Bias issue with Marshall Super Lead RI

[schwarzerkaffee]

Dear forum,

I am currently struggeling when biasing my Marshall SLP 1959 RI (90's). I bought a matched quad some time ago from TAD and when I intended to bias the tubes, I observed that the bias current on V7 and V6 is increasing while it is quasi constant on V4 and V5. To be more precise: I use a TAD Bias master to measure the bias current of all four output tubes simultaneously. I set the bias current quite cold and these are some readings (mA) after warm up and 15 minutes later:

V7: 23.9/31.1
V6: 25.7/33.2
V4: 27.3/28.1
V5: 27.0/27.1

The plate voltage (pin 3) on V6 changed from 457.3V to 455.7V in the same time range. Control grid voltage (pin 5) is -40.8V spot on for all four tubes. Has anybody an idea? I have attached the schematic of the power section. Is it possible that the coupling caps C12/C10 cause any problems? Apart from the power tubes all parts are original. You'll also find an image of the measurment setup in order to make sure that the valve numbering is correct.

Best,
Klaus

[sluckey]

I don't see a problem. You can swap places with V4/5 with V6/7 to see if the issue follows the tubes or stays with the sockets.

[stratomaster]

Check C12 for leakage. You'll have to lift the leg at the junction with the 220k and give the floating leg a ground reference through a large-ish resistor.  Take the power tubes out to be safe during this measurement.  Since you're measuring the same voltage at all grids this is highly unlikely to be the problem.  It's more likely you have a set of tubes that was matched without burn in or that have drifted as they've aged.

If no leakage, then swap one set of tubes per side for a better average match.

[schwarzerkaffee]

Hi guys,

thanks for the quick replies. I will do as you suggested. I just would like to add that the current in V6/V7 was still rising after 15 minutes. In an earlier experiment it went up over 50mA. I turned the amp off then, since i didn't want to risk higher currents. So my values are no steady state. Whats more, the support guy from TAD said, that for their matched tubes differences of more than 1-2mA should not occur.

I'll be back

Klaus

[tubeswell]

Those 1959 SL100 RIs are notoriously difficult to bias.  Been there. You need to alter the values in the bias circuit to get it to bias colder/stabilise or get a more closely matched quad of colder tubes (preferably genuine Svetlanas from back when they were still in business*). The rule of thumb of being within 5mA doesn’t apply to these amps- they are running those EL34s over the edge of sanity. They need to be within 1-2mA otherwise the hotter pair in the quad will suck all the current when you dime the amp, and replate - while screaming insanely sweetly just before they take out your OT.


* Marshall ordered a special run of EL34s from Svetlana made to go in those RI amps back when they made those RIs (before Svetlana closed up shop).

[shooter]

while screaming insanely sweetly just before they take out your OT.

talked with a lead guitarist after his set, let him know, "Pretty sure your PA tubes are going"
his reply "OH man, you noticed!  don't you love the sound of a twin dimed with dyeing tubes???"


He only has his tech install tubes that got pulled, says he gets 1-2 shows out of them and a sound no one else has, musicians!

[stratomaster]

Hi guys,

thanks for the quick replies. I will do as you suggested. I just would like to add that the current in V6/V7 was still rising after 15 minutes. In an earlier experiment it went up over 50mA. I turned the amp off then, since i didn't want to risk higher currents. So my values are no steady state. Whats more, the support guy from TAD said, that for their matched tubes differences of more than 1-2mA should not occur.

I'll be back

Klaus

Measure the grid voltage again as you notice the upward current drift.  I'm having a hard time believing that is stable and the same across all 4 tubes as your bias current increases. 

The first 2or3 replies to your question seem to presume that to be true since that's how your post read. 

[schwarzerkaffee]

Hi guys,

as stratomaster suggested I also remeasured the grid voltages (pin 5) and indeed there is a drift in that voltage as well. I apologize for the bold assertion in my original post. So here are the readings

           V7      V6      V4      V5
grid     -38.7  -38.7  -38.7  -38.7
bias     24.1   25.3    27.4   26.4
---------------------------------------
grid     -37.9  -37.9  -38.5  -38.5
bias     27.6   29.4    28.5   27.4
---------------------------------------
grid     -34.3  -34.3  -38.2  -38.2
bias     48.0   50.1    28.4   27.4

The three measurements are roughly a quarter hour appart each. So it seems that there is a problem with the coupling cap C12, right? I measured the two 220k resistors R25/R26 which are both in spec. 

[WimWalther]

A leaky cap is the first possibility to eliminate. Lift one end of C12 and see if the bias voltage & current stabilizes.

The other possibility is that one of the tubes (V6/V7) is drawing grid current, but it's much less likely.

[WimWalther]

Also, just a tip.. Once you have this fixed and bias is stable, take an accurate AC current reading for the entire amp and record the AC line voltage at the socket.

This way, you have a quick & easy sanity check if you think bias has drifted or something else is awry.

[schwarzerkaffee]

Hi WimWalther,

thank you for the valuable advice. Just two (maybe stupid) follow-up questions

1) If I am going to replace the 22nF coupling caps, what voltage rating should I go for? 600V? - I can not find out from the schematic I have.
2) I am a bit confused about the second advice. Do you mean the wall power reading in my place? Could you point out how the sanity check would work.

Best
Klaus

[WimWalther]

Hi WimWalther,

thank you for the valuable advice. Just two (maybe stupid) follow-up questions

1) If I am going to replace the 22nF coupling caps, what voltage rating should I go for? 600V? - I can not find out from the schematic I have.

What voltage are the original parts? Let that guide you. Otherwise, a general guideline is 15-20% above the highest B+ supply. Remember, you entrust your tubes to those caps.

2) I am a bit confused about the second advice. Do you mean the wall power reading in my place? Could you point out how the sanity check would work.

Yes. Precisely measure & record the AC line voltage and current draw of the fully warmed-up and correctly biased amp.

This way you have a benchmark to quickly check for bias drift or other issues, as the current should only change if line voltage does. No need to use the bias probe, etc.

[stratomaster]

Hi WimWalther,

thank you for the valuable advice. Just two (maybe stupid) follow-up questions

1) If I am going to replace the 22nF coupling caps, what voltage rating should I go for? 600V? - I can not find out from the schematic I have.
2) I am a bit confused about the second advice. Do you mean the wall power reading in my place? Could you point out how the sanity check would work.

Best
Klaus

600v or 630v are good standard ratings for that role. It's good to understand what that cap is doing so you can answer those questions for yourself.

The cap blocks the phase inverter plate voltage from the bias voltage at the grid of the power tubes.  The bias voltage is negative on fixed bias amps, so the total voltage across the cap is the pi plate voltage minus the bias voltage.  Subtracting a negative is addition so you have over 300v on plates + up to 70v at the grid from the bias.  This can be greater than 400v across that cap in some amps.

In lower voltage amps or cathode bias amps you can sometimes get away with 400v caps there.  Or if you're Mesa Boogie and love playing with fire on fixed bias amps. 

[PRR]

It is well to think about the start-up phase. In some amps the B+ at start can be 20% or more above the running voltage, and there is no drop across plate resistors. Agree "greater than 400V in some amps", especially at the driver stage.

[shooter]

i built with stiff PS's.  always measure startup, AND note on schematic 

[schwarzerkaffee]

Hello again,

so I finally found some time to swap the coupling caps. I used a pair of orange drop caps rated 600V as you can see in the image. After startup with stand-by off I measured the grid voltages on all power tubes at -38.7 V. So far so good. But unfortunately with stand-by on, the bias again drifted on tubes V6 V7 as before with the grid voltage going down slowly. By the way: in the meantime I also swapped the tubes in order to rule out a problem with the tubes.

What possible further causes might explain this situation?

Best,
Klaus

[tubeswell]

Increase the (negative) bias voltage

[WimWalther]

Something is drawing current through R26. Have you checked / replaced it? If it's drifting up in value as it warms, that would do it.

Otherwise, with the amp showing the fault condition, watch the grid voltages as you pull & replace V6 and then V7. Does removing one (or both..) correct the grid voltage?

It sounds like you have the issue localized to R26 and the sockets for V6 & V7. It's possible you have a bad socket, but it's very unlikely.

So where is that current going?

[PRR]

> -38.7 V. ......with stand-by on, the bias again drifted .....going down slowly.

"Down" where? To -38.0V? To -30V? To zero?

Voltages drift. It's all relative. A little drift is normal. A huge slump is bad. Which is it?

[stratomaster]

Clarify what you mean by "standby on".  Does this mean the amp is in play mode (i.e. voltages present where they need to be in the amp to produce sound)?

If you mean standby on, as in B+ interrupted and no sound being made, then yes the grid voltage will change between the two "standby" switch settings.  The way standby works in this amp is by disconnecting the high voltage AC input to the rectifier.  This means no plate or screen voltage in standby which means no conduction/current draw. 

Measure and set your bias with the amp in play mode and preferably after it has had a few mins to warm up and stabilize.

[WimWalther]

My understanding is that the drift issue is limited to the V6-V7 pair. All four power tubes show the same g1 voltage initially, but with time V6-V7 drift - g1 becomes less negative and Ip drifts up in proportion.

He seems to have isolated the problem to the tube sockets & grid resistors.

[shooter]

leaky caps in the bias supply been eliminated? 


bias might be "stable" without tubes, once you load down a PS, interesting things can happen.
1ohm 1% PA cathodes to ground provides a simple monitoring point that doesn't unduly load a grid with a sketchy meter.

[tubeswell]

One half of the OT primary has lower DC resistance than the other. Tube current gradually beds down as the tubes heat up and so the tube current on this side creeps up a bit more. This isn’t unexpected, but at the voltages the reissue 1959 SL 100 runs at (plates at 508 to 510, screens at 498 to 506 - you haven’t told us what these voltages are), the effect is more noticeable especially with EL34s - whose grids are wound in close proximity to the cathodes (in order to provide increased gm), but which also makes these tubes prone to damage from overheating warping the grid (and grid support rods). Under these conditions, ‘hotter’ tubes will drift more and poorly made or damaged hotter tubes will start to run away (where slightly cooler tubes might otherwise find a stability point). Try increasing the bias range to see if that helps. The tubes could be too far gone for that amp. YMMV