Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: hesamadman on May 11, 2017, 07:57:36 pm

Title: Debugging a 2204
Post by: hesamadman on May 11, 2017, 07:57:36 pm

Having some odd issues with my latest build. The amp actually works, it just doesn't sound like it should. It doesn't seem to be producing gain like it should. Really my main question here is what would cause plate voltages to drop dramatically when you install a 12ax7. I'm only getting about 160volts on the plate of v1. The voltage I have on the other side of the plate resistor is around 350v. I feel like even with a 220k plate resistor, that's way too much drop.

Title: Re: Debugging a 2204
Post by: shooter on May 11, 2017, 08:28:29 pm

have you;
tried multiple tubes?
monitor the 350Vdc over time and see if there's leakage?

160vdc at plate of V1 should not effect gain that much, IIRC you're starting with 50mV going to like a volt or so.

Title: Re: Debugging a 2204
Post by: hesamadman on May 11, 2017, 08:42:46 pm

I've tried EVERYTHING except whatever my hidden problem is. Maybe the voltage isn't my problem. It just caught my eye because the schematic says there only about a 30 volt drop not over a 160 volt drop.

Title: Re: Debugging a 2204
Post by: 2deaf on May 11, 2017, 09:06:17 pm

What is the value of the cathode resistor?

Title: Re: Debugging a 2204
Post by: sluckey on May 11, 2017, 09:14:14 pm

Quote

I'm only getting about 160volts on the plate of v1. The voltage I have on the other side of the plate resistor is around 350v. I feel like even with a 220k plate resistor, that's way too much drop.

That's perfectly reasonable. There's only .86mA flowing through that 220K resistor.

So why are you using 220K in the first place? JCM-800 2204 uses 100K plate resistors in the preamp. What other "improvements" have you made? Show us your as built schematic.

Title: Re: Debugging a 2204
Post by: hesamadman on May 11, 2017, 09:47:41 pm

http://site.triodestore.com/JCM8002204Stock.pdf (http://site.triodestore.com/JCM8002204Stock.pdf)


I originally had 100k in and I had even more of a drop. I would have thought it would have been less. Around 105vdc. Is this too low? At this point I've tried multiple values of everything. Beginning to think this isn't even my/a problem.

Title: Re: Debugging a 2204
Post by: sluckey on May 11, 2017, 09:59:18 pm

Hi/Lo input jacks can be tricky to wire correctly for the 2204. I seem to remember you wanting to use a relay or toggle switch for this. Did you?

Title: Re: Debugging a 2204
Post by: hesamadman on May 12, 2017, 01:39:39 pm

No not on this one. I did do that though and it worked great. But It probably is a silly mistake. I'll go over it more thorough.

Title: Re: Debugging a 2204
Post by: Ritchie200 on May 12, 2017, 06:21:31 pm

Check that .68uf bypass cap.  Also verify the 10K and the 2.7K values at V1.

Jim