Here are some mods fron the 18watt forum and the gearpage. Phils suggestions are in the middle starting with several cap values and end with taste.
I'm really not qualified to answer but just for the sake of my own education, I'll offer up my suggestions:
1. Move the "top" end of the .0001 cap between the first and second gain stages from the top of the volume pot to the more upstream end of that 68K resistor - this may only give a very marginal increase in brightness/
2. reduce the value of, or remove the .0047uf cap on the anode of the second gain stage.
You could also reduce the values of the cathode bypass caps in the preamp from 22uf down to 1uf or .68 but that's not more treble so much as less bass. Same goes for reducing those big .047 or .1uf coupling caps.
Unbeleivable what a better design that is than some of the modern epi or even some modern Marshall and Fender schems floating around, not sure I get why they put that .22uf cap on the grid of V1 though.
Simplest thing might be to just experiment with some different tubes or
speakers.
EDIT: by the way, that tone knob, if I'm looking at it correctly, doesn't actually give you
any treble boost, it only cuts treble out.
Several cap values need to be reduced. Change the cathode bypass caps in the preamp to around 1uF or 2.2uF. You could also experiment with lifting the 2nd preamp stage's bypass cap for some NFB. Change the two 0.047uF coupling caps between the preamp and PI to 0.022uF or maybe 0.01uF. The two 0.1uF caps going to the grids of the EL84s also need to be reduced to 0.022uF or even 0.01uF.
A couple of resistor values could also do with being changed. The 1.2k resistor on the PI cathodes will tend to make the PI distort in a fizzy way, and needs to be reduced to 820 ohms (like an 18W) or 470 ohms for a bit more crunch. I would also suggest reducing the 2nd preamp stage's cathode resistor to something between 620 and 1k ohms. 820 ohms might be a good value to start with. Lower values will give earlier distortion, and vice versa.
These are just some fairly obvious fixes that need doing, with some room for tweaking to taste.
It's an AC15, with an extra gain stage cascaded on the front end. PI and power amp are 100% AC15. The gain pot is a dual 500K that serves as simultaneous voltage dividers after each gain stage.
The amp is actually biased at over 15W per tube static. It's insane. I swapped the 100 ohm cathode resistor for a 180 ohm, made no difference in the sound but made the tubes a lot happier.
It selects a different HV tap on the PT secondary. One leg remains tied to one side of the bridge rectifier, the other side has two taps and the switch selects between them. The result is something like 250VDC rather than about 325VDC.
It affects the entire amp, not just the power tubes, and it does NOT do anything to the bias state - so, in stock form the amp is severely (IMO) underbiased in 15W mode, and just about right in 7W. But I think the low voltages affect the feel in a negative way.
I replaced the power switch with a DPDT and also switched a second cathode resistor in parallel when I selected 7W. That made both modes idle happily.
You're right, I omitted the master from my drawing. It's a 500K dual-pot post-PI master with an extra set of .1uF caps.
The power switch is on my drawing, it's on the HV secondary. It selects between two 'ends' of the winding - HV winds up with about 325V and LV about 250V.
I don't know how you'd affect only the voltage in the power amp since the switch selects a different HV tap on the power transformer.
180 ohms is a good choice at 15W power; I replaced the power select switch in mine with a DPDT (Mouser sells the same switch brand and family) and used the other pole to place a 100 ohm resistor in parallel with the 180 when I went down in power. Worked great.
I don't think there's much to gain with cap swaps, myself. And both of my transformers were crooked too!
....this time about the half power switch. I know how it works an all as i've read the schematic but has anyone with the amp noticed a discernable change in tone and gain when switching between half power and full power?
I ask this as theres a rather interesting mod someone has suggested which allows you to change the bias of the Tiny Terror when switching between the power levels.
Basically you change out the current cathode resistor (120 ohms) to a 180 ohms cathode resistor which runs the EL84s less hot on full power mode and change the power swtich to a double pole version. One pole handles the power switch while the other pole can bring in a 100Ohm resistor in parallel with the 180ohm cathode resistor. That way you have a consistent bias level between the different power levels. That mod would be simple for a tech to fit.
Several months back when I joined this forum, I posted a link for mods to the tiny terror, which included these above, since then they've posted a lot more.... Yes you do get a discernable change in tone ,switching from 7 to 15, and the bias voltage is the reason.
it's at the music electronics forum:-
http://music-electronics-forum.com/show ... php?t=1477
that's if the link still works...TTFN