EITHER dynamic box speakers, OR the ESL57's. So nobody thinks Im running them in parallell. I did not try but I guess the OPT would not like that.

Martin
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Default Quad II question: High B+ a problem?

On Tuesday, May 13, 2014 9:35:49 PM UTC+1, wrote:
Just caught an error: Of course I meant I can use EITHER dynamic box speakers, OR the ESL57's. So nobody thinks Im running them in parallell. I did not try but I guess the OPT would not like that.



Martin


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