The Amp Garage

Has anyone tried this tube yet, and specifically, can anyone speak to its differences/similarities with the EL34B-STR? I really like the STR version, but am curious about the BHT....

Cool. Thanks. I picked up some high idle current EL34B-STR tubes, and they bias up just fine with the circuit unchanged. So, I think so long as I look for higher idle current EL34s, I should be fine. As for bias level....I'm liking 35ma at 420v. But I see your 28ma at 410 is much lower. I was told b...

Is the consensus, then, that the Mullard RI is fine in the V2 spot in a JTM-45? I'd like to try one in that spot, but don't want to cook it....

You probably want to change the bias pot to 100K to give you more sweep range, and then the resistor before the diode sets the extent of that sweep so you might want to change this too to get a good usable range for the tubes you want to use. Good deal...I think I'll just start with getting some hi...

So you want to get about 31mA with the SS rect then, correct? I would change the 56K to maybe a 33K or 39K. Now you could most likely fine a value that will allow you toggle between both tube types if you want to experiment. TM What if I just replaced the 56k resistor/50k trimmer with a 100k trimme...

That helps a ton, ToneMerc. However, I do have the newer, low-voltage PT, so my plate voltage is around 420vdc with EL34s biased up at 31ma, given a 5AR4 rectifier. With a solid state plug, I can get about 37ma, but again, that's with the trimmer maxed, and with the highest-idle-current EL34s I coul...

What does the bias network look like starting from the rectifier? For example, a Marshall JTM 45 RI schematic shows: Rectfier/HT, 220k, diode, 15k, 56k, 22k trimmer, ground. If you can't get enough idle current maybe there is a resistor between your trimmer and ground? If so that's the one you want...

Hello all, I have a Germino Classic 45 (JTM-45 clone-sh) and I really like how it sounds with EL34s. Unfortunately, I need to find high idle current EL34s in order to have them bias up in my amp, and that's proving somewhat hard to do. So, I'm contemplating changing the bias circuit to allow me to u...

Cliff Schecht wrote:It'll look fine, the issues are internal and (usually) hidden from sight.

Right, I meant how would it look on a DMM or a scope? What am I looking for...or can it be tested outside of it being in the circuit and the problem being presently manifest?

What would a faulty standby switch look like? I've never had one go bad before...

1. I love your username... and 2. Heck yes it'd be wonderful if that was the problem. I think I have a spare switch somewhere...but if not, that's about as affordable a fix as I can conceive of. The standby switch does usually pop when I "re-set" the amp, and I do get some "voltage drop fizz/crackle...

Hmmmm. What you mentioned about the standby switch has me wondering about the switch itself. A poor connection inside the switch dropping voltage could cause the problem you're experiencing. If that is the only way you are able to make the problem correct itself that just might be very telling. Dav...

I'll post an updated picture of the amp, but since that picture was taken it had been shipped back to Greg and he'd replaced a lot of the CC resistors with CF ones...leaving only a select few in the circuit, and only after they'd tested within spec. I'll be using the amp tomorrow, so sometime next w...

Attached is a picture of the chassis, post caps. There are two SM caps...one under the voltage divider resistors, and one in the tone stack.

...but it does sound like a leaky cap that taking the amp into standby mode is allowing the DC charge on it to diminish. Now we're talking. So putting the amp on standby will allow DC on a leaky coupling cap to "drop off"? That's describing the problem pretty well....it's like the amp starts to go ...