Bias Drift?

[Buddha's Guitar Tech]

Marshall JCM2000, 100W

Over the course of 30 minutes or so, with new, and matched JJ EL34s, i noticed the cathode currents rising.

A few minutes after applying HT, i set the bias controls to match. I started pretty cold - 40ma per side.

Over 30 minutes, that 40ma became 68 and 62. The 62 bias control was maxed out. i had a little play with the 68 side, but the point of the experiment was to see the rise of the two sides.
And it seems one pair was rising faster than the other.

As anyone knows, these modern amps are a real pain to work on.

Any ideas for shortcuts before i begin the man-going-bald process?

As one might expect, the bias voltages went down - fairly consistently for both sides. And of course the HT node went down a little, due to the rising current in the valves. (480 - 472)

All screen voltages reacted consistently - going from 470 to 462

Bias voltage dividers? Caps?

Thanks.

[martin manning]

Leaking PI coupling caps is a likely cause.

[Jana]

I have heard (read) that some of these amps had a NTC thermistor or something along those lines installed by mistake in the bias circuit. The effect is that as the amp warms up the bias shifts. Theory is that someone installed the wrong tape of resistors in the stuffing machine and a whole bunch of amps got the wrong type of resistor.

I haven't seen the error myself but have read it from some sources that seemed reliable. I wish I could remember where. A google search might find it.

Edited to add this link: http://www.lynx.bc.ca/~jc/TSL122.html

[Structo]

Wow, my opinion of Marshall has sunk to a new low.

Way to get out ahead of the problem Marshall.

Sounds like you have to find the right guy and beg to get a new board to replace the defective one.
These amps are not cheap ones either.

In this case the PCB was poorly designed as well as the material it was build from.

Probably another case where the bean counters controlled the quality of parts.

[vibratoking]

From the article:

...Being of the dual type (not a good idea to begin with) in which two trimpots mounted on an external board connected to the mother board via a multi-wire connector..."

... My original beef with general dual-biasing is outlined in my Tube Amp book (IF&MTA),...

IF&MTA = "Inside Fender and Marshall Tube Amps".

Does anyone know about this book or what his beef is with dual type biasing? There are alot of guys who would strongly disagree that dual biasing is problematic. In fact, many think it is a big improvement.
[/quote]

[NickC]

Wow, my opinion of Marshall has sunk to a new low.

Way to get out ahead of the problem Marshall.

Sounds like you have to find the right guy and beg to get a new board to replace the defective one.
These amps are not cheap ones either.

In this case the PCB was poorly designed as well as the material it was build from.

Probably another case where the bean counters controlled the quality of parts.

That is sad, sad, sad. Looking at the gut shots of that Marshall model, reaffirms my decision to build-my-own amps rather than buy all that PCB, ribbon-cable, flimsy, mass produced stuff.

But I doubt the bean-counters deserve the brunt of the blame in this case (nor in most). I've made a 30+ year career of bean-counting (audit) in manufacturing (pharmaceuticals), building materials industry, and service organizations (including brief stint in government). My experience is the accountants care about quality, but are often thrust into the situation of making the numbers "work" when marketing departments collide with real-world necessity for the Board of Directors to return value to shareholders. I've never seen the bean-counters be the ones driving decisions to cheapen quality to drive profits (which is short-sighted and long-term destroys companies) ..... in fact, I've seen the opposite.

On the other hand, I have seen ruthless venture capital groups exploit well-earned reputations for quality (by cheapening quality, maximizing profit, and riding the good reputation) driving formerly excellent companies into the ground. Seen that up close and personal, still makes me sick to this day.

[Buddha's Guitar Tech]

That article could have been made just for me. Sounds exactly like what this one's doing.

Negative temp coefficient, eh? Might be worth replacing them i guess.

Grumble, moan, if this were an old Fender, moan moan... easy, moan, but not PCB stuff, grumble, whine.

Sometimes i think there's no way i'm going to get rich, fixing amps. The gods lied to me.

[NickC]

..................

IF&MTA = "Inside Fender and Marshall Tube Amps".

Does anyone know about this book ...............

Inside Fender and Marshall Tube Amps

Jean-Charles Maillet. ISBN 0-9684849-0-5; 124 pages. $40. Contents: BASICS AND HEURISTICS. Triode Gain Stages. Common-Cathode DC Transfer Characteristics. Variants on 12ax7 Common-Cathode transfer Specs. Common-Plate DC Transfer Characteristics. Variants on 12ax7 Common-Plate transfer Specs. Differential Triode Circuits. Differential-to-Single-Ended Circuit Conversion. Fender Power Supply Circuits. Audio Circuits as Power Supply Loads. The RC-Ladder Filter Network. Rectifier Turn-On characteristics. The CBS Power Supply: Fender Design Alterations. Fender Push-Pull Output Stages. Fender Vibrato Circuits. Bandwidth and Fidelity. BANDWIDTH CONTROL: Tone Stack Component Values. Transient Fidelity in Guitar Amps - Dynamic Waveshaping. Interstage resistance: Tweed and Blackface Designs. Shadowing: Dielectric Adsorption in Signal Caps. Wiring Parasitics. Class-AB Biasing. Making the Notch Go Away. Maintaining Power Tube Matching Over Long-Term Usage. Daily Drift of AC Line Voltage. Fender Amp Mods. NEUTRAL MODS: Single-Ended Tweed Filament Hum Cancellation. Silverface Filament and Bias Circuits. Late 60s and Later Silverface Bias Circuits. Improved Fender Biased Circuit. Alternative Bias Circuit Sourcing. Lead Dress for Improved Hum Cancellation. Tonal and Dynamic Enhancement Mods. Silverface Power Supply Enhancements. Silverface Signal Path Enhancements. Balance of Overall Overdrive Characteristics. Balance of Overall Circuit Gain Against Tubes and Speakers. Fender 6L6 Silverface - MOD A. Hot Blackface-style 6L6 Pentode Ratios. Fender 6L6 Silverface - MOD B. Extreme Single-Channel Blackface Performance. Fender 6L6 Silverface - MOD C. Tweed Signal Path and Direct Coupled Kolkki Reverb. Fender EL34 Super Reverb - MOD D. British Super Tweed Reverb. Fender 6v6 Silverface - MOD E. Deluxe Reverb: The Stranglehold Variation. Low-End bandwidth Extension. Power Supply Revamp. Fender 6L6 Silverface - MOD F. Sampo Kolkki Reverb Re-route. Marshall Amp Mods. Marshall Circuits Overview. Bridgeable Volume Control. Master Volume Control. Marshall power Supply and Filter Distribution Network. Marshall Circuit Mods. Hot-Rodding and Restoration Mods. Dual-Mode Marshall Switching Circuit. What about the stock JCM800 Normal Channel. Adapting the switching circuit to JCM800s. Adapting the switching circuit to JCM900s. Annexe. Traynor Amplifiers - The Bassmaster. Speaker Cab Considerations. Vacuum Tube Modeling for Pspice Simulation.

http://www.lynx.bc.ca/~jc/ifmta.html


I couldn't find a link anywhere where a copy could be purchased.

[Buddha's Guitar Tech]

Sittin' here with the amp in front of me.

It's a JCM2-60-00 board. R68 is a 33K CF resistor. I've replaced it with (ashamedly) a 10k and 22k, (both CFs) cuz that's what i gots.

As a happy accident, they stand off the board.

I'm running it without HT for the moment, in the hopes that simply warming the amp will cause some fluctuation. (and save the plates some abuse)

G1 voltage started at -45.1 (I maxed the trimmers) and after ten minutes it's reading -45.4 Looks like a step in the right direction.

If no results, i'll flick the HT and play a bit.

[Prairie Dawg]

Just had a thermal runaway on one JJ 7027 out of a matched quad that was brand new out of the box in an Ampeg V4. Over the space of about ten minutes the current draw kept climbing on this one tube until it started to redplate which was then when I shut it down. Lucky for me the customer was watching and he'd supplied the tubes.

[Buddha's Guitar Tech]

Alas, my work so far hasn't done what i'd hoped.

The voltages crept downwards. (rather, upwards toward zero.)
One side falls quicker than the other - about 3 volts in 20 minutes versus 1 and a bit.

Replaced the four 220k grid stoppers with 5k6 - I figured having two 220k resistors in series can be a bit like a voltage divider and if one starts to drift even a little, the relationship is changed more drastically than with 220k/5k6.

But... no appreciable change in condition. Perhaps i'll change the last two 220k resistors in that group and see where that takes me.

Interestingly; you know the wee bias trimmers... I don't know if it's me, but the left trimmer seems to control the right pair of EL34s and vice versa.

Back to it tomorrow.

[tubeswell]

FWIW it is normal for some bias shift to happen between a stone cold tube and a properly warmed up one - that is why you should wait a few minutes when adjusting the biasing.

My worry would be if the bias voltage didn't stop falling at some point. Does it settle down eventually (now that you have replaced those resistors)?

[Buddha's Guitar Tech]

When first fired, the cathodes will read around 40ma per side- very cold i know.

About half an hour later — with the bias voltage maxed — i'll read 65ma on one side, 45ma on the other. It's at that point i get jittery. Again - bias voltage maxed.

I figure the JJs will probably handle a bit of extra draw, but that's a big swing in half an hour.

The fact that it's uneven tells me something's not quite right.

With roughly a volt dropped every five minutes, it shows no sign of settling before i get the jitters and switch it off. (and sob silently in the night.)

[jelle]

Test with known good powertubes for bias stability. This will exclude or point to the tubes. Jelle

[Structo]

If the voltage is dropping, wouldn't that indicate that the current is rising?

It gets tricky to discuss bias voltage because it is negative voltage.

In other words, -50v is less than -40v.

More negative if you will.