6G15/5G15 Reverb - trashy thin distortion

[theunrulychef]

This is when I really wish I had a scope so I could see what's going on in there a little better. If I could isolate where the offending distortion is originating, then I could try to figure out how to tame the gain on that stage(s).

Tubeswell, your unit - does it overdrive at high dwell settings? Diablo, are you using a nice 12AT7 or a cheap one like me? I'm just wondering how much getting a better tube in there might help smooth some of that distortion out so it's not so offending - more overdrive rather than fuzz.

Mine sounds similar to what I've had in the past from cold bias situations (on power tubes), or even when I had a partially blown OT that plagued an amp build. Sounds raspy, grainy & unpleasant, but only with higher dwell & mostly on the attack.

[loudthud]

Here's a question I'd like to raise. What is being used for the drive transformer? When you see the listings from companies selling transformers for guitar amps, there is usually one one listing for "Reverb driver transformer". It seems like the one used in Fender combo amps (pri 15K?) wouldn't match the 6K6 (wants 5K-9K) very well. The idle current is 24mA. I would think something closer to a Champ OT is what you want to use. I'm asking the question because I've never built an amp with reverb. Could this be the real problem here?

[theunrulychef]

I'm using Weber transformers. I was actually just wondering about this myself. Says it's 7k primary : 4 ohm secondary - my tank is 8 ohm. So does that mean impedance mismatch or is it close enough?

I certainly trust Ted to know his products & have it all listed right on his site - they've always done me right.


EDIT: Here's the listing on the Weber site - Output Transformer for Tweed Champ, Champ, Vibro Champ, and reverb driver for Weber 5G15 and 5H15 (ReVibe) chasses. 4 ohms output.

[loudthud]

If that's what Ted supplies with his kit, it's not an issue.

[tubeswell]

Tubeswell, your unit - does it overdrive at high dwell settings?

Not unless I have the hot cake in front of it.

[Diablo]

Diablo, are you using a nice 12AT7 or a cheap one like me? I'm just wondering how much getting a better tube in there might help smooth some of that distortion out so it's not so offending - more overdrive rather than fuzz.

I've tried various nice US-made 12AT7 tubes, Tungsol, RCA...no difference.
My OT is a Heyboer - same general spec as the Weber, at least it looks about the same.

I did another mod to tame the fuzztone. I added a 68K resistor between the wiper of the dwell pot and pin 7 of V2 (12AT7). I'm also using the unbypassed 1.5K resistor on pin 3 of V2 to lower the gain. Until someone comes up with a better fix, I'll live with this one. Obviously, it knocks down the reverb, but I still have plenty for normal playing....maybe not enough for the twilight zone.

[theunrulychef]

D'oh. So I think I've got a bad OT. Before I write Weber to get a replacement, can you guys double check my testing method - from here.

Pulled 6K6 & reverb tank lead. Then I tested for resistance between leads - get 345 ohms. Test to ground on each lead - open. Then I moved to the secondaries & it seems like they're shorted. I disconnected one lead from the RCA jack and tested again. Dead short - less than an ohm. If it's such a blatant dead OT, I'm wondering how it was driving the reverb pan as well as it was?

I pulled the tranny, and remeasured. Everything is the same - 345 primaries, short on the secondaries. It's dead, right?

Thanks,
Jay

[Diablo]

D'oh. So I think I've got a bad OT. Before I write Weber to get a replacement, can you guys double check my testing method - from here.

Pulled 6K6 & reverb tank lead. Then I tested for resistance between leads - get 345 ohms. Test to ground on each lead - open. Then I moved to the secondaries & it seems like they're shorted. I disconnected one lead from the RCA jack and tested again. Dead short - less than an ohm. If it's such a blatant dead OT, I'm wondering how it was driving the reverb pan as well as it was?

I pulled the tranny, and remeasured. Everything is the same - 345 primaries, short on the secondaries. It's dead, right?

Thanks,
Jay

Put your transformer back in....it's fine. The secondaries will have very low resistance, less than an ohm. Check each lead relative to ground and there should be infinite resistance. My OT primary resistance is 257 ohms - Heyboer.

[theunrulychef]

Damnit, I thought I figured out why it's distorting (OT is such a good scapegoat too). Thanks for the quick reply Diablo. I'll have to check out your tweak to tame the gain.

This stinkin' build is driving me crazy, but I know having problems like this is how I'll gain experience for "next time".

[Diablo]

Here's another test you can do to show your OT is fine. Unplug the reverb input that goes to the pan. Plug in an 8 ohm speaker...around 15 W or greater. Now your 6g15 is a Champ guitar amp. The Dwell knob is the volume control. I just did this test with my Tore-Vibe to see if I could get clean guitar at high dwell. I still got a little distortion when cranked up full. My thinking was that if the guitar was clean, then it had to be saturating the reverb transducer in the pan. So much for that theory.

Oh, and this distortion problem appears to be characteristic of the circuit, because it seems we're three for three - check this one out from a year ago:
http://music-electronics-forum.com/s...highlight=6g15

[theunrulychef]

Too funny. I did that last night too. The distortion was a little hard to pick up through the speaker, but it was there. The tone was quite thin, although the speaker was loose & sitting on the bench, not in a cab. I also ran out through the tank in my PR & back into the return section of the amp (had the other tank plugged into the PR rev out as a dummy load). Got distortion there. I also did the reverse & it was ok going through the 5g15 return section. Then I played straight princeton reverb through the 5g15 tank for a bit just to make totally sure it wasn't the problem - it sounded great. I tried the 56k off the dwell & didn't like it too much. I also tweaked the resistors on the 12AT7 so my voltage readings are really close to stock now. The 6K6 is still a little high, though Tubeswell said his is working ok with the same voltages.

I bought a scope today on the bay. Hopefully I can remember how to use it well enough to find the problem quickly (been about 12 years since I used a scope for anything).

I'm gonna say screw it & work on something else for a bit till the scope arrives. It's certainly playable, just not perfect.

[Diablo]

I bought a scope today on the bay. Hopefully I can remember how to use it well enough to find the problem quickly (been about 12 years since I used a scope for anything).

I'm gonna say screw it & work on something else for a bit till the scope arrives. It's certainly playable, just not perfect.

I can hardly wait for your scope results....

[tubeswell]

Hi TUC

I just had a thought that it might be that one (or more) of your plate resistors might be bad. I had a problem once with a bad plate resistor that made one of my amps sound terrible until I figured out what it was and replaced it. (Turns out I had overcooked the carbon comp while soldering it - don't know if you used CCs or not tho')

[theunrulychef]

Hmm, that's a good one... I literally just buttoned it up in the box, so maybe tomorrow night. A burnt CC is a real possibility. I love my Hakko iron, but it is a beast & surely could fry a resistor without dropping a degree if I spent a little too long on there.

One question though, was that burnt resistor microphonic? I've poked around in there with the ol' chopsticks trying to get noise from anything & haven't had any luck.

[tubeswell]

The burnt CC resistor gave me all sorts of weird noises til I sorted it. I didn't quite realise how bad it was until I was de-soldering it - it was so badly compromised, it virtually crumbled when I came to yank it out.