Tube Amp Archive

I think people have missed an important factor here. The most likely reason that this amp squeals in one positions and not another is that cables are getting moved slightly within the amp. Any amplifier should not require shielding to prevent oscillations. I would recommend using a chopstick to move...

Yes, the LarMar MV is simply a Ken Fischer Type II master volume. Dual ganged 250kA pot with 2M2 between the wiper and one leg. It replaces 220k grid leak resistors and Vbias is applied to the same leg as the 2M2 is connected to. Typically mounted in Marshall style builds by removing one of the spe...

Using a bulb limiter drops the operating voltage in the amp proprtional to the current drawn throught the bulb. Valve amps typically have half the normal operating voltage, which is fine for them. Something at the back of my mind was telling me that some SS devices dont like being operated under vol...

Ah dammit, I was afraid someone would that.
Would such SS devices exist in audio circuits?

Yes, it's safe to use with SS equipment. The D.U.T. likely won't work well (or at all) with the bulb in series, but as has already been said, it's just a quick-and-dirty overload check. :wink: --mark Cool thanks. That's pretty much what I thought, but I was afraid that certain SS devices may be dam...

I'm at work so can't see the schematic but if I'm picturing it correctly then this is exactly what I'd expect to happen. The 220k mix resistor acts as a load to ground via the other channel (6SN7) volume control. This is forming a low pass filter filter which is attenuating highs. There should be a ...

I usually use a SPDT center off switch so that you can select 2 different NFB resistor values, or have it completely out of the circuit. This is a great way of getting the open/loose vibe of Vox type power amp, or the the tightness of 6L6 type power stage, or somewhere inbetween, like an early Plex...

The bulb limiter, I believe, is just a safety device used on initial start up to take the load and indicate by glowing bright that you have a short the needs to be corrected. If it goes bright and then dims to a soft glow there are no shorts and should then be removed for further tests and voltage ...

I worked the current limit out to be about 104mA @ 25W around where I'm from (240Vac), guess you're on 120Vac? I was just worried that running SS componants on too low a voltage could cause damage to them? I'm pretty sure that switching circuits etc. would not work due to gate voltages not being mad...

Out of interest; is it safe to use a bulb limiter to test solid state equipment?
A friend had an old stereo which he thought was valve based. It was blowing fuses but I was fearful of potentially causing further damage by using a limiter.

You can achieve even further noise improvements by connecting the centre of the 100 ohm resistors to a positive DC voltage instead of ground. I hooked it up to the power valve cathodes in my build and have virtually noise, with 4 gain stages. There is only slightly more noise when connected to groun...