I just got a pair of used Quicksilver Silver Mono amps rated at 90watts each. Being a tweak, I opened one up. Component and built quality seems good. I noted a simple all in one four pin bridge rectifier just before a 5AR4 rectifier tube. I have replaced similar bridge rectifiers with HexFreds in solid state gear with an increase of resolution. My question is has anyone done this to their tube amps and was it worth the effort?
I have changed normal ss diodes in high voltage supplies and heater supplies against faster ones (hexfreds or schottky) and have heard the same sort of improvement as in ss gear - even in the heater supply!. So I definitely think it is worth the effort.
I dont know the circuit of the Quicksilver amp, but it seems to me that the rectifier bridge might be doing the rectifiing and the 5AR4 might work only as a delay for the high voltage. Some people did that because it was near impossible to get reliable 5AR4/GZ34 some years ago.
I have it the other way around, replacing SS HEXFREDs with a 5AR4. I got about a 20V decrease in B+ because the voltage drop over a tube rectifier is greater than the SS diodes. However, I was able to compensate for the decrease in B+ by changing other circuit parameters. If you change to HEXFREDs, you'll probably get a 20V or so increase in B+, which may be too much for the components and could damage something, maybe.
Personally, I like the 5AR4s. The sound is somewhat smoother and the bass seems to be "rounder" to me. HEXFREDs seem to give a tighter more dynamic bass with more impact, but I think you loose a bit of the midrange liquidity of tubes. Compared to regular N4007s, the sound of HEXFREDs was much more dynamic and resolved. I've also tried UF4007s with filtering, which DocB kindly suggested (See the latest issue of VALVE). These are supposed to be more tubelike in their sonic characteristics due to the decreased switching noise. I did find the sound to be smoother than with HEXFREDs while retaining the resolution. In general, my opinion is that you'll get tighter bass, but loose some of the tubiness of the 5AR4s(GZ34s).
Another idea is to try other tube rectifiers such as the 6Y3, GZ37 or 5U4. This would involve no soldering. Most of these will probably give you less B+ depending on the conditions. I'd be very interested in what you find.
I think it depends on what kind of sound you want. If you are really curious, I'd go ahead and try changing to HEXFREDs, but be carefull you might blow something out. Ideally, I'd measure the B+ while bringing up the power with a variac (I'm saying this without actually having done so since I don't have a variac :-). Also, see the safety tips posted elsewhere here or in the tweaks section. There's a useful tip mentioning the use of a light bulb for limiting current. You also should see what the maximum ratings of the various components are. There should be some room to push the limits a bit.
Other things to tweak include the PS caps, coupling caps and resistors. These components should also result in large sonic differences.
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