Trem-O-Nator experiment in one-channel AB763 build

[joulupukki]

I recently completed a AB763 one-channel amp kind of as a mixture betweeb Robrob's Blackvibe and the Hoffman single channel. It has reverb and trem. It has the bias trem as in Hoffman's 1-channel schematic which sounds extremely smooth and nice, but I was looking to see what I might do to make it have more depth. So, I switched out the bias trem with the Trem-o-nator (the original circuit on the left shown here: https://sluckeyamps.com/trem/trem.htm).

I was hoping to have it be really smooth like the bias trem with the added benefit of it having a large range in speed. The speed range is great. But, I don't find it to be a very smooth trem like the bias trem was. The volume adjusts more abruptly. There's no thumping or ticking so it's definitely an improvement over what you'd have on a typical Deluxe Reverb but I'll likely change it back to the bias tremolo. ...unless I've done something wrong? Should I have expected it to sound smooth like bias trem?

I inserted the signal right before the phase inverter's 0.001uF capacitor.

[sluckey]

I don't think you did anything wrong. I know that different types of Vactrols sound different due to the response/recovery times of the LDR. I wonder if different brands of VTL5C1 sound different too? I used a Vactec Vactrol brand VTL5C1 and consider it to be relatively smooth, probably as smooth as the 6G16 circuit in my Hampeg conversion, although I did not do a side by side comparison. The Vactec Vactrol VTL 5C1 is still available on eBay.

Listen to the sound clips on my Trem-O-Nator site, especially the one with the AB763 amp by purpletele. Do they sound choppy to you?

[joulupukki]

I don't think you did anything wrong. I know that different types of Vactrols sound different due to the response/recovery times of the LDR. I wonder if different brands of VTL5C1 sound different too? I used a Vactec Vactrol brand VTL5C1 and consider it to be relatively smooth, probably as smooth as the 6G16 circuit in my Hampeg conversion, although I did not do a side by side comparison. The Vactec Vactrol VTL 5C1 is still available on eBay.

Listen to the sound clips on my Trem-O-Nator site, especially the one with the AB763 amp by purpletele. Do they sound choppy to you?

Thanks for the extra info. It’s probably a bad generic brand of the VTC5C1 (got a 5-pack off Amazon). Before I take the mod out I’ll see if I can track down a 50K-RA and a better Vactrol and see if there’s an improvement.

The sound clips sound pretty good and that’s what inspired me to try it out in the first place. To my ears the clips don’t sound exactly like the bias trem that was already working in my amp. I think it’s at least in the ballpark. If I can figure out how to make it so that it doesn’t reduce the volume output as soon as I switch it on I’ll probably keep it in there for a while.

[joulupukki]

I should have started a new thread, but I think, after doing more reading on the forum, it sounds like what I should probably do is switch back to the 6G16 trem and try some of the depth mods. I think this Trem-O-Nator is perfect for someone who wants a better trem than the AB763 for sure though.

[sluckey]

I can split the topic and put all your stuff in a separate thread if you wish. If so, what subject title would you like?

[joulupukki]

Maybe, "Trem-O-Nator experiment in one-channel AB763 build" ? I dunno. I may or may not be beneficial to others who come across the topic. I am going to switch back and use this for reference (https://el34world.com/Hoffman/files/Hoffman_AB763_Mods.pdf) and start with the LED mod. The only thing I thought my stock 6G16 trem was missing was more depth.

[sluckey]

The only thing I thought my stock 6G16 trem was missing was more depth.

That trem can be finicky to set up. You have to compromise between good trem effect and desired bias for the output tubes. Fairly easy to accomplish with 6V6s but more difficult with bigger tubes. And when you change tubes you'll have to juggle again. But it does sound good when you get it in the sweet spot. I suspect this is the reason Fender changed to the roach trem circuit.

[joulupukki]

I’ve restore balance to the force and the bias trem is wired back up and sounding great with the LED instead of the bypass capacitor & resistor on the cathode. :)

[joulupukki]

Ok, just with my phone’s camera and sloppy playing, but it’s back to sounding smooth again and with these two mods it’s running a bit deeper if I want it. I’m not entirely sure what to think about the bias … is there a point I should be worried? I’ve got it biased at approx 22mA, 61% dissipation. With the trem on and intensity maxed it reaches 30mA. I don’t think I’d run it at max. More likely around half way, but who knows.

Mods: I’m using a red LED on the cathode and I dropped one of the 470K resistors to 270K.

[sluckey]

Sounds good. I'd call it done.

[tdvt]

I think that sounds great ... & timely as I just finished the same circuit week before last & would like to deepen the trem.

Nice detail with the cabinet edge stepped back above the knobs.

6V6 or 6L6?

Speaker?

[joulupukki]

I think that sounds great ... & timely as I just finished the same circuit week before last & would like to deepen the trem.

Nice detail with the cabinet edge stepped back above the knobs.

6V6 or 6L6?

Speaker?

Thanks! I repurposed a stainless steel panel from an old tankless water heater to build the chassis and didn’t bend the front panel to the angle of a typical Fender amp so I had to figure out an alternative and I think that step back edge turned out working pretty well. :)

It’s a 6V6, with a 330V power transformer using power dropping resistors like Rob Robinette’s Blackvibe 6V6. I built it all from scratch to be the width of a Princeton Reverb but a couple inches taller to have plenty of room for a 12” speaker. I’ve got it currently loaded with a Celestion Creamback Neo 8 Ohm which has a lower sensitivity so it doesn’t quite get as loud as a DR but I really like the mellow tone – and the whole thing weighs 30 lbs with that speaker.

I’ve also tried a Mojotone Greyhound 12” 8 Ohm in there and that sounds great too – and much louder, but it also increases the weight by about 6 lbs. If I needed it to be louder I would switch it back to that.

What configuration (output tubes & speaker) did you end up building?

[tdvt]

What configuration (output tubes & speaker) did you end up building?

Well....

I started with a Classictone 6L6 transformer set that I had bought before they went out of business, to go with Doug's turret board, all still in boxes on the shelf. Decided it was time to finish it up.

Bought a blank Bassman chassis on Ebay for it all to live in.

Near the end of putting it together, I thought I would probably use 6V6s in the end, so I set up the power rail & OT for that configuration. The amp worked well, but as I thought about it more, it seemed I should at least try it out at full capacity, while it was still getting buttoned up.

So, I switched things to the original design & it presently has a new 6L6 tube set, which I am pretty happy with.

I asked about speakers as I don't have one yet. Tried a few by patching them in, played it patched through a friend's Celestion Vintage 30 (in a Deluxe Reverb cab) the other day & will probably order one of those as that sounded pretty nice.


Final configuration will be quite like the single-channel Custom Pro Reverb Fender put out recently.