Need some help with an X202C repair

Hi folks,

My first post on the forum, but it has already been a great help getting up to speed with a X202C I'm repairing for a friend after one channel stopped working.

I found a failed cathode resistor (R87 in the schematic I found at fisherconsoles.com). Interestingly, both channels had a 15 ohm resistor installed rather than the 12 ohm on the schematic. I haven't seen that as a recommended mod; the PS voltages all seem a little higher than spec, but the amp was apparently working reliably for years before the recent failure.

With R87 replaced the left channel was distorted and the waveform was pretty asymmetrical. I tested all 4 of the output tubes (worn lettering, but I think they are all old GEs) and one of the two left channel tubes tested bad. So, at least one new tube pair is needed.

Now the questions:

There doesn't appear to be any bias adjustment on this amp, so is there any way to check that the current is split evenly between the two tubes in each pair? The manual suggests using the DC balance control with an IM analyzer, but is there a simpler way? If I replace the tubes with a matched pair (or quad), can I just assume that getting the bias voltage the same on pin 6 of both tubes is a good starting point before tweaking for minimum distortion?

The grid resistors are all still 330k, so I will probably replace all of them with 200k or so (and the coupling caps). I understand from reading this forum that most new tubes don't like the 330k resistors, but even if I replace them I'm not sure which of the various tube options will work. Seems like the JJs are universally disliked, the EH are possibly too big for the X202C, so that leaves NOS or the new Tung Sol 7591. There was a a great post on an early test of the TS 7591 by Dave Gillespie, but does anyone else have experience with them in this amp?

Any consensus on the tube choice? Keep looking for NOS GEs, go to the TS, or something else?

Thanks in advance for the help!

Peter

Your best option is to add separate bias controls, and install individual cathode resistors so that the amp can be properly biased and balanced. I believe that Tube-a-lou has done these things to his X202C, so hopefully he'll chime in to give you his experience in doing that. The TS 7591s are excellent tubes, but were in short supply recently, and also seemed to require somewhat greater negative bias voltage to produce a more normal quiescent operating current.

Good luck with it!

Dave

Love those T-S 7591's in my 800-c. Dave's comment is correct in that they require more negative bias voltage to produce normal quiescent currents.

Update on the X202C

Dave and Larry,

Thanks for the replies. I ended up going out of town for a few weeks, but I'm now getting back to the X202C. I had a few things to replace in the amp (in addition to the lower grid resistors and caps, there were a couple of burned out resistors across the heater supply) but I finally installed the T-S 7591s in one channel.

The quiescent current is quite a bit higher than with the old GEs that are in there (as you predicted). The single cathode resistor in each channel had already been changed from 12 ohms to 15 before I saw the amp, but even with that change the current is somewhere around 110mA for the pair of tubes vs about 65-70mA for the pair of GEs.

The voltage labels on the Fisher schematic suggest that the quiescent current should be about 40mA per tube. Sound right?

I found another post about changing the values of R75 and R76 (and adding a pot) to give me more negative bias voltage, and I think that's the next step. Changing the cathode bias approach per Dave's post seems like a mod that might change the character of the amp more than the owner wants (although it would make the amp easier to work on!)

Thanks again for the advice,
Peter

Peter -- adding the small cathode current sampling resistors doesn't change the bias scheme, but merely allows for an accurate DC balance condition to be achieved with your DVM, rather than setting the control for minimum hum in the output by way of an indicator connected across the speaker outputs.

If you do go to the T-S output tubes, you will definitely need to alter the bias voltage to the balance controls, or install controls so that the current draw can be accurately zeroed in on.

Do determine why the heater resistors burned out. In a worst case scenario, that can be because of a short condition within the power transformer.

Good luck with it!

Dave

Dave,

Yes, I think adding small cathode sampling resistors makes a lot of sense, so I will go ahead with that. Given that the amp originally had no facility for setting the bias, it probably also makes sense to add a bias adjustment pot (or two). I bought a matched quad of the T-S 7591s, so I guess it's a must-do!

The 220 ohm resistors across the heater supply are a bit of a mystery. I think they basically failed after years of being overheated. They are right next to one of the cathode resistors, but in normal operation that shouldn't have been hot either. I will check that the heater voltages seem right, and that the supply isn't at some weird potential wrt the chassis.

Now that I have spent some time with my first Fisher amp, I'm keen to hear it once I'm done...

Thanks,

Peter

This amp project gets more and more mysterious

Dave,

You were right that the failed heater resistors were worth some investigation.

The X202C has two heater supply circuits. One is a simple AC power transformer tap that feeds V5-V10. The other is a DC circuit that uses the same tap as the bias circuit, and feeds V1-V4 in series-connected pairs.

Well, somewhere in the amp's past the two heater supplies were connected together! So there was a 27v DC offset applied to the heater circuit for the output tubes, and of course the 220 ohm resistors were fried, since they were feeding more than 100mA to ground and dissipating over 3W each instead of their 0.5W rating.

The mysterious part is that the wiring for V1-V4 looks original, but they are wired in parallel rather than series.

Is this some mistaken variation of a standard mod for the amp?

In any case, it looks like I'm not done yet...

Peter

I have no idea what someone was trying to accomplish, but if the supplies were connected in parallel -- as it sounds like they were -- then such a bone headed move would have also removed most of the bias to the output tubes as well, no doubt causing them great distress. There is no way this was a factory variation but rather, someone who clearly didn't quite understand whatever it was they were trying to do!

Dave

Peter;

Good catch on the miswiring of the heater circuits. They definitely have to be separate. As Dave said you never know what someone else may have done to the unit before you began checking it. My experience with an X-101-C was a real workout due to so many modifications having been made to it previously and many missing parts. I sincerely hope you do not have missing parts in this one!

Good luck.

Joe

The interesting thing is that the amp was working for a long time. My guess is that once the 220 ohm resistors burned out there was no further current to ground, and the AC heater supply for all of the tubes was biased to about -27V.
I don't remember enough about tube behavior to know what impact a negatively-biased heater would have. The datasheet for the original TS 7591 says the heater can be up to 200V below the cathode, so the maximum rating ins't being exceeded.

So I think the tap that is intended to supply DC to V1-V4 isn't doing anything other than holding the heater circuit at this negative voltage (and supplying the output tube bias voltage, of course).

Peter

I finally finished the work on the amp, and since folks on this forum were key to getting the x202C back up and running I thought I would report on the results.

Here's what I ended up doing:

Replaced the burned out cathode resistor in one channel and the vaporized 220 ohm heater termination resistors.
Changed the grid resistors to 200k and increased the value on the associated caps
Added cathode sampling resistors so I could match the DC bias
Replaced the output tubes with a matched quartet of Tung-Sols from The Tube Store
Changed the bias supply to provide more negative swing for the Tung-sols and added a bias adjustment pot (just one)
Rewired all of the DC heater supplies for the preamp tubes back to the schematic (since they had been paralleled and connected to the AC supply for the outputs)
Set the bias to ~35mA after an hour or so of operation. I know that's a little cool, but the distortion is pretty low at that level...

The amp was finally plugged into my system and powered up on Sunday afternoon. Right from cold the sound is shockingly good. There is the usual rich mid range and bloom around instruments that only tubes seem to the able to produce, but there is also an amazing sense of speed - a real snap to percussion instruments. It really makes music!

I also made some distortion measurements while I had the amp on the bench. At somewhere around 1W into a 4 ohm resistor load the distortion was almost all 2nd and 3rd harmonic. The 2nd was at about 0.06%, and the 3rd was a little lower at about 0.04%. The resolution limit on my spectrum analyzer on that quick test was somewhere between -80dB and -90dB (0.01% to 0.003%).

For an amp built around 1965, this is a seriously well-engineered product! I hadn't encountered a Fisher amplifier before, and I can see why these things have devoted fans.

Thanks for all the help,
Peter