express build: growling issues

[rawnster]

I’ve got an interesting problem. For a bit now, my amp has emitted a “grrrrrr” sound, or a growl type sound when the amp is pushed a bit…volume above 10 oclock with the guitar dimed. It will start to “pop/low freq crack”, then go into a machine gun type “grrrr”. I’ve looked at it and chopsticked for hours with no luck. I’ve replaced most of the wiring. I’ve replace ALL of the caps. I’ve refloated most every solder joint. Nothing has stopped the problem. Last night, when working on it again, I thought to try and change the impedance selector to 16 ohms. Wow, the growl went away!. Then I tried 4 ohms. The growl stayed away. I flipped it back to 8 ohms and the growl came back. I thought I may have a bad switch. Since I had extra switch handy, I replaced. Tried the amp again at 8 ohms, and it “grrrrrr’d” as usual. The switch change did not help.

I guess this is an obvious question, but did my output tranny go out? It’s the vintage tranny set I got from Toneslut a few months ago.

A little history:
When the amp was completed, the only cab I had at the time was a 2x12 openback cab loaded with two 8 ohm speakers wired in parallel for a 4 ohm load. I used the amp with this cab until just a few weeks ago when I picked up a used avatar closed-back loaded with two 16 ohm greenbacks wired for an 8 ohm load. This is when I noticed the growling problem.

To sum up, it sounds fine when using 4 or 16 ohm load. It SUCKS when using the 8 ohm tap.

Any ideas???

[Jana]

just thinking out loud here:

If there was a problem with the secondary winding of the OT, then it *should* show up in the 16 ohm setting too. The 16 ohm winding uses all of the 8 ohm winding and then a bit more. Unless of course there was a problem with the wire that is connected to the 8 ohm tap.

The 8 ohm tap is the one that is connected to the 100k feedback loop, right? Don't know if this is merely coincidence but seems worth exploring. What happens if you disconnect the feedback loop? Still growls at 8 ohms? What happens if you change the 100k to 47k and connect to the 4 ohm tap for feedback? Not saying any of this is a solution, but merely things to do to possibly find the cause.


You say that you originally tested with a 4 ohm cab going to the 4 ohm setting on the amp and it was fine.

You say that an 8 ohm cab goes grrr on the 8 ohm tap but is fine on 4 or 16 ohms.

what happens if you connect the 4 ohm cab to the amp with the amp set on 8 ohms? This would be a similar mismatch as when you connect the 8 ohm cab to the amp set on 16 ohms and it is fine.

just searching for more clues.

[rawnster]

Thanks Jana, great things to try. I should of have thought of some of those, but it was past 12am last night and I getting punchy and a little angry at the amp.

[hendo]

I'm going to have to come over and give you a hug ... just think of it this way you probably have master soldering skills now after rebuilding that amp a few times ... hope it wasnt the attenuator

[rawnster]

I'm going to have to come over and give you a hug ... just think of it this way you probably have master soldering skills now after rebuilding that amp a few times ... hope it wasnt the attenuator

I could always use a hug or two. ...and no, I'm not switch hitter.

I doubt it was the attenuator. I'll do some modding to the presence and NFB setup. Hopefully the problem lies somewhere therein.

[rawnster]

Just wanted to let you know of my progress here and what I’ve done to troubleshoot the problem.

First, I removed the presence pot and .1 cap from the grounding buss wire contraption, and connected the cap directly to the wiper and ground tabs on the presence pot. Then, I connected a separate ground wire running from the closest preamp board leg post directly to the presence pot ground tab. This didn't help remove the "grrrrr" at all.

Next, I removed the NFB wire from 8ohm tap....thus NFB wire not attached at all. This did not help. Still growls at medium volumes.

Next, I connected the NFB wire to the 4ohm tap. This did not help at all. Still growls at medium voume.

Next, I plugged in the amp to another cab with a 16ohm speaker load. It growls as usual with the impedance set on 8 ohms. Something new here is that it now growls on the 16ohm impedance setting with this cab, albeit to a lesser degree.

I’m at a loss here as to the problem, other than it's got something to do with the output transformer.

The amp sounds GREAT at lower up to medium volumes. The cleans are articulate; the lows are tight and punchy. As the amp gets louder, it breaks up nicely, with some complex distortion/hair. At the point just prior to it going into "grrrrr", it's fairly loud and rocking. It's just when you push it past this point, say, for some sustaining solo work and such, is when it goes bad.

To sum up what I've done:

- Refloated about every solder joint
- Rewired the entire preamp board with new caps
- Rewired the All of the pots
- Tried different tubes
- Reconfigured the presence/cap ground connection
- Connected NFB wire to 8ohm and 4ohm taps, and no tap at all
- Tried two different cabs with different ohm loads (8 and 16)
- Lifted the amp off the cab to reduce vibration
- Stood on one foot with tongue hanging out

In talking extensively with Rooster here on the forum, his conclusion is that it's tranny issue; and a problem that he's never run into before. Nice.

FYI: Moose (Chris) at Tonesluts has been more than gracious and is very willing to swap out the trannys…that is when he is stocked again.

So, there you have it. If you can think of anything else that may have been missed in troubleshooting this thing, I’d be ever so grateful to you if you’d share!

Ron

[rawnster]

double post