Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: proaudioguy on January 13, 2021, 02:29:59 am
-
The twin reverb project I have been working on since the 1990s may be nearly done.
The kind man that built my turret boards (when I didn’t know what a turret board was) in 2004 sent me layouts and schematics as well. The first pic is how I had it when I couldn’t get the remolo working well enough and I ripped it out, tried the bug, then frustrated with that tone killer, ripped that out and then life happened so it got shelved again. Parts came in today I I notced the bias to the intensity was not the same as hoffman. Pic one has the way I was told to do it and pic two is how it is done now. Seems to work. Still need to set the bias. Any thoughts on this? Hoffman also has the bias supply cap in a different place.
Also does anyone know what cathode current I should be looking for in the Twin reverb? I have 1Ohm resisters inline I can measure.
While I was in there I did the robrob channel mixing mod which made perfect sense to me. Its how I always thought it should be done. I haven’t taken out the original 220K on the tremolo channel after the reverb yet, but I’ll do that tomorrow. Normal channel works great. Getting some signal on tremolo channel when volume is all the way down. Not sure what that’s about. ANy ideas where to look? I am going to sleep now. Too tired to look at it anymore tonight.
So looking at the before and after, do you think this was my problem all along?
-
Update. Bias has to be pretty cold to get the tremolo to work well. Is it possible other values on this amp are off by some amount?
Thoughts?
Tremolo works best at around 10% plate dissipation!!
These are Sovtec 5881/6L6WGC that I bought in the 1990s. We didn’t have the internet so my choices were limited.
Pin 3 to pin 8 is about 435V on the tubes I have.
I would like to have it both deeper at a higher bias setting and I would like it slower at the lowest setting and faster at the fastest setting. The sound of the tremolo is terrific though.
-
You are witnessing the reason that Leo abandoned that bias vary tremolo. Works fine with 6V6s, but 6L6s are more difficult to wiggle.
You can significantly increase the tremolo signal amplitude by replacing the resistor/cap on the oscillator cathode with a red LED. Then tweak the bias. You may find a happy spot for this set of tubes. But there will always be a compromise between good tremolo effect and good bias. Each set of tubes will be a bit different too.
Leo's answer was the roach. Works good enough and not nearly as many customer complaints. Still using the roach after nearly 60 years. None of his other 6L6 tremolo circuits lasted more than a couple years.
-
Sluckey - why not recommend your Tremo Nator? I've tried all the things mentioned to make my Hoffman SR trem better, but it is still weak, although it does sound nice.
-
I’m just trying to get that Vibroverb VIBE going.
So I’m getting a POP that varies in intensity when switching the tremolo OFF with the footswitch. Switching it ON is silent.
Also I had just a touch of signal coming through on channel two with the volume all the way down. Any thoughts on that? Looks OK and I do not recall that ever being an issue.
What RED LED specifically, do you recommend? I assume you run it in the logical direction with the anode toward the tube? I have some diodes here, but the only red one is probably an old radio shack that was a panel mount complete with chrome edge and threads to mount. Its pretty small.
I have a lot of 6V6 tubes and I’m totally down with using 2 of them and running the speakers 16Ohm. Would take only a few minutes to set up. I’m just worried the plate voltage is too high for the 6V6GTs. They are all either used or NOS. Nothing modern. I have GE (pair), Motorola (pair), RCA (quartet), and a pair of something that is not marked but I have to assume they are also 6V6s. I also found an old Rectifier.
I have tons of 12AU7s and some 12BH7As if anyone is interested in trading.
-
I took out the ROB ROB channel combining mod. It was a tone suck. I rather have out of polarity channels and only one with effects than have the tone suck.
I am now working on the tremolo switch. If I switch it OFF (unground), I get a pop in the reverb return. If the reverb tank is not connected I get a thunk thunk thunk thunk sound in the reverb return when the trem is ON.
I can’t imagine the VIbroverb did this. Any ideas?
I have the correct TWIN REVERB foot switch with the correct cable and the reverb side has the shield around it.
-
I am now working on the tremolo switch. If I switch it OFF (unground), I get a pop in the reverb return. If the reverb tank is not connected I get a thunk thunk thunk thunk sound in the reverb return when the trem is ON.
I'm unsure which trem circuit you have at this point. Hoffman's trem (same as Vibroverb 6G16) is off when the switch is grounded. A real AB763 trem is off when the switch is ungrounded. This makes me think you have switched to the "roach" trem? Please clarify.
Why would you ever operate this amp with the tank disconnected? If you must disconnect the tank then also switch the reverb off (grounded).
-
Please recommend what I should use to CLEAN the pots which are barely used but some apparently found dust.
Please recommend what I might do to fix the SHHHH coming from the reverb. Swapping V4 didn’t fix it.
If you want to skip the rest, that is the gist of what I need to do next.
Now here is tonight’s story.....I just came in from the garage and I was going to say, the trem is OFF when not grounded which is backward but you confirmed its right and I’m just a dummy.
I got rid of the pop. I required the footswitch jack to footswitch jacks and the footswitch plug to footswitch plugs. I figured simple is better than easy. These reproduction footswitches are easy to get now. When I started this project I had to by the fender reissue footswitch (TRS) so I had a TRS installed. It was one with switches in it so I might have boogered something or maybe it wasn’t my fault. I didn’t bother to check. I cut it off and tossed it in the recycled parts bin.
I have a SHHHHH (not hiss) coming from the reverb return. Its only there when the reverb is turned up and it varies with how much its turned up. Its still there if the reverb switch is set to OFF (no reverb sound). I swapped the tube to an NOS 12AD7 with no real improvement.
I noticed the V1B, V2B, and V4B plates (I think, hard to see pin numbers) are all microphonic. I found this tapping with a chopstick. Tapping that particular wire when the vol or reverb levels were up produced sound. I tapped the tubes and they were quiet. Its very low level but I would love for it to be gone. I didn’t find that anywhere else in the amp. I previously had a lot of microphonics and that turned out to be a reverb transformer. I changed that last time I was messing with this amp, must have been like 8 years ago. That cleared up a lot of issues. I must say re-using old parts with new is way harder than starting from total scratch. I built a princeton reverb and had 1 issue and had it fixed within a few minutes of discovering there was an issue. This amp has been a little something here, and there all along the way. I spend a few weeks messing with it then it goes away. I want it DONE this time. LOL