I knew this amp was good !

[cbarrow7625]

I could see how the cleans wouldn't be so great. I would expect that from an amp with relatively small grid resistors and large-ish plate resistors.

To troubleshoot your broken amp put a signal into the input and follow it through the amp from input jack to speaker until you get to the point where it disappears (or looks lower than normal). Luckily for you you have an exact copy amp to compare it with so it should be pretty easy. Obviously you are missing something somewhere just looking at the DC voltages. If you get full measured signal output from the power tube plates then you might have a bad tranny (or at least a mischevious cross-dresser )

[daz]

If you get full measured signal output from the power tube plates then you might have a bad tranny (or at least a mischevious cross-dresser )

How could i check this w/o a scope?

[cbarrow7625]

Just use an AC voltmeter. All you are looking for is the presence of an AC signal throughout the signal chain. At this point we don't careabout what the waveform looks like just whether it is there or not.

Anything could be your signal generator including just plugging in a radio at low volumes (just make sure the input level is in the guitar range of 100-200mV). No hi-tech gear required. Just find out where the signal quits and you will know what to fix (most likely).

Besides, you can't find mischevious cross-dressers with a scope......

[defaced]

I uploaded a 1kHz sine wave yesterday that you can use with a computer/mp3 player for your signal generator:
http://music-electronics-forum.com/s...ad.php?t=12945

[daz]

Thanks guys. I actually made a 1k sine wav in cool edit pro becaiuse that download one is too short and blips every 1/2 second when i set the player to infinate repeat. Anyways, can i get some tips on what kind of AC values to look for and where? I can tell you the AC on the el34 grids goes to a high of 80V, which is a lot more than it is at 10:00. so i assume it's happening AFTER that point, which i take it is probably bad OT news?

[daz]

geez....i posted that last one, then when i went to post again that post was gone. Now it's back and the one i posted after is no gone !!! WTF is going on here?!

anyways, now i had to recheck all the voltages because i put them in the last post to disappear. See if this tells you anything because i have no idea what kind of A/C voltages i should be looking for...

All voltages at max master volume

input to tone stack: 47VAC
master vol out: 10VAC
EL34 grid: 75VAC
speaker out: 8VAC

[octal]

That sounds like plenty o' grid drive to the EL34s, so I would suspect the OPT, the tubes, speaker jack or impedance selector switch if fitted.

Double check that you are seeing the proper voltage on the screen grids. When EL34s blow, they often toast the screen resistors.

Good luck, I'm sure you'll be able to sort it out.

Nathan

[daz]

I was afraid of that. I don't think there is any issue with the other things you mentioned, but i will hard wire the 8 ohm tap directly to the jack to eliminate the switch. the tubes i alreay swapped with all different ones and the 1k screens read fine. I hope it's not the OT because i would feel obligated to get him a new one, but then i have to worry about what caused that. I CAN say that after unhooking the OT primarys i read them and it was 45 ohms on one side and 35 on the other. don't know if that is telling in any way but thought i'd mention it.

[daz]

Well, the speaker jack is fine as is the selector. (both bypassed) I swapped tubes and screen resistors read 1k and are delivering proper voltage to the screens. Does this mean it's got to be the OT? Any way to tell besides swapping OT's?

[cbarrow7625]

Daz, you said that you have an identical amp of your own right? That one is working properly right? Just use the same test set-up and measure the working amp and compare the results. You should know immediately where your problems are.

[daz]

I know, but it's not exact in the way it sounds and it does have a different layout and some different brands of caps. Plus it's not in the same location. But yeah, thats my next step when i can. Right now i'm investigating a problem that may be the cause.....it seems the preamp has an issue with no clean worth mentioning even with the master and gains reversed. but if thats the case, why would that not still get louder since the drive is good at the el34's as octal told me? My amp gets very clean at that setting. It seems as tho the PI could be the issue if it has almost no headroom, so i'm going to check all the components there and the V3 socket.

[cbarrow7625]

Did you check all of the DC voltages on all of the preamp tubes and the PI? Make sure they are all normal. maybe you have a leaky cap or something (unlikely if you used modern caps).

Have you tried just swapping out the tubes one at a time? Maybe its a bad tube.

[daz]

I checked the caps and voltages everywhere, tho only swapped the EL34's and PI. I'll swap V1 and 3 just to be sure.


EDIT: tubes are all fine