Seymour Duncan 84-40 HT too high

[oc disorder]

Hello
I've got a Seymour Duncan discontinued (4x EL84's) amp here
and the o/p transformer is shorted on one side.
http://www.seymourduncan.com/images/84_40.pdf (595kb)
Having disconected it and a temp replacement hooked it
I' ve discovered over 700volts dc on the c/t..
Now I know it has a voltage doubler cct in the psu but this seems very
high considering the HT starts life as aprox 260v ac ..
The schematic is a little hard to read as broken into 2 parts but below the
o/p transformer shows 550volts (Bit high for EL34's) ht input but doesn't
show that point on the psu schematic but i presume its prior to R62.
I'm putting it to one side at the moment as i have a mental block to
how this can occur ..apart from some kinda double fault in the psu.
All food for thought appreciated while enduring my momentary lapse of reason..

[Enzo]

The 550V node in the B+ supply is the first cap, right after D5, and yes, before R62. You wouldn't run the B+ through a 10k resistor for the output plates.

Can I assume this 700v you found was with the power tubes removed? Or with the OT disconnected? (Disconnect the OT and the power tubes can't conduct.) B+ levels with the tubes not conducting don't mean much of anything. Add in the current draw from operating tubes and that B+ will fall to more reasonable levels.

[oc disorder]

Thanks Enzo for reply

Hi again
Thanks Enzo for your reply ..I have been away and now have revisited the
SD 84-40. The fault was with my meter - under load the HT measures about
545 volts but it seems the EL84's (sovteks) are not too partial to this
voltage.It seems all the EL34's are rated to a maximum of 300v on the plate.
One started glowing in the plate and had to be replaced and now have a faint hum reminiscent of lack of bias and heading towards runaway and burnout.
Have checked bias caps and filter caps (by substitution and measuring ESR
Dick Smith Aust sell a great little esr meter) and am coming to the conclusion
will have to redesign the power supply maybe removing the voltage doubler
and replacing with a more conventional supply.
Have access to a CRO and have the bias adjusted so there is some crossover
distortion ie going very easy on the tubes so I am wondering if (despite the
10 ohm resistor from the cathode to ground)reducing the ht is the way to go ? Thanks

[Enzo]

Overly high voltage will arc the tube over. Plates glowing red hot are conducting too much current. Back off the bias. Looks like there is only about 17-18 volts to work with there to start with. A little crossover distortion never hurt a guitar amp. This ain't no hifi.

The screens are substantially lower than the plates. The numbers in the tube manual are for applications like the tubes were designed for - TVs, Table radios, record players. They were for designs that would provide long term reliable service. Dad was NOT going to replace all the tubes in his TV every six months. But in guitar amps we bust specs all the time. Leo Fender made a career out of over voltage 6V6 circuits.

We want to get the idle dissipation down to about 8-9 watts. At 545 volts on the plate, that means about 15ma per tube. You have four tubes, so about 60ma through that 10 ohm cathode resistor. That means about .6 volts or 600mv.

Try that. Monitor the voltage across that cathode resistor and set the bias for .6 volts there. Forget the crossover notch. Set it close, then let the tubes cook for 15 minutes. Monitor the current. After it is good and warm, tweak the setting.

S-D made a lot of these amps, and they didn't all blow up. You COULD redesign the power supply, but I see no need to do that.

If it seems hummy, it could be out of balance. Matched tubes?

[tjgleck]

seymour duncan 84-40 low sound level

I have a problem where my 84-40 got noisy and then the volume cut big time and remains very faint. I brought it into an amp repair shop that had previously done good work for me. They proceeded to put in a new input jack, replaced the pots, and put in all new tubes. None of this fixed the problem, and they didn't have a ready answer as to what would fix it. I know very little about electronics. Others have told me there is a good chance the output transformer needs replacing. I have some transformers from old stereos lying around, but the wires don't match up. Should I try swapping it myself? If so, are there any hints for what wires to try matching up? Are there any safety things I should be aware of? I have been putting a ground jumper on a cap after powering down and before messing with anything. Thoughts appreciated!
Ted

[Enzo]

For what it is worth, if those other people were not tech, and they didn't put their hands on the amp, then they are just guessing. And guessing with YOUR money instead of theirs. Transformers can fail, but it is not very common for that to happen.

When they replaced the tubes, did they check for burnt out screen resistors? Did they check voltages all around the power stage?

Plug your guitar into the effects return jack instead of the regular input. Is it still really weak? The controls won't have any effect, and it won't be full power, but if it sounds clear and reasonably strong, OK. That tests the power stages.

Now plug a spare cord into the effects send jack, and run it to the input of another amp. That is the preamp signal, and we can listen to it from the second amp. If it is strong, then the preamp is working. If ot sounds liek your problem, then teh preamp is at fault.

In fact, did they check the efects loop? Plug a cord into the effects send jack, and the other end right back into the efffects return jack. If this restores the sound, then the return jack needs service.

If the preamp is bad, then we applu a signal to the input and trace it stage by stage through the amp. ANy competent tech ought to be able to do that.

The whole point of troubleshooting is to isolate the problem. Once you narrow it down to one little section, then there is not many things it could be.

[tjgleck]

seymour duncan 84-40 low sound level

Enzo -
Wow, thanks for the quick, knowledgeable response, youdaman!
Plugging into the effects return with the guitar gave me that sweet sound that I haven't heard out of that amp for 3 years. So I guess the transformer isn't the issue.

Plugging into the effects send jack, then into another amp gave me quiet, noisy sound on that amp, while the Seymour remained loud. The effects and return jacks were a bit noisy moving the plugs around, and when I went to tighten the nuts with my fingers I found that they had been stripped and will spin, but not come off.

Plugging a spare cord from the effects send to the effects return while having the guitar plugged into the input gave me almost no guitar sound. There was some noise, but when I turned the reverb down, the noise went away.

What do you recommend for my next step please? The first tech that I brought it to for the work I mentioned below charged me $160 to do that work and not fix the problem. The second tech I brought it to kept it for 2 weeks and charged me $40 to tell me he would have to work on it for hours, it probably needed a transformer, and with the part and his labor rate it wouldn't be worth fixing.

If I should try bringing it into a third tech to look at the preamp stage, is there anything specific I should tell him, and what would seem like a reasonable cost?
Thanks,
Ted

[Enzo]

One has to wonder why a tech would not have at least tried the loop jacks to see which half of the amp was troubled. He would have instantly realized the transformers were fine.

At this point, without it in front of me, I might start to suspect the integrated circuit. or the +/-12VDC power supplies for it. it is just basic troubleshooting to apply a signal and follow it through the amp and see where it turns ugly.

Did we do that effects send test right? Plugging into the normal input on the front of the DUncan gives weak crummy sound right? That is our symptom. Now IN ADDITION, we run the cord from effects send to the second amp. I would not expect the DUncan to become loud at this point. The effects send test is not done with anything in the effects return.

[tjgleck]

seymour duncan 84-40 low sound level

You are good at picking up details as well as being very patient! I did indeed do the effects send test wrong. I had the guitar still plugged into the effects return, and then ran a separate cord to the other amp.
Doing it properly now, the Duncan remains soft, while the other amp has a soft signal with lots of hum and noise.

[Enzo]

Patient? I have put up with my wife for 20 years or so, this place is easy.

OK, if the preamp out signal from the loop send is weak, then the preamp has the problem. Plugging into the effects send should not affect what comes out of the Duncan. If you wanted to hear what the other amp sounded like without hearing the DUncan, connect the effects send as before, to the other amp, and then plug something into the effects return jack. That will shut off the signal on its way to the Duncan power amp.

At this point we have to go stage by stage through the preamp, and follow the signal path. This will require a scope or a signal tracer at least. And knowldge of voltmeters.

[tjgleck]

seymour duncan 84-40 low sound level

Thanks Enzo -
That gives me enough hope and direction to bring it to a third guitar shop to see if they can fix it. I'll let you know how it turns out.

[tjgleck]

seymour duncan 84-40 low sound level

OK, the frustration with the local music stores continues. The place I tried this time turned out to use the same tech I had last time, working for them on weekends. I unfortunately didn't find that out until after I had dropped the amp off. His diagnosis:
1. The effects return jack is bad. (How can that be when we got good tone plugging into this jack instead of the input jack?) They didn't have any of this type of jack in stock and would have needed to research to find and order them.

2. Replace 11 caps on the preamp circuit. That is almost every one immediately connected to a 12ax7. They didn't have all the caps they needed for this either.

Estimate - $100 for parts and another $100 for labor.

I told him it wasn't worth it, paid my bench fee, and brought it home.

Any ideas on what to do next?

[Enzo]

Plugging into the jack is not the same as relying on the jack's internal contacts to carry the signal. The return jack breaks the signal path. When you are NOT using the jack, little contacts come together to carry the signal past the jack. If those contacts are not OK, the signal does not get carried past.

I can see $100 labor, maybe, to pull a board and replace a lot of parts, but $100 for 11 caps and a jack? Those caps are almost $10 each!!! I wich I could charge that much for a cap, I could retire.

What area you live in?

[tjgleck]

seymour duncan 84-40 low sound level

Green Bay, WI
Is it probable that all of those things went bad at the same time to make the amp go from fine to garbage in an instant? I obviously know very little about electronics, but I would have guessed that the signal would go kaput at a specific point, and although the other caps may all be degraded, and not at their best, that it may not have a significant impact on the problem.