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I was looking at the tweakers notes on the schematic and noticed that there is no notation for adding a cap on V3 pin 8 where it shows adiing a cap in parallel to R18 . Is there a cap specified for this or was that something not used after all? I was thinking of trying some of these tweaks for fun . Which would see the most gain in distortion( the good kind)and which would see the least gain?

That particular cathode cap (0.047uF) just makes the amp sound brighter. Some early 70s Hiwatts had it installed - some didn't. When we were experimenting with the voicing of the Triwatt, we ended up taking it out, as it made the amp sound too shrill. If anyone's using KT66s, which are naturally a bit darker than 6V6s, they could install this cap if they wanted a little more brightness.

The most common way to give a Hiwatt (or a Triwatt) more gain is to swap the 12AT7 in the PI for a 12AX7. The early '70s DR preamp used in the Triwatt is generally considered to be the cleanest of all the Hiwatt preamp versions (until you switch on the OL stage). Adding cathode caps to any of the preamp gain stages will increase overall gain, and you can see these in the tweakers table. Small values like 0.047uF add mor brightness. Large values like 64uF add more bass. Just for fun I might be inclined to try a 1uF on V3 pin 8 or V2 pin 3. That should give you a real gain monster of an amp.

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Great tones come in small glass jars!

Thanks Z! I will give it a try when my footswitch kit comes in. I was even thinking of trying a rotary switch with a selection of caps like the TC15.

Just try tacking on one or two cap values and see how/if you like it. I doubt that you will ever want a rotary switch, as the difference gets pretty subtle. An ON-OFF-ON 3-way toggle switch will probably be all you want, so that you can select 0.047uF, no cap, or 1uF, say.

PS: Just make sure you do a tidy job when you mod that amp, OK?

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Great tones come in small glass jars!

You got it Zaphod!Thanks for the advice as usual.