Josh's 36W Tremolo TMB Xtra Reverb

[JMPGuitars]

jmpguitars-ttmbxr.jpg

This is the amp I made with mostly SMT stuff. I still need to add the FX loop, then I'll take some more photos. I'll also post a demo. Soon?

The voltages aren't what I was expecting, but they're all within tube spec, and the amp sounds really good. I may eventually swap tubes around and/or adjust to get the PI voltages up, but, I'm really happy with the sound. So, I dunno. We'll see. I certainly don't need more headroom for my office.

Power tubes are biased at about 83%, so that's all good.

I had what looked like an oscillation around 30MHz, so I added grid stoppers and bypass caps to both PI inputs. This resulted in what I think might be the cleanest single-tube reverb I've heard. I used 10KΩ resistors and 100pF caps. I think that combo blocks everything above about 100kHz, but there are other factors that would affect the range. In any case, it's well beyond the (desired) audible frequency range of a guitar amp.

Thanks,
Josh

[Bieworm]

Super nice man! Lots of knobs and switches… you have to be sober when playing that thing

[JMPGuitars]

Super nice man! Lots of knobs and switches… you have to be sober when playing that thing

Depends on your skill level.

[Bieworm]

Super nice man! Lots of knobs and switches… you have to be sober when playing that thing

Depends on your skill level.

I’m from belgium… I’m pretty skilled all those Trappist beers

IMG_8059.jpeg

[JMPGuitars]

I swapped out the LTP tail resistor from 15K to 39K, and now my PI voltages are much better. I'm getting about A 207, G 45, C 67.5 per triode. That puts my cathode at about 32% of the anode voltage, which should be close enough to the 30% goal.

My V1 plates are a little low, and V3 plates a little high, so I'm going to swap those tubes around and see if the voltages stay the same, or follow the tubes.

I'm looking forward to hearing it tomorrow when I can crank it up.

[JMPGuitars]

V2 voltages were actually higher % off than V3, so I swapped V1 and V2. And there was a decent change.

V2 plates were high by 30V, and now they're only high 20V.

V1 P1 was low 19V, and now it's high by 7V. That's a big jump from just a tube swap. V1 P6 was low by 10V, and now it's high by 4.5V. That tube is definitely going to stay there.

V3 voltages are likely a little high because of the tube driver voltage, which is about the same voltage I'm getting at P1. I may add a resistor before the tube driver source voltage, or adjust the tube bias...or try a different tube. Options, options...

[Bieworm]

How is the cascaded mode sounding? Not too harsh preamp OD ?

[JMPGuitars]

How is the cascaded mode sounding? Not too harsh preamp OD ?

The amp sounds great in general. I like the P/C preamp switch. My main amp is an Xtra TMB, this is the same thing with the Tremolo added.

The last thing I have to hunt down is a very subtle high pitch noise coming from the reverb when cranked. I'm going to try and decouple the reverb driver and see if that has the desired effect.

[Bieworm]

It’s not the tank resonating? I always put some isolation foam on the flat surface of the tank. I’m sure you mean something else, but just mentioning this in case it would be useful info

[JMPGuitars]

No, I'm pretty sure it's a parasitic gremlin. I have a reverb tank mounted in a tank bag, with 2 layers of cardboard in the bag. I also tried a smaller tank, same thing.

The high pitch bitch starts around 60% or so on the reverb knob, then when you get to about 80% there's a very faint warble in the background, but only for a small portion of the knob.

The tube layout on this is not ideal. V3 was added over V1 in a small chassis. I might need to redo that section to get rid of it. I'll try a few other things first.

Everything sounds awesome, until the reverb is turned up too high. It's minor, but it's big enough to annoy me.

It could also be that I'm pushing the reverb driver too hard. I should probably look at the spec sheet for it.

[JMPGuitars]

Well, MM said the max voltage is 600V for the FBFRT reverb driver, so that's not it.

I pulled the tube, and the high frequency noise is still there when the reverb pot is turned up, so it's not the tube either.

Chopsticking the wires doesn't do jack either, so it looks like it's scope time.

[Bieworm]

The mystery whining

[TriodeLuvr]

Nice looking amp!

"The high pitch bitch starts around 60% or so on the reverb knob, then when you get to about 80% there's a very faint warble in the background, but only for a small portion of the knob."

Yep, that sure sounds like a parasitic. I'm surprised the chopstick doesn't affect it. Do you have a schematic for this section of the amp?

Jack

[JMPGuitars]

Yep, that sure sounds like a parasitic. I'm surprised the chopstick doesn't affect it. Do you have a schematic for this section of the amp?

Nothing public yet, but if you take the Tremolo tube schematic from the Tremolo TMB, and shove that in front of the preamp for the Xtra TMB, then that's basically this amp. There are some adjustments, and such, but that's the general idea.

I think there was a flaw (or five) in my PCB design. I changed potential issues I overlooked, improved some layout stuff, added a few options, and ordered some more. We'll see what happens.

[JMPGuitars]

I experimented with a few things on this new version, and it paid off. It came out awesome. Can anybody see what's different? One thing you can't see, I used a 2M Reverb pot.

ttmbxr-01.jpg

ttmbxr-03.jpg

ttmbxr-02.jpg

[JMPGuitars]

Here are my voltages.

For my first voltage test, you can see V1 was too low, and V2 was too high compared to their targets:

before-tube-swap.jpg

I swapped V1 and V2 and the voltages look great now:

Josh TTMBXR Voltages.jpg

My bias is slightly hot at around 91%, but that's fine. Basically the cathode voltage was about 1.5V higher than normal at the time of testing. This could be due to me switching to a different pair of power tubes after 1. discovering one of the previous tubes might be a noise issue; and 2. breaking said tube while trying to remove it. Oops.

I used my tube tester, and at a plate voltage of 350VDC, the old tubes gave about 400mA current at 0V grid, and I think it was set at 310V screen. The tubes I put in now, give around 450mA at the same voltages. In other words, the Mullards were not as hot as this specific pair of TungSols.

On a side note, I added a 10K resistor at the reverb driver on the PCB. That lowered the V1 P1 from 300 to 260. I think a 4.7K there would hit the target dead on, but it's arbitrary and sounds great. I'll try and record a demo this weekend.

Thanks,
Josh

[JMPGuitars]

Guess who forgot to use a shielded wire on the gain pot to the board?

[Bieworm]

Guess who forgot to use a shielded wire on the gain pot to the board?

On a regular tremolo tmb it’s asking for trouble.. I can imagine the problem it is on that amp

[JMPGuitars]

Guess who forgot to use a shielded wire on the gain pot to the board?

On a regular tremolo tmb it’s asking for trouble.. I can imagine the problem it is on that amp

Yup. It gives a nice high pitch ringing. Chopsticking it minimizes it until you crank the gain all the way, but obviously I'm going to replace it with a shielded wire.

[JMPGuitars]

Shielded wires on the Gain pot helped a little, but now it looks like I need to shield the V2 plate wires. A future version of this PCB will have shield connections for all 3 preamp tube plate wires.