The Amp Garage

I've found the chassis awkward to work on. Being a deep U shape, the components at the bottom of the U (jack sockets, fx loop , speaker etc - back of the amp) dificult to get to with a soldering iron, necessitating quite a bit of dismantling to get to them, or trying to work through a forest of flyi...

Something else you might like to try, if you like experimenting, is to use a second stage with feedback from the plate to the grid. That way you can select the gain you want, without resorting to attenuators after the stage. There are pros and cons, as always. The gain is approx = Rf/Rin, it can hav...

Why not put it on a switch.

for example the ampmaker nsx kit shows 2 stages with switched bypass caps on the first 2 stages.

http://www.ampmaker.com/images/ak01kit/n5xsch2.jpg

I think you could increase the cathode bypass cap on the first stage, at the moment it's only partially bypassed and whilst this cuts the bass on the gain channel, it does the same for the clean. Some texts recommend fully bypassing the first stage, for best noise rejection. I think you'll find that...

Those green mylar polyester caps, I've only ever seen them rated at 100v.

Q1 is hard on, RDSon looks to be about right, so what are the driving voltages. What amp is it, and where is the loop being inserted.

How has the bypassing been achieved, perhaps a pic of the wiring might help debug the circuit, or rather the installation of it. It's a very neat design, using so few components in the send stage, but achieving all that's needed. I noticed that the gate of the first stage hasn't got any protection d...

I'm too far down the line to do a 5F2, maybe another time. I need the space of the ultra chassis, the new amp will be a fairly standard design, trying to keep most preamp stages centre biased, I'm not too keen on the sound of cold clippers and asymmetric clipping, 3 stage preamp, 2 tones stacks with...

Thanks Franco, It has been good to have this discussion, I've learnt something (I hope). Although I worked as an electronic tech., I know there are some fundamental gaps in my knowledge. I am cautious, so tend to drop down to the easiest solutions which I am more confident about. But I still make ba...

Franco, It is puzzling, I didn't expect to have to do that. Maybe it's to do with the extra gain when using the ultra side of the valve. I did wonder whether it might be due to the way the heater noise interacts with the spiral filaments and the cathodes in V1. And somehow there's an imbalance betwe...

Yes, I understand what you say. I think that the stock circuit is safe as it is, but I do think it is not recommended. As you say the 100 ohm resistors save it. I think it is just easier to leave them out, there might not be a need for them at all. I've found removing the bridge and all DC associate...

Maybe it works ok as designed, though I have always thought there was something odd about it. Reading Merlin Blencowe's website and book he states several times that there must be no ground on the Ac side if the DC side has been grounded, or vice versa. Maybe the two 100 ohm resistors and 'fake cent...

I don't have a layout, so can't help you there. I have a sp6Ultra, and I don''t find the chassis the easiest to work on, and often thought about putting it into a different box. If you do go on to build one, you might want to modify the heater supplies. They are grounded on either side of the rectif...

I think that without the 10nf decouple cap it will mess up the bias of the cathodyne stage that follows and the mid circuit won't work as effectively without it. Without it there's a DC path to ground via the Bax 120k resistor and the bass pot, which will tie the grid to ground, preventing the catho...

Merlin, I agree with your first two paragraphs, but in the last two you are descending into fanning the flames territory. Mods: A few suggestions if I may be so bold. 1. Please can this thread be closed, it is bringing out the worst in forum behaviour. It is just as distracting and irritating as the...

I think most of the the preamp distortion is generated in V2b. I also don't like to think of the 'clean' volume control as a master volume, because V2b follows it and it has a lot of gain. With both the drive and volume maxed (and boost engaged) I was getting approx. 15 volts RMS at the send jack, e...