Thunderbass Quantum Project

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Hi! First post here. First I want to thank this site and members for posting info about Guild stuff - its been great to find some info for this amp I would otherwise be lost without.

Frontdirty_zps88619ee0.jpg

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So, I'm a hobbyist with a bit of experience building and fixing amps and I agreed to fix up a Thunderbass head for a friend. I am posting the progress here and hope to contribute some info and learn more and seek help etc. etc.

When I first researched a little about this amp I found mixed reviews about "Oh - its a good amp but the chassis is flimsy, they are hard to work on, they take weird tubes, and so on. Well after spending some time with it I find most everything about it cool and different and I can't wait to get it working good.

I brought it up on the Variac and found the amp actually worked (not well) and had really dirty pots and switches etc. I played it for a few minutes and decided to take it out of the cabinet to see which version it was (I found other good posts here about what version is what - Thanks capnjuan and other posters!). Based on what I saw I did not have a clue what I was looking at. Someone had converted to EL34's, replaced the 7247 driver tube with a 12AU7, and added some parts to the bias supply. After some more checking I found the 6GF7 voltage regulator and that led me to find the correct Quantum schematic on the Prowess Amplifiers website.

Some measurements (630 Volts on the plates!) and the Transformer part number on the schematic confirm this amp is supposed to have the long discontinued 8417 power tubes. I dont know how the EL34's survived at that voltage especially because when I checked the pin 4 of the output tubes I find the regulator is not working. Pin 4 is supposed to be 300V and I'm getting the same 630V on that pin as the plates. Hmmm...... Also the extra parts added turned out to be a voltage doubler circuit obviously added because the EL34's needed more negative voltage to stay alive! The schematic below is what I traced for the voltage doubler. It also ties to the regulator adjustment pot through a 22k resistor. I'm not sure if the alternative wiring for the doubler will cause the regulator to not work. Any suggestions?

My intention is to put it back stock with the 8417's which should be fairly straightforward. Parts are on the way - more to come.

VoltageDoublerschematic_zps899b02ce.jpg
 
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Well – it turns out the amp really does have more issues than I thought. While I was waiting the few days for the parts I ordered to come in I did some more cleaning and checking things out.
When I pulled the tubes to test them I found the 7247 driver tube was installed in the V1 preamp position (go figure).
The parts finally came in so I began by rebuilding and checking the components around the voltage regulator tube socket. Turns out the 150k resistor on the voltage divider (connects to adjustment pot) was cooked open. I replaced that, 2.0uF cap at the output, and the 22k resistor (bottom half of the voltage divider on the adjustment pot) then moved on to rebuild the bias supply.
Bias supply got rebuilt as shown on the schematic: new caps, 1N4004 diode, 8.2K resistor, and 6.8 ohm 1% metal film resistors at the output tube cathodes.
Next – I put the tubes in and fired it up while monitoring the regulator output and bias voltage.
Big disappointment!
I found I am getting only around -22 volts bias where I should be getting (according to the schematic) around -54 volts (at anode of diode) and the regulator output is around 150 (supposed to be 300V). It was 10PM so I gave it up…..
I have to do some more experiments but the first thing I did was disconnect the bias winding from everything to check the AC output. I measured 42VAC end to end – no load, which if I’m correct – is way too low. It appears the winding is shorted inside the transformer. More checks to do…
 
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Sure Enough!
I powered the bias circuit using my Variac set to 100V and like magic all voltages - Bias, regulator output, etc. are all spot on. I also checked to see if the shorted winding had shorted to any of the other windings and lucky enough measured infinity on every other tap. So the good news is I can use a small 100V transformer to put things right.
 
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OK - Time for another update:
I ordered a 100V Hammond transformer and installed it on the top of the chassis. It actually measured 115V which seems ok without any circuit changes. I now have all voltages set exactly to the schematic. I now get -61V at the 1st cap (schematic says I should have -54), -21V for bias, and the regulator allows adjustment all the way from 272-374V. Things appear to be working just fine.
Here is a picture of the transformer - a little close to the preamp tubes I know but its the only place it would fit. I don't seem to get any hum from it.
100VTransformer_zps14c38644.jpg

Here is a picture of the rewired bias supply:
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After playing it for a few minutes I find sounds great but it still needs a little more work. I have an intermittent problem where the output level drops then comes back along with a "ghost" note/distortion. Also the amp sputters. I'm planning on a cap job (it still has originals) and tracking down whatever component's' or solder joints are bad. Wish me luck.
I found 250uF/500V caps at AES for $10.95 each! Bummer is the closest 4-section cap they have (40,40,30,30/450V) is $35 - on the expensive side but it should work ok.
 
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One more thing - after taking another look at what I thought was a voltage doubler someone had wired in (hand drawn picture above) is not a doubler at all. If I have it correct the two diodes are simply in parallel and do not double anything. As for the rest of the circuit I don't know. My guess is that's what shorted the winding and fried the resistor on the regulator...
 
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So – I did some more poking and prodding and found one of the 12AX7’s was extremely microphonic so I replaced it with an old GE I had lying around. Then next step was to install all of the new caps. Everything with the re-cap was pretty straightforward except replacing the cathode bypass caps on the small elevated circuit boards. Watch out for the ground wire’s’ attached under the driver tube PCB on a little star washer thing. I didn’t notice it and reassembled the PCB onto the standoffs without the ground in place. Needless to say the amp just hummed when I fired it up. After the cap job the hum/buzz in the output was greatly reduced and now the hum balance control works much better – before it was dimed to one end of the adjustment and still hummed – now the sweet spot is back within the range of the pot.
After all that I tested the amp some more and found the output tube sockets were causing some issues – when the amp vibrated or was “knocked” the socket connection was intermittent. I cleaned and re-tensioned but it only lasted for a day – the next time I turned the amp on a knuckle wrap on the chassis or chopstick tap on an output tube would produce a pop in the speaker and the bias voltage would move around. More parts – Angela Instruments sold ceramic replacements that fit the chassis holes on eBay for around $12 each – super expensive but nice silver plated ceramic and no chassis modification.
Now that the new sockets are installed that problem is fixed. Some more testing reveals I still have some kind of small issue with the “Bass” channel. I still get some sputtering/volume drop/distortion intermittently in that channel. Almost done! Wish me luck – sometimes the small problems like this are the hardest to track down. What else from a 44 year old amp!
 
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