Marshall 1959HW Bias runaway

[Mathias]

No DC differences but let me check if that voltage recovers back. I haven't thought about that.

The pin 5 voltage tells me that the tube sockets have become conductive. Voltage is leaking from 4 to 5 which causes current to flow through the 220 k resistors in a direction which is causing an opposition voltage to the negative bias voltage.

That's why I suggested the 56k resistors in place of the 220 k. You don't have to pull the resistors, you could tack on a couple of 82 k resistors in parallel with each if the 220 k resistors.

What this does is lower the voltage drop across the 220 k resistors caused by the leakage. The conductive path from pin 4 screens is shunted to ground via the bias supply with a reduction of 400 percent leakage voltage, which should be enough to tell you if it is the tube sockets. I just did this to a pair of JCM 2000's that same through the shop.

Don't worry about the screen voltage being over the handbook voltage ratings. This has been going on ever since they started making tubes. There are thousands of guitar amps out there running these voltages.

- Added 82k's on the 220K's but to no avail... The current is still rising.
- I rearranged some wires but no success neither.
- Pin 5 does bump back to its initial -49.6V.

 

[Mathias]

Look for any signs of arcing across the tube sockets , than can sometimes leave behind a slightly conductive carbon residue .

So this is a Marshall amp that youve rewired point to point ?

Maybe the amp is osscillating or unstable at a very high frequency ,
the purple feedback wire from the transformer secondary can be a source of this trouble if its badly routed in the chassis

The purple one, coming from that 47k to the speaker jack?

 

[Mathias]

This is how the wiring is done around the power transformer. Not sure if this is causing any oscillation.

 

[Mathias]

Tubetech mentioned poor lead dress in the NFB connection causing an oscillation (good call!). I'd thought about this next thing earlier but didn't say anything, because from your description the tubes were already hotplating while just playing the amp normally. For future reference, I've had a few Marshalls oscillate when the power tubes are plugged into bias probe sockets. If you disconnect the NFB wire from the OPT to the PI it generally stops, so I just preemptively unsolder it before biasing new tubes.

I've starting to think this is a possible cause... If I take a voltage measurement on pin 5 I can see the idle current jumping on the TAD Bias master.
I've ran it without the DMM to eliminate the DMM being the source but still rising current. I'll check if that resolves the issue.

Just to be sure, that's the purple one from the 47k to the speaker jack, right?

 

[CJ]

You would hear it if the amp was oscillating and the tubes would all red plate within a matter of seconds. And your plate voltage would drop do to the excessive current draw.

How are you measuring the current? 1 ohm resistors on the cathodes? Check those resistors. Remember that you double the plate current for a pair of tubes, so you want about 75 ma per side.

You can double check by turning the amp off, measuring the DCR if the output transformer and measuring the voltage drop across the windings.

You can also try lifting the 220 k junction and neg bias wire off the PC board and see if that changes anything.

 

[Matador]

If the bias voltage is changing, it can only be a few things:

1) The bias supply itself is changing (some issue with the rectification and filtering of the bias supply)
2) The coupling caps from the PI are leaking, and dragging up the negative voltage
3) There is grid current in one or more power tubes, which will drop across the 5.6K's and 220K's

These are easy to test separately:
1) You can use an external negative supply built from 5, 9V batteries (to make a -45V supply). If the bias remains stable then the issue is in the negative supply.
2) You can lift both coupling caps, and see if the bias is stable.
3) You can measure the DC across each of the 5.6K grid stopper resistors, or across the 220K grid leak resistors: there shouldn't be any appreciable DC drop across any of them.

I have seen severely contaminated wax-cardboard eyelet boards on old Fenders become conductive, since on the standard Fender layout a B+ eyelet and the negative voltage tap eyelet are right next to each other, and the board itself can drag up the junction of the 220K grid leaks up by a lot. But I don't think I've ever seen this on a Marshall PCB before.

 

[oldude]

My 2203 did this years ago the first time I checked the bias. I guessed that it was because I had 8v on the filaments and it was getting too hot. My wall voltage is about 125vac in the US. I bought a variac and dialed in 6.5 on my filaments then did the bias. I did have to change a resistor in the bias circuit to stay in range.
I never really checked to see if that actually fixed it. I’m interested in knowing what you find out.
Hope you find results.

 

[CJ]

It could also be the filter CSPs in the bias supply. If they have leakage current then this could drag the bias down. Did you replace them? They go positive to ground.

 

[Mathias]

If the bias voltage is changing, it can only be a few things:

1) The bias supply itself is changing (some issue with the rectification and filtering of the bias supply)
2) The coupling caps from the PI are leaking, and dragging up the negative voltage
3) There is grid current in one or more power tubes, which will drop across the 5.6K's and 220K's

These are easy to test separately:
1) You can use an external negative supply built from 5, 9V batteries (to make a -45V supply). If the bias remains stable then the issue is in the negative supply.
2) You can lift both coupling caps, and see if the bias is stable.
3) You can measure the DC across each of the 5.6K grid stopper resistors, or across the 220K grid leak resistors: there shouldn't be any appreciable DC drop across any of them.

I have seen severely contaminated wax-cardboard eyelet boards on old Fenders become conductive, since on the standard Fender layout a B+ eyelet and the negative voltage tap eyelet are right next to each other, and the board itself can drag up the junction of the 220K grid leaks up by a lot. But I don't think I've ever seen this on a Marshall PCB before.

- I placed in a set of colder TAD STR EL34's just as a sanity check to rule out tubes. Problem not solved.

- No DC on the 5.6K's but YES there is DC creeping up on those 220k's together with the rising current.

- I lifted both PI coupling caps but that did not resolve the issue. The DC voltages across the 220k's keep on creeping up:

@18mA around 0.09V
@28mA around 2.3V

Upon switching the standby off that DC voltage rolls slowly off.

To rule out some things, could this be caused by:

- Improper lead dress under the board
- Any faulty pre amp tube (even with the PI couplers disconnected)
- Could a faulty PT/OT be the culprit of the DC leaking in those 220k's

Any advice is much appreciated

 

[Mathias]

******* I might just found out that the TAD Bias Meter probes are causing all of this...
I said the hell with it I want to play it for a couple of minutes and removed the probes.
Measured no DC at all now on the 220K's. The B+ is stable, the grids are stable... and it seems to sound fantastic as super bass.
Played it for 20 mins... voltages stay dead on stable.

I'd say that follows CJ's theory about the dropping screens and bad sockets, except it's in the TAD Bias Master probes. (I assume)

I'll definitely install some 1 ohm resistors on the cathodes to read bias from.
And do some longer period playing with bias on coldest and measuring to make sure I'm not dreaming all of this up.

The conversion to Super Bass sounds fantastic by the way. I have some Hammond/Philips JAN NOS pre amp valves in there. Just wonderful!

I appreciate all of the input from you guys! I guess I've had an intense crash course on Marshall bias issues...

 

[Khron]

I guess I've had an intense crash course on Marshall bias issues...

You misspelled "TAD Bias Master issues"...

 

[Mathias]

You misspelled "TAD Bias Master issues"...

 

[CJ]

Sounds like you got it!

Me bad for not seeing the tad bias meter which was clearly mentioned in your second post. But good learning experience, and a reminder to always think outside the box.

Weirdest problem I ever had was a filter choke making an amp sound bad because it was arcing out at high volume wtf? DCR and inductance measured fine.

 

[Matador]

I wonder what could be inside that bias meter to cause this. I always thought it was simply a 1 ohm resistor inserted into the cathode.

 

[panman]

Weirdest problem I ever had was a filter choke making an amp sound bad because it was arcing out at high volume wtf? DCR and inductance measured fine.

So, how did you find that out? I may have a similar problem with a SF Twin Reverb.

 

[CJ]

Checked everything else.

This problem was on a Twin Reverb. Forget what vintage.

 

[NoisyIndividual]

I recently changed a reverb transformer on a Fender Reverb, it was actually shorted primary to secondary ground, shorting B+.
If this is the case with yours, you could try lifting the secondary and the symptoms may go away, or at least change.

This was the replacement:
https://www.tubeampdoctor.com/en/reverb-drive-transformer-fender-boogie



Edit: I say this because I seem to remember the secondary is way easier to disconnect than the primary.
(both ends of the secondary need disconnecting in this proposed scenario)

 

[panman]

I guess it`s worth checking out. Thanks!