MK III biaset zoomz to 26v during warmup

[- - : R A T B o y : - -]

I'm rebuilding a pair of MK III's. Just now working on the first.

My plan was to replace all tubes AND:

-1 Replace selenium stack w/ diode & bias caps/resistors with a kit from
Uncle Neddy. Test voltages.

-2 Replace quad cap with triple cap + 1 cap with another kit from Ned. Test
voltages.

-3 Replace driver board with stuffed board from Classic Valves. Test
voltages. Enjoy some Dave Brubeck on my rebuild 60 honkin' watt amps.

Of course, I never took baseline voltage measurements (for I am RAT Boy...
and I knows it all). I replaced the bias circuit (being very careful to get
it all wired back correctly). Using original tubes, I powered it up and
checked the biaset... 1v, 1.9v 2v, 4v, 26v!!! PULL THE PLUG!

I powered it up again and began to check the voltages and compare them to
those listed in manual... 5AR4 voltages are OK on pins 4 & 6 but pins 2 & 8
are sky high. PULL THE PLUG!!!

Replaced the Quad filter with Ned's (triple + 1) kit... same issue.

Is my PT fried? What should resistances at PT leads be? Could the choke be
causing this? Ideas?

[[email protected]]

- - : R A T B o y : - - wrote:
>
> Is my PT fried? What should resistances at PT leads be? Could the choke be
> causing this? Ideas?


Hi RATs!

RATboy, you are playing with powerful forces. Please do the basic
homework before you kill yourself, or worse, ruin a good amp.

An email to this herd of wretched posers is not they way to learn basic
electronics.

Sorry. Read some books. Build some circuits.

Ignorance is bliss. Bliss is for idiots.

Happy Ears!
Al

[Jon Yaeger]

in article , - - : R A T B o y : - -
at - - : R A T B o y : - wrote on 10/1/06 2:11 PM:

> I'm rebuilding a pair of MK III's. Just now working on the first.
>
> My plan was to replace all tubes AND:
>
> -1 Replace selenium stack w/ diode & bias caps/resistors with a kit from
> Uncle Neddy. Test voltages.
>
> -2 Replace quad cap with triple cap + 1 cap with another kit from Ned. Test
> voltages.
>
> -3 Replace driver board with stuffed board from Classic Valves. Test
> voltages. Enjoy some Dave Brubeck on my rebuild 60 honkin' watt amps.
>
> Of course, I never took baseline voltage measurements (for I am RAT Boy...
> and I knows it all). I replaced the bias circuit (being very careful to get
> it all wired back correctly). Using original tubes, I powered it up and
> checked the biaset... 1v, 1.9v 2v, 4v, 26v!!! PULL THE PLUG!
>
> I powered it up again and began to check the voltages and compare them to
> those listed in manual... 5AR4 voltages are OK on pins 4 & 6 but pins 2 & 8
> are sky high. PULL THE PLUG!!!
>
> Replaced the Quad filter with Ned's (triple + 1) kit... same issue.
>
> Is my PT fried? What should resistances at PT leads be? Could the choke be
> causing this? Ideas?
>



Looking at the schematic, the Mark III is an ultralinear amplifier with
fixed bias.

The recommended bias voltage for this configuration is -70 volts, +/- 25%.
When you say 26 volts do you mean POSITIVE 26 volts?

If so, check the orientation of the replacement bias rectifier - it is
probably installed backwards. The B+ of an amplifier will be very high if
the output tubes aren't conducting. There is no conduction with a positive
bias . . .

Check the silicon bias diode for proper orientation.

Jon

[Jon Yaeger]

in article . com,
at wrote on 10/1/06 5:02 PM:

>
> - - : R A T B o y : - - wrote:
>>
>> Is my PT fried? What should resistances at PT leads be? Could the choke be
>> causing this? Ideas?
>
>
> Hi RATs!
>
> RATboy, you are playing with powerful forces. Please do the basic
> homework before you kill yourself, or worse, ruin a good amp.
>
> An email to this herd of wretched posers is not they way to learn basic
> electronics.
>
> Sorry. Read some books. Build some circuits.
>
> Ignorance is bliss. Bliss is for idiots.
>
> Happy Ears!
> Al


Al,

Some assistance here would be better than poetic derision . . . .

Jon

[Eeyore]

Jon Yaeger wrote:

> There is no conduction with a positive bias . . .

Eh ?

Graham

[Jon Yaeger]

in article , Eeyore at
wrote on 10/1/06 5:54 PM:

>
>
> Jon Yaeger wrote:
>
>> There is no conduction with a positive bias . . .
>
> Eh ?
>
> Graham
>


If you bias a grid positive with respect to the cathode, it will not
conduct.

[Jon Yaeger]

in article , François Yves Le Gal
at wrote on 10/1/06 7:02 PM:

> On Sun, 01 Oct 2006 17:22:18 -0400, Jon Yaeger > wrote:
>
>> The recommended bias voltage for this configuration is -70 volts, +/- 25%.
>> When you say 26 volts do you mean POSITIVE 26 volts?
>
> I guess that there's 26 V on the "biaset" connector on the front panel,
> which should give a 1.56 V reading (same as an unloaded D cell) when bias is
> properly set.
>

I was talking about the grid of the output tubes. Guess I should have
clarified that.

[[email protected]]

Jon Yaeger wrote:
> in article . com,
> at wrote on 10/1/06 5:02 PM:
>
> >
> > - - : R A T B o y : - - wrote:
> >>
> >> Is my PT fried? What should resistances at PT leads be? Could the choke be
> >> causing this? Ideas?
> >
> >
> > Hi RATs!
> >
> > RATboy, you are playing with powerful forces. Please do the basic
> > homework before you kill yourself, or worse, ruin a good amp.
> >
> > An email to this herd of wretched posers is not they way to learn basic
> > electronics.
> >
> > Sorry. Read some books. Build some circuits.
> >
> > Ignorance is bliss. Bliss is for idiots.
> >
> > Happy Ears!
> > Al
>
>
> Al,
>
> Some assistance here would be better than poetic derision . . . .
>
> Jon

Hi Jon,

OK. I was caught up in the romance of the moment.

-

Ratboy,

Back up a few yards and start fresh :)

1. Did the amp work before you tried to improve it?

Happy Ears!
Al

[Eeyore]

Jon Yaeger wrote:

> in article , Eeyore at
> wrote on 10/1/06 5:54 PM:
>
> > Jon Yaeger wrote:
> >
> >> There is no conduction with a positive bias . . .
> >
> > Eh ?
> >
> > Graham
>
> If you bias a grid positive with respect to the cathode, it will not
> conduct.

So how is it that many high-power RF amps operate in this way ?

Graham

[Jon Yaeger]

in article , Eeyore at
wrote on 10/1/06 9:30 PM:

>
>
> Jon Yaeger wrote:
>
>> in article , Eeyore at
>> wrote on 10/1/06 5:54 PM:
>>
>>> Jon Yaeger wrote:
>>>
>>>> There is no conduction with a positive bias . . .
>>>
>>> Eh ?
>>>
>>> Graham
>>
>> If you bias a grid positive with respect to the cathode, it will not
>> conduct.
>
> So how is it that many high-power RF amps operate in this way ?
>
> Graham



Graham.

My sincere apology. I thought we were talking about a Mark III audio
amplifier.

Just for my edification, how many AUDIO frequency amps operate like your
subject RF amp?

I've not seen a push-pull output stage or a SE for that matter, with the
plate at B+ and a grounded cathode that conducts when the grid goes
positive.

Please enlighten me. I'm ready to give up my hobby once you show me the
light.

Jon

[Patrick Turner]

Jon Yaeger wrote:

> in article , Eeyore at
> wrote on 10/1/06 5:54 PM:
>
> >
> >
> > Jon Yaeger wrote:
> >
> >> There is no conduction with a positive bias . . .
> >
> > Eh ?
> >
> > Graham
> >
>
> If you bias a grid positive with respect to the cathode, it will not
> conduct.

On the contrary Jon,
If a grid is biased positively and cathodes are ar 0V,
then the tube will be turned on fully and the there will also be grid
current.

Maybe the cathodes are not grounded properly
and the 11.2 ohm current sensing resistor is open
because getting 26V across the 11.2 ohms is a very large amount of
current.
But if the 11.2 ohm is open, the voltage at the cathodes will measure
very high without the
tubes conducting any current.

What is the applied NEGATIVE applied bias voltage at the grids?

If the bias supply diode has been reveresed to produce a +ve supply
instead of a -ve supply
the cathode current will be maximal saturation current.

If the current of 2 amps is maintained longer than 20 seconds
in the two output tubes the PT and OPT could be seriously damaged
because these ancient bits of old audio junk don't have automatic over
current
detection that turns off the amp if the cathode voltage exceeds the
1.56V that should appear across the 11.2 ohms
by 50%.

Having only ONE bias supply for 2 output tubes is a very poor way to
design any amp
and it should be altered so there is a bias set pot for EACH tube.
When installing revised bias set circuits only 3 watt wire wound pots
should be used.

Patrick Turner.

[[email protected]]

Jon Yaeger wrote:
>
> I was talking about the grid of the output tubes. Guess I should have
> clarified that.

Hi RATs!

I withdraw my apology. Asking the ignorant sluts on this NG to teach
you basic electronics is suicide,

So, RATboy, did the amp work before you decided to improve it?

Happy Ears!
Al

[Jon Yaeger]

Patrick,

You are absolutely right. So are you, Al.

Don't know what I was smokin' or thinking.

Apologies to all.

Jon

[Eeyore]

Jon Yaeger wrote:

> in article , Eeyore wrote
> > Jon Yaeger wrote:
> >> in article , Eeyore wrote
> >>> Jon Yaeger wrote:
> >>>
> >>>> There is no conduction with a positive bias . . .
> >>>
> >>> Eh ?
> >>>
> >>> Graham
> >>
> >> If you bias a grid positive with respect to the cathode, it will not
> >> conduct.
> >
> > So how is it that many high-power RF amps operate in this way ?
> >
> > Graham
>
> Graham.
>
> My sincere apology. I thought we were talking about a Mark III audio
> amplifier.
>
> Just for my edification, how many AUDIO frequency amps operate like your
> subject RF amp?

None of them do so intentionally AFAIK !


> I've not seen a push-pull output stage or a SE for that matter, with the
> plate at B+ and a grounded cathode that conducts when the grid goes
> positive.

It gets pretty non-linear AIUI but the RF guys aren't especially bothered
about that.


> Please enlighten me. I'm ready to give up my hobby once you show me the
> light.

About positive grid bias ? I only know of it in passing really. I wouldn't
want *anyone* to give up their hobby btw.

Graham

[Eeyore]

Patrick Turner wrote:

> Having only ONE bias supply for 2 output tubes is a very poor way to
> design any amp and it should be altered so there is a bias set pot for
> EACH tube.

That's much better pratice.


> When installing revised bias set circuits only 3 watt wire wound pots
> should be used.

We discussed this at least a year ago funnily enough. I'd actually use a
cermet pot out of preference, they're easily as reliable and also have a
finer setting accuracy. Also it's advisable to wire the wiper of the bias
pot to the the most negative part of the -ve bias circuitry via large
value R in the rare event that the wiper should ever go open circuit.

Graham

[Sander deWaal]

"- - : R A T B o y : - -" <- - : R A T B o y : - >
said:

>I'm rebuilding a pair of MK III's. Just now working on the first.
>
>My plan was to replace all tubes AND:
>
>-1 Replace selenium stack w/ diode & bias caps/resistors with a kit from
>Uncle Neddy. Test voltages.
>
>-2 Replace quad cap with triple cap + 1 cap with another kit from Ned. Test
>voltages.
>
>-3 Replace driver board with stuffed board from Classic Valves. Test
>voltages. Enjoy some Dave Brubeck on my rebuild 60 honkin' watt amps.
>
>Of course, I never took baseline voltage measurements (for I am RAT Boy...
>and I knows it all). I replaced the bias circuit (being very careful to get
>it all wired back correctly). Using original tubes, I powered it up and
>checked the biaset... 1v, 1.9v 2v, 4v, 26v!!! PULL THE PLUG!
>
>I powered it up again and began to check the voltages and compare them to
>those listed in manual... 5AR4 voltages are OK on pins 4 & 6 but pins 2 & 8
>are sky high. PULL THE PLUG!!!
>
>Replaced the Quad filter with Ned's (triple + 1) kit... same issue.
>
>Is my PT fried? What should resistances at PT leads be? Could the choke be
>causing this? Ideas?


Have you accidentally reversed the polarity of the bias diode?
Remember that bias voltage in a tube amp is negative relative to
ground.
The cathode of the diode (what used to be the selenium rectifier) has
to be in the direction of the transformer, the anode to the
electrolytic capacitor.

How did you measut=re voltages on the 5AR4?
2 and 8 are filament, but 8 is connected to the cathode as well and
willl as such carry B+ when warm, relative to ground.

--
"Due knot trussed yore spell chequer two fined awl miss steaks."

[Patrick Turner]

Eeyore wrote:

> Patrick Turner wrote:
>
> > Having only ONE bias supply for 2 output tubes is a very poor way to
> > design any amp and it should be altered so there is a bias set pot for
> > EACH tube.
>
> That's much better pratice.
>
> > When installing revised bias set circuits only 3 watt wire wound pots
> > should be used.
>
> We discussed this at least a year ago funnily enough. I'd actually use a
> cermet pot out of preference, they're easily as reliable and also have a
> finer setting accuracy. Also it's advisable to wire the wiper of the bias
> pot to the the most negative part of the -ve bias circuitry via large
> value R in the rare event that the wiper should ever go open circuit.
>
> Graham

Cermets are OK but a wire wound of 10k with a range of -30V to -50V
is usually quite OK for tube biasing.
The range of voltage adjustment should never allow the tube to be biased
fully on, or biased right off.

Owners of tube amps routinely make terrible mistakes with bias pots
and owners are responsible for much of the smoke clouds seen in loungerooms
from time to time.

All old amps like the crap from Dynaco should be fitted with 10ohm R to each
output tube cathode
and biased with separate bias pots and have active protection measures
against
excessive cathode currents.
Fixed bias isn't self regulating!!!!

As tubes age and approach the end of life the applied grid voltage fails to
control
the anode current which suddenly can begin to rise and send a tube into the
thermal run-away
condition and the tube may sit there red hot until it dies maybe after an
hour of sitting
there with glowing red hot anode and 250mA instead of 40mA.
There is no cathode resistance to offset the effect.
Even with a cathode resistance, the same thing eventually happens, but
at least the Rk can be rated to fuse open if the Ik rises enough, thus saving
the
OPT primary.
Excessive Ia may cause the OPT primary to go open or get really hot, causing
insulation breakdown
of the enamel wire giving shorted turns,
but not cause enough current to flow to make a fuse anywhere blow.

Hence maybe a large cloud of smoke and large repair bill.

Old amps need active protection more than new amps.

Who would drive an old Mustang and use it without a working
water temperature meter and oil pressure guage?

Patrick Turner.

[Bob H.]

I had wandering bias on my Dynaco ST70, which I fought for some time.
It turned out to be the rectifier tube socket. Try swapping out the
sockets, if you haven't done so already.
BTW, all the cleaning in the world didn't fix my problem. Only a new
socket did.
FWIW.

Bob H.



- - : R A T B o y : - - wrote:
> I'm rebuilding a pair of MK III's. Just now working on the first.
>
> My plan was to replace all tubes AND:
>
> -1 Replace selenium stack w/ diode & bias caps/resistors with a kit from
> Uncle Neddy. Test voltages.
>
> -2 Replace quad cap with triple cap + 1 cap with another kit from Ned. Test
> voltages.
>
> -3 Replace driver board with stuffed board from Classic Valves. Test
> voltages. Enjoy some Dave Brubeck on my rebuild 60 honkin' watt amps.
>
> Of course, I never took baseline voltage measurements (for I am RAT Boy...
> and I knows it all). I replaced the bias circuit (being very careful to get
> it all wired back correctly). Using original tubes, I powered it up and
> checked the biaset... 1v, 1.9v 2v, 4v, 26v!!! PULL THE PLUG!
>
> I powered it up again and began to check the voltages and compare them to
> those listed in manual... 5AR4 voltages are OK on pins 4 & 6 but pins 2 & 8
> are sky high. PULL THE PLUG!!!
>
> Replaced the Quad filter with Ned's (triple + 1) kit... same issue.
>
> Is my PT fried? What should resistances at PT leads be? Could the choke be
> causing this? Ideas?