I would paint the inside of that little box with white radiator paint ..
My VOX AC10C1
9 days later
hi.. you didn't answer the other message, but can I just ask , are there 321 volts going through the EL84 ? or perhaps I've read it wrong
Cerise
I think you have it wrong? Volts are potential, not current.
Cerise
Voltage does not "go through."
I assume you are asking about the potential between the EL84 plate and ground.
It was 321 Volt on that day.
a month later
hello. have you finished modifying this amp ? you mentioned the pre amp tubes and the likely hood of a pocket of hot air building up in the top half of the case. I tried tube shields, often made by Belton -without the internal spring...regardless, they run a lot cooler than the two EL84's
No, I have not done further work on the amp yet.
2 months later
TAMING THE FIRST GAIN STAGE
The first gain stage being via a LND150 FET, on surface mount PCB, it is not configurable by swapping tubes or components on the fly. Resistor R95 is the “Plate Load” resistor, 220 kOhm. Q4, below right of it, is the MOSFET, and the small surface mount resistors below it, R62 and R63, are the “Cathode” resistors.
I want to reduce gain on this amp, or, have the gain kick in more gradually and smoother. I decided to alter R95, lowering it’s value by parallelling on a 266 kOhm resistor to lower the Plate load to about 120 kOhm. Oh, yes, R100 is also in that circuit. I guess the original Plate load would then be 230 kOhm, and I reduced it to 130 kOhm.
Laboratory equipment would have been nice, but, I shall have to play a lot to evaluate the result.
Initial quick test was with a SE Tremonti neck humbucker at full volume, guitar tone turned down, amplifier gain full open, master volume full open, Bass full up and Treble full down – It was rather noisy, and proved the system still works.
Some sane playing indicates the Gain pot now needs to be open more before sound eminates from the speaker, and it might have a softer ramp-up to it?
I expect overall volume to be about the same: The Master Volume pot only increases volume up to half way, after that volume does not increase anymore. I would like to have an oscilloscope to see what the signal does here.
And from the Pre-Amp side, once the signal is clipped to whatever voltage swing is available in the Valves, more Gain will not increase volume either, just distort more. Which it does, at full gain…A scope would be nice.
If I have to say, after brief playing, I might want to use a 170 kOhm resistor here, but time with different guitars will tell.
So far so good.
Cerise
Playing the amplifier results in the top panel not being very hot to the touch, so, I have decided not to drill holes as yet. I might still do so at a later stage, when I have the chassis out.
a month later
- Edited
sorry, I've not been logging on recently, first time I've seen this. I don't get too deep into changing resistors and caps. However the last thing I would do is lower the gain as it's not that deep as it is. That's the first time I've seen the MOSFET. Is that the gain pot above it to the right ? they put that in so as to achieve gain at lower volumes and I believe to avoid having a third pre amp tube.
My chief gripe is that it's too loud, maybe a load box or attenuator could help. One amp tech said the pre amp is set up 'too hot' on these. They were built to a price, but it sounds good and sells well. The worst aspect is the closed back-which i modified on mine...
I'm also looking to use a cab with a 12" speaker...
- Edited
Just to add to my previous, you state at one point, ''the LND150 FET is not configurable by changing components on the fly'' it is in a way. If you change the nearest tube to the guitar input to a lower gain one -like a 5751, it will cut gain or distortion by a fair amount. I tried it ,and after a time changed back to the 12 AX7 / ECC83, the best all round pre amp tube - IMO
5 days later
Cerise
I have changed the tubes to lower gain ones, as documented earlier on.
I am referring to adjusting the gain on the MOSFET itself - not the circuit after it. Altering the stage after the MOSFET (and gain pot) does not do anything to it's gain, or the effect of the gain pot. It merely reduces the signal to the inverter.
Just curious, if the pre-amp is set up "too hot" (?), and the master volume does not reduce the volume to your desired level, did that amp tech have any suggestions about how to solve your problem?
- Edited
what he said was that it causes the amp sound to ''clip'' earlier , which I've heard . At times it can sound harsh, but you're saying changing resistors around the MOSFET can reduce this
I think what it comes down to is the amp is just too loud even at 10 watt, as are most tube amps compared to the solid state equivalent . Luckily with the AC10 it sounds good even with the master volume at about a quarter turn. With it being single channel if you reduce the gain by changing resistors as you have, then you may end up with too little gain. Depends what you want. Controlling gain is easy for me. I crank the gain higher than the master, like gain at 3/4 turn and master 1/4 or less then i get a good crunchy overdrive. That is when playing at home. Gain always needs to be higher. The opposite applies for a clean sound, although I can have the gain at mid day and Master at 9 am and it will still be fairly clean
you've disappeared again . has what you have done changed the tone of the amp?
Cerise
I have not played it much yet, cannot comment on the tone.