[Patrick Turner]

I have a VAC 7070 here for repairs, upgrades, and
for instalation of active protection circuits.

I have just about completed the circuit changes which included
removal of output stage FB capacitors to "neutralise" the 300B input C,
re-working of the values of RC used in the 6 way switchable NFB networks
to give unconditional stability, including phase/gain shelving networks
for LF and HF, and including 8ohms plus 0.22uF across the 8 ohm outlet
to load the amp at HF. Finally the amp was able to
give a very nice stable response
from 7Hz to 84kHz with 6dB NFB, and Rout of about 1 ohm.

The noise the amp makes with 4 x 300B in PP for each channel was the
main obstacle left.

I started by rectifying the heater supplies for the two 6SN7 used for
input, CPI,
and balanced amp. The circuit used is virtually identical to the
original Williamson amp from 1947.

I added a CCS to the tail of the balanced amp which much reduced the
common mode 2H which otherwize appears at each phase of output.
The loading on the balanced amp 6SN7 is 25k dc supply R and cap coupled
100k
grid bias R for the following output stage.

I got the driver stage to make 91Vrms at 3%, and its about as good as
anyone could
with R loading instead of choke + R loading as seen in schematics at my
website.

67Vrms is needed to drive each of the output grids where the Ek = 93V,
Ia at idle is 59mA,
and Ea = +416V, so the 300B are very comfortably set up to give a lot of
class A PO,
with each 300B dissipating about 25 watts.

Anyway, I finally got the amp with zero NFB to give a full BW and
operate without low level LF instability, and to give hum&noise less
than 1mV.

The original VAC had the 300B idling at an unsafe 37 watts.

Before I was asked to intervene, one 300B turned white, and just quit,
then another after a red hot episode,
and the amp is sent to me for a fix. Two others were dodgy.
I discover one 300B develops a slightly positive grid, and hum appears
at the output
when it is used, so I apply 27 + 27 ohms as a divider to centralise the
hum on all the heaters.
Each output tube's dc heater supply is made by rectifying
a Vac winding with a bridge and 33,000uF to make 5Vdc with 66mV of hum.
I try another of the original 8 x 300B and find one shorts from anode to
grid
when heating power is applied, Hum sure increases, and 450Vdc is seen
across the 10W x 1k5 cathode R.
I turn it off, label the 3rd crook tube, and plug in the last I can
select, and then the amp
stops humming. For how long?

The anode supply is an SS bridged rectifier feeding 4 x 330uF in a
series/parallel
set up to give a total C of 330uF, and the ripple is about 1.5Vrms at
the OPT CT.
If one 300B is left out of the circuit, hum at the output increases
alarmingly, because Idc is very unbalanced.
So the amp depends on its cathode bias and output tube bias stability
for low hum. During the many tests over a few hours I performed, the hum
varied considerably.

One would need to only use speakers with sensitivity below 89dB/W/M if
one expects
silent operation. For horn speakers, amps need to be very quiet indeed.
The VAC would be totally unsuitable for horns, but a single 300B with
squeaky clean PSU
would be fine.

The 300B were Sovtek types, and very microphonic.
Tapping them lightly made them ring audibly like a bell,
and this created wave forms on the output in excess of 20mV very easily.
( Is the subtle actual microphony in tubes responsible for the different
"sound" we hear
with different brands of tubes? )
The TRIODE output stage means Ra is very low, and with a
low impedance power supply the 100Hz hum voltage at the OPT CT creates
a common mode hum current in each output tube of about 5mA, and
when inspecting the cathodes at the top of each 220uF cathode bypass cap
there is 22mV of 100Hz hum, and at low level use this intermodulates the
incoming signals, and when observing the THD of the amp at low levels
normally used for listening
I found there was slightly more amplitude modulation of the distortion
wave form than the
actual harmonic distortion itself.
This would not be the case if the PSU was better filtered.
So the voltage measurement of whatever harmonic or IMD was present
showed that
it rose faster and sooner than I would ever like to see between 0.0 Vo
and 2Vo,
and this is supposed to be the really clean region of any amplifier
working because
most listening is done with signals below 2Vrms.

So, would I bother ever using 300B for anything I'd sell anyone?

Not likely, because one MUST better filter the heaters to reduce hum
10mV
and prevent its modulating Ia, and better filter the B+ applied to the
OPT CT.

The filtering of the 5Vdc cathode suplies is most effectively done using
CLC,
and where you have an amp with 8 x 300B, its a lotta gear you have to
use.
The VAC has 4 small 12VA transformers each with 2 x 7Vac windings to
make 8 windings.
There are 8 bridges and 8 x 33,000uF caps.

It looks like they didn't want to do it right and have 8 windings for
300B heaters
on the main large PT. This would have left room for the filter chokes
for the B+.

Using KT88, 6550, or better still, KT90
would be cheaper, easier, less microphonic, and give longer tube life
without what appears to be the fragility of the 300B.

The amp owner has given me a fresh set of Sofia 300B output tubes that
do look nice,
and I hope they live up to their looks.
At least they won't be idled at levels which invites failures,
and if one fails, the amp will politely shut down
without smoke or burning out cathode resistors and caps and starting a
fire
and scorching a portion of the 4mm thick fibre board used with turrets
for connecting point to point.

I hate the use of boards in all tube power and preamps.

They just don't belong.
There isn't any access to the blind side of the turrets, and although
the board
is held in the amp with a few screws, its a terribly horrible job to
remove the board if one has to.
Accessing the tube sockets between the board and top metal part of the
chassis is an awkward
job.

Sockets should always be bolted to the metal plate that forms the
chassis toplate.
Terminal srips which can be turrets if you like should be on strips
of 8mm x 8mm fibreglass rods held away from the chassis to allow wires
to pass under.
The board actually reduces volume in which parts can be mounted.
In the VAC, I was stuck with the damn board, because the alternative of
chucking all VAC's work under the chassis into the bin and starting all
over
was much more work than replacing all the GE capacitors with Wimas.
Just why so many 300B failed isn't clear, but I have replaced all
coupling caps
in case some fail again soon. Several had already been replaced by some
other tech before me.

Last year I repaired a pair of Sun Corp amps with 2 x 2A3 per channel
there was ac heating,
and a hum nulling pot was used for each heater circuit which has cathode
biasing.
I gave the owner a pair of headphones to use while adjusting hum to
minimum levels every few weeks.
Where you might have 4 x DH triodes per channel and ac heating,
adjusting hum
on the 4 tubes would not be reliable, and not easy, because the
adjustment is interactive, and because of the
problem with IMD with incoming signals.
Hence octal tubes with IDH cathodes are much better, unless you reduce
hum on the cathode
Vdc supply to really low levels. This could be achieved CLC as I said,
and the L doesn't need to be very large, but otherwise done with a
regulator
circuit using bjts, but if anything goes wrong, they can get zapped a
lot easier
than a choke.

Eventually, I will get the VAC to be as good as it can be, and I may yet
have to
put in a pair of small chokes for the B+, 1H at least.

So far, the VAC with 4 x 300B hakes almost as much maximum power as my
SE845
with a pair of 845 giving 60 watts max, A1 PO.

The 845 SE amps have 7dB global NFB and VAC now has 12dB max global NFB,
but the
VAC makes twice the THD at 5V into 8 ohms than the SE amp.

Just because an amp is a really gee whiz super duper push pull class A
triode
concoction, it does not mean that it will measure better in *any* way at
listening levels
compared to many SET amps.
it takes 12dB GNFB to get Rout of the 4 x 300B PP to be just undr 0.5
ohms.
Its 2.4 ohms without GNFB.
The SE845 has Rout = 0.35 ohms with only 7dB NFB and 0.9 ohms without
NFB.

So no wonder the SET amp which has equal max PO to the max PP amp PO
sounds better. There is less distortion of all kinds.

When we listen to the utter drivel Trevor Wilson sprays about the Net,
we can all see just how utterly incorrect he is,
as is born out again by my comparitive observations of fundamentally
good amps like the VAC
and including ones I design and build myself.

I do like the 300B; I am fond of it, it does well with audio signals.
The VAC sounded very well just before it came to me, with at least 2 of
8 tubes
not operating, or operating poorly.
But you do have to look after the 300B well to get it to perform at its
best,
and that means providing voltage supplies like those from good
batteries.
Use piles and piles of capacitance, and chokes where appropriate.

In Quad-II amps, the B+ is applied to the OPT CT filtered only by 16uF,
and thus the Vripple = 17Vrms at the OPT CT. ( Quad-II is a chic toy
thinge from 1955 )
In my measurements, the IMD caused by interaction of riple voltage and
signal
means total distortion measured with a sine wave signal about equal to
the THD alone.
But the KT66 has a high value of Ra, and thus common mode ripple voltage
current flow in the
tubes isn't great. It would be if you run the output tubes as triodes.
So I don't recommend it unless you upgrade the PSU with CRC or CLC B+
filtering
all as shown at my website.

Patrick Turner.

[RapidRonnie]

In a correctly designed tube power amp with filamentary output tubes
AC power can be used with perfect silence. It's easier with push pull
but even SE amps can be done. One key is the use of a fully balanced
AC transformer with a center tap at perfect null for DC and AC. There
should be a ground shield between the primary and secondary.

If DC power is used the filaments will die early unless polarity is
swapped from time to time.

[glenbadd]

On May 20, 3:53 am, RapidRonnie wrote:
...
If DC power is used the filaments will die early unless polarity is
swapped from time to time.

Documentary evidence? If there is significant grid bias, the
drain of electrons from the filament will be fairly constant
along its entire length.

[Patrick Turner]

RapidRonnie wrote:

In a correctly designed tube power amp with filamentary output tubes
AC power can be used with perfect silence. It's easier with push pull
but even SE amps can be done. One key is the use of a fully balanced
AC transformer with a center tap at perfect null for DC and AC. There
should be a ground shield between the primary and secondary.


Last year I repaired a SunCorp amp with 2 x 2A3 DHT in PP.

Both 2A3 had ac powered cathodes from 2 windings on a separate
PT just for heater power made by Hammond.

There were I think, 22ohms + adjust pot + 22ohms so that a null of hum
could be obtained by adjusting the pot to EACH 2A3.

There was only one position on each pot which gave the best null
of hum.

If a CT of a winding was used, the amp would hum.

I gave my customer an adaptor to connect a pair of headphones
across the output terminals so he could adjust for this best
null position of hum.

If DC power is used the filaments will die early unless polarity is
swapped from time to time.

What do you mean by "die" ? is it failing emission,
or going open circuit?

I cannot quite see your reasons for your sugestion.

Patrick Turner

[Nick Gorham]

RapidRonnie wrote:
In a correctly designed tube power amp with filamentary output tubes
AC power can be used with perfect silence. It's easier with push pull
but even SE amps can be done. One key is the use of a fully balanced
AC transformer with a center tap at perfect null for DC and AC. There
should be a ground shield between the primary and secondary.

If DC power is used the filaments will die early unless polarity is
swapped from time to time.

Whats your definition of "perfect silence"? -40dB, -60dB, or what?

Even if you do get the hum down to whatever level you don't notice, it
still doesn't have any effect on the modulation caused by the heating
effect of AC on the fillament

http://members.aol.com/sbench/humbal.html

--
Nick

[Patrick Turner]

Nick Gorham wrote:

RapidRonnie wrote:
In a correctly designed tube power amp with filamentary output tubes
AC power can be used with perfect silence. It's easier with push pull
but even SE amps can be done. One key is the use of a fully balanced
AC transformer with a center tap at perfect null for DC and AC. There
should be a ground shield between the primary and secondary.

If DC power is used the filaments will die early unless polarity is
swapped from time to time.

Whats your definition of "perfect silence"? -40dB, -60dB, or what?

Even if you do get the hum down to whatever level you don't notice, it
still doesn't have any effect on the modulation caused by the heating
effect of AC on the fillament

http://members.aol.com/sbench/humbal.html

--
Nick

Thanks Nick for reminding me of what Steve Bench has to say about
hum and IMD.

The page above of his tells us how to sort of couple the
grid of the DHT to the cathode using diodes that some how
reduces IMD and gives a cleaner outcome for music presumbably.

In the VAC I have repaired, its rasther too much work to adopt Steve's
ideas.
And there is already dc applied to each cathode of 8 tubes, and there is
cathode bias, to avoid having the nightmare of 8 bias adjustments
for the two channels with 4 x 300B each.

With fairly well smoothed 5Vdc applied across each cathode with
only 66mV of ripple, AND with now well smoothed B+ with
0nly 30mV of ripple, the IMD I was seeing with 1.8V of ripple at the B+
at CT
has now vanished with the hum.

What constitutes a quiet amp?

Well, both VAC channels now have 1mV of hum or other noise per
channel,
and my THD tests suggest the noise does not increase while in class A
and only slightly
when forced into class AB.

Since 19Vrms is available into 8 ohms, and assuming noise was 2mV,
the SNR at clipping is 0.002V / 19V = approx 1 / 10,000, or -80dB,
unweighted,
which is quite good for any tube amp, and this is without GNFB.

With no signal present, on the CRO the noise reduces as GNFB is switched
in
so the SNR gets better by about the amount of NFB used; in my case 12dB,
so with GNFB, SNR becomes -92dB.

However, one never uses an amp at 19V of output into 8 ohms.
Inevitably, THD and IMD artifacts at high level completely swamp the
noise levels.

Ppl use more like an average of 1V, so if there was 1mV of noise, SNR
is only 0.001V / 1V = -60dB unweighted, and this is without GNFB.

With GNFB it becomes -72dB, unweighted.

It means there is 0.25mV of hum present with 1V of signal.

Typical THD at 1V with GNFB = 0.02%, which is -74dB.

If we assumed IMD was also of similar magnitude
with say the standard 4:1 LF:HF test,
then total THD and IMD might be -71dB.
So the whole lot of noise and distortion could add to about -68dB.

It means with 1V of signal about 1/2 a millivolt of noise and
distortion.
If it was possible to play such an amount on its own in a speaker,
you'd not hear it.

The N&D could be up to about 2mV before it could be audible on its own.

If average listening levels were 1V, the permissable ratio of N&D :
signal
could be 0.002V / 1V, = 1/500 = -54dB, unwieghted, or 0.2%.

I found the VAC with my mods and without GNFB easily was able to
comply with this proviso for artifact levels.
THD at 1V, 8 ohms on the 8 ohm outlet = 0.05%,
and noise 1mV.

With 12dB GNFB, the THD at 1V = 0.015%.

So the sonic purity is definately good enough.

Whatever differences ppl percieve in sound with or without NFB isn't
much due to
artifacts because they are so darn low in both cases.

I suspect microphony may measure more than THD/IMD if one was able to
measure it while music played.
Microphony is a form of noise, or non-quietness.
Its effects are directly reduced by the amount of NFB applied.

But many ppl prefer no NFB; they hear a pleasant difference
which they say is more like the real music they hear at a concert.

I just build amps as well as i can on a shoestring budget.

I leave the public to argue about how they hear it but
if I make my amps follow the rules above for low THD&IMD and
hum&othernoise,
the public like what I make.

Its very difficult to make a bad sounding triode class A amp
if you have followed my reasoning above.

Patrick Turner.

[Nick Gorham]

Patrick Turner wrote:


It means with 1V of signal about 1/2 a millivolt of noise and
distortion.
If it was possible to play such an amount on its own in a speaker,
you'd not hear it.

Yes, I try and aim for 0.5mv hum and noise at the terminals as well. I
find that once it gets to that level the amp is quiet with just about
any speaker. It does help is its a nice smooth sine + rabdom noise as
well, no nasty power supply induced harmonics.


I suspect microphony may measure more than THD/IMD if one was able to
measure it while music played.
Microphony is a form of noise, or non-quietness.
Its effects are directly reduced by the amount of NFB applied.


I agree, it certainly is in there. Its one of the reason I like teflon
sockets instead of ceramic ones, it seems to have a slight damping effect.

--
Nick

[Patrick Turner]

Nick Gorham wrote:

Patrick Turner wrote:


It means with 1V of signal about 1/2 a millivolt of noise and
distortion.
If it was possible to play such an amount on its own in a speaker,
you'd not hear it.

Yes, I try and aim for 0.5mv hum and noise at the terminals as well. I
find that once it gets to that level the amp is quiet with just about
any speaker. It does help is its a nice smooth sine + rabdom noise as
well, no nasty power supply induced harmonics.


I suspect microphony may measure more than THD/IMD if one was able to
measure it while music played.
Microphony is a form of noise, or non-quietness.
Its effects are directly reduced by the amount of NFB applied.


I agree, it certainly is in there. Its one of the reason I like teflon
sockets instead of ceramic ones, it seems to have a slight damping effect.

Usually the input tube is the source for microphony, most especially in
phony and microphone amps,
but also in line and power amps. But IDH tubes do have low microphony
compared to DH tubes.

Having the tube in a soft plastic socket doesn't do much and
in some old mic amps and tape recorders I have seen the socket mounted
on
soft springs. One can encase the tube plus socket it in a foam block.

But 300B are output tubes and are quite microphonic compared to
IDH octals etc which usually are much less micro.

Its possible the N&D created by the 300B is less than the microphony
if the level is just right.

Certainly, one wouldn't ever want to use a 300B in a guitar combo amp.


Patrick Turner.

--
Nick

[BretLudwig]

The WE made 300Bs and their UK made STC 4300 (?) equivalents were pretty
non-microphonic if they were made much after WWII. The modern ones being
made only for hobbyists are all over the place.

There are other reasons for not using triode outputs in commercially
manufactured guitar amps-first few would like the sound and second no
dealers would sell them because they'd have to stock "oddball" tubes.

Gibson Guitar Inc. (sic) learned this with the Red Bear line of combo
amps they imported in the Yeltsin era when Russian goods were
super-giveaways wholesale. They used Russian types that were cheap.
Dealers would not sell them. They could be converted to common tubes but
dealers said, why bother? Also, getting parts like transformers was
impossible.

Commercial products are not so much about what consumers like but what
retailers want to sell.

That's why GM failed ultimately with diesel cars in the 1970s and before
that with the Corvair.

--
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