The Amp Garage

True about the bias voltage. But this is something that puzzles me: when you increase the bias voltage to say, -40, the plate and screen voltages drop considerably, from 400 to say, 360-370. These are just estimates, I don't h e the amp in front of me to test. Is this normal? Is it just a matter of ...

Hey thanks for all of the suggestions. The PT is not warm ever, and when the master and gain are maxed out, there is barely any ac voltage on the output. Just a tiny bit from hum. I am determining the current across the 1ohm resistors. And yes, they are definitely 1ohm exactly! I had another amp tha...

i'm aware that tubes get very hot under normal operating conditions, however my issue is that the tubes should not be on the verge of redplating at 27ma. maybe i should rephrase and say "extremely, abnormally hot". :) r11 does not get hot. removing the pi tube doesn't affect the power tubes heat dis...

Yes, I've swapped the tubes. I have a pair of JJ's that I can get around 38-40ma of current from, but this is still not even 60% dissipation at idle, and still, running extremely hot. Measuring current across the 1 ohm resistor on the cathodes.

OT is hammond 1650N. 4.3k primary. Not a class A amp, I think Steve misread something.

Stevem, this is a push pull fixed bias amp! The irons used are the ones recommended by the schematic.

Just finished another ax84 50w build. Having some issues with redplating tubes and low output. Here's a quick link to the schematic: http://www.ax84.com/static/corepoweramps/50W_PP/AX84_50W_PP_Poweramp_Schematic.pdf I wanted the amp to run 6L6, so I switched the grid resistors to 470r. Also, for L1 ...

So basically, you just leave it disconnected in this case. Since I didn't have those two pots, I just used resistors in place of the pots. Only had a single 1meg pot, so I used it for treble, seems to work fine so far.

i know this is definitely a no-brainer for most people here, but I've having a difficult time understanding a particular symbol on this ax84 schematic. It is a line with an arrow across the wiper terminal on the mid and bass pots: http://www.ax84.com/static/corepreamps/Simple/AX84_Simple_Preamp_Sche...

This happens alot when the OT secondary winding is not referenced to DC ground/common on the chassis. If your 1/4" jack is mounted on your chassis, just connect the - terminal to the sleeve of the jack. Just make sure to follow a known good grounding scheme, or use star grounding.

Thanks for the lesson there, tubeswell. All of my resistors are Vishay 3watt, for peace of mind. I don't understand how the cap/circuit isn't affected by having a 100k resistor in parallel. Guess I have some reading to do!

So, I need a 600v cap. I've got plenty of nice 47uf/450v caps sitting around that I thought I might be able to use. Here is the idea, take four 47/450 caps. Wire each pair in parallel, giving me two 94/450 caps. Then, wire those pairs in series, providing me with one 47/900 cap. It sounds like a gre...

After I grounded g3, the whooshing noise is gone. But I don't know if its because I replaced the 600v cap, cleaned all pins and sockets or what. Also, the hum is greatly reduced. I think I'm going to tackle the bias circuit next. Something seems to be amiss, in order for it to burn out tubes like it...

Purchased this amp from a local retailer that moves alot of gear. This had been sitting for a few months and I think he just wanted it gone. Even if this was a private sale, like Leo said, he could have said no. Also, a large 30uf, 600v cap had a big hole in it, as if it exploded. Turns out it was t...

I never really suspected the OT to be bad. These have a reputation for having pretty rugged iron, so yea. Just haggling. Something weird though, 3 of the 7027 tubes were in fact toast. I've never actually seen a tube burn up, so that was kind of alarming to see 3 dead ones. I have two matched 6L6gc ...