Thunder 1 - 12 coupling caps

[Brendan]

Greetings!

After playing with my Thunder 1 for a few years in stock configuration, I want to try different values for the coupling caps to get some earlier breakup. I was expecting 2 coupling caps, but after looking at a schematic, I think I am seeing three, but I am not sure. I've circled the suspects below:

It's the cap on the bottom right that I suspect is not a coupling cap. Am I correct? Much appreciated!

 

[Default]

Hi Brenden! They are all coupling caps, that bottom right one is part of the phase inverter, so you'd want to make that the same value as the top right cap. Which way do you want to go with the amp, brighter or bassier?

 

[beinhard]

I am seeing 5 coupling caps there, and a few more if I look at the rest of the schematic.
I don't think replacing any of them will result in earlier breakup, though.

beinhard

 

[Default]

Changing the caps would possibly make it brighter if the old caps were leaking dc. You might have better luck if you reduced the value of the cathode resistors on the preamp tubes. Of course, that might make it louder than stock, too. 6GW8's are kind of known for being a hi-fi tube that breaks up late on the volume control.

 

[Brendan]

Thanks for the responses. Changing the coupling caps to get earlier breakup was a recommendation from Capnjuan a few years back on this thread:
viewtopic.php?f=7&t=4431

So this wouldnt' be a good approach?

 

[Default]

It's worth a shot, especially if it's done for experimental reasons. Caps are cheap. 8)

Also, change those 32 mfd/ 25 volt caps on the preamp tube cathode resistors. If the seal on the end is bulging out, they are dead and need to be replaced. The effect gain as well.

 

[Brendan]

It's worth a shot, especially if it's done for experimental reasons. Caps are cheap. 8)

Also, change those 32 mfd/ 25 volt caps on the preamp tube cathode resistors. If the seal on the end is bulging out, they are dead and need to be replaced. The effect gain as well.

Oh no! I didn't know that John had passed away. What a terrible loss. I was so thankful for his knowledge and patience. Clearly an awesome guy on all levels. I had a PM from him dated in December - I think it was from the last day he posted. I didn't realize what had happened, so when I was unable to open the PM, I deleted it.


Default, thanks for the input. I'll give this a try. Would you recommend going with a larger numeric value on those cathode caps? The current ones are good, but those are easy to swap out.

 

[beinhard]

Coupling caps are supposed to:
- block DC, so each tube stage can be biased independently of the others
- pass AC (the guitar signal) from output of one stage to input of next

Together with the input impedance of the following stage, a coupling cap forms a filter that lets high frequencies through but blocks lower frequencies and DC.
In most amps the caps have values that place the cutoff frequency below the lowest guitar note. Change to a lower value cap and it may start to affect your bass notes.

In a high gain amp or overdrive channel it can make sense to use small coupling caps, not because the caps themselves provide more gain or breakup, but because sometimes less bass can make distortion sound better.

To get earlier breakup (distortion) from an amp you should look for ways to alter the gain structure, or use different tubes, change tube biasing or reduce supply voltage.

If you really feel like experimenting, there is a 68k resistor that provides negative feedback to the cathode of V2b. Tweaking that should affect gain and breakup.

Note, however, another quote from our much missed and very experienced friend John (capnjuan):

I admit to being a low-tolerance person when it comes to 'try-this, try-that' ... especially when it comes to trying to make any amp into something that it isn't.

Replacing old electrolytic cathode bypass capacitors is a good idea. These do go bad with age, and while the amp can still work, a bad cap here can significantly reduce gain.

beinhard

 

[Default]

You reminded me of something practical, beinhard. I had forgotten about the negative feedback resistor, even though I swapped resistors in my Vibrolux (which got a lot ballsier). Try disconnecting that 68K resistor first. Don't run any negative feedback at all. The whole point to the negative feedback loop was to reduce distortion.

On my Fender, the tone changed and became more bassy and thick, so you may or my not like it. It was set up to be a jazz amp, so YMMV.