Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: hesamadman on December 14, 2016, 10:34:19 am
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I just added a gain stage on the front end of my 2204. It's odd because as soon as i turn it on, it works. Then you hear it fizzle out to nothing. The mod doesn't consist of much so I replaced everything short of the tube socket. I took measurements on the new tube and it's off because I get only around 100vdc at the plate. On the other end of my plate resistor I get 295vdc. 195vdc drop can't be correct using a 100k resistor can it? At the next few gain stages there is about a 10vdc drop across plate resistors. I've tried many many tubes. I was looking for sag in the filaments thinking the filament winding couldn't handle it but it's a new Hammond PT. Surely it can handle one extra half triode. The unused half has pine 1,2,3 connected together. I did have them floating with no connection but thought maybe this could have cause some weirdness but it didn't help.
If I wait a while it will work for a few seconds after I turn it on until it fizzles out. But again I've removed the cathode bypass cap. Replaced the new coupling cap. Removed the extra added filter cap. Only thing I have not done was check the cathode bias resistor. It should be 2.2k. But maybe I didn't pay attention and installed something crazy.
Any thoughts.
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Sounds like it's missing the grid load. Put a 1M from grid to ground.
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Oh man!!!!!!!! I have one on the input jack. However that one was for the first gain stage. Well since I added a stage, the first became the second and I Decided not to add the extra gain pot which would have been my drop down resistor for the (now second) following stage!
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Sounds like it's missing the grid load. Put a 1M from grid to ground.
Insightful call!
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Take a snapshot.....may be the last time u ever see me answer answer a question let alone get it right ! :laugh: (seriously, i know next to nothing but i've experienced that enough times to know what it was)
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Very nice man. Most mistakes are silly ones. Amp works great now. It's the type of gain I've always wanted from my own hand built amp. The original 2204 can be modded for more gain without adding a tube but too much gain in one stage can really muddy it up. This is definitely the EVH gain I've wanted to finally achieve. Thanks again for the help.
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You should share what you did, and how! I'd love to see it, 2204 is one of my favorites and I hope to own or build one someday!
-Brett
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Sure Bret. I'll make a post about it soon
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Very nice man. Most mistakes are silly ones. Amp works great now. It's the type of gain I've always wanted from my own hand built amp. The original 2204 can be modded for more gain without adding a tube but too much gain in one stage can really muddy it up. This is definitely the EVH gain I've wanted to finally achieve. Thanks again for the help.
I don't really need EVH levels of gain, but i know what you mean. Mine is much like 2204 with a regular gain stage instead of the cold bias stage. The max gain i can get with 3 stages a retain a quality tone and no issues like blocking or oscillation is less than a EVH but more than i need 95% of the time. And when i do want more i have a clean boost. But the thing is, i DO have a 4th 12ax7 i added for a effects loop but after trying a few i ripped the last one out because they just never are 100% transparent and i can't deal with ANY loss of tone quality. So i have been debating what to do with that tube and i have considered using 1/2of it for a 4th switchable stage. By the way, it took me a long time to nail it, but you CAN get 3 stages with plenty of gain w/o mud. But you can't get EVH levels of gain anyways so you needed that extra stage for that rather than eliminating mud.