36 Watt EL34 Superlite TMB

[JMPGuitars]

Well, here it is. My take on the Superlite TMB with EL34 power tubes. I designed this (with feeback from Zaphod_Phil who created the original Superlite circuit) because I love the Superlite TMB, and I love EL34s.

The build I did included reverb, but the rest is completely the same. If you build this and your voltages are slightly off from mine, post them here before you worry about that. Obviously the extra tube in my build could change things a bit.

Here's my build:

_JMP3151.jpg

Here's the docs:
Layout: files/JMPGuitars_36_Watt_EL34_Superlite_TMB_Layout.pdf
Schematic: files/JMPGuitars_36_Watt_EL34_Superlite ... ematic.pdf
Voltages: files/JMPGuitars_36_Watt_EL34_Superlite ... ltages.pdf

If you decide to build this, please post final results here for discussion. If you need help troubleshooting, start your own thread first.

Thanks,
Josh

[Riffmonster]

Super cool, Josh! I really wanna build this one when I have some money. How do you like the sound? Does it sound like a JTM or more like a super lead?

[JMPGuitars]

Super cool, Josh! I really wanna build this one when I have some money. How do you like the sound? Does it sound like a JTM or more like a super lead?

I like the sound a lot! I'll post a demo soon. I've never owned or played a JTM or a super lead, so you'd have to tell me. ...actually, I've never really liked anything from Marshall except the 18 watt stuff, and I think the community on this site makes them better than Marshall does. I feel like the company ruins things for the mass production business model. That's my opinion anyway.

[geoff 1965]

awesome!
thank's for posting the layout/schematic Josh.

[colossal]

That is an interesting amp. Almost Class A at idle. With 350V plates and 310V screens, 20V on the cathodes, that's 133mA for the pair. I bet it sounds great.

[JMPGuitars]

Here's a mediocre demo of my 36 Watt EL34 Superlite TMB.

2:54 reverb turned on (with the reverb off, the reverb portion of the circuit essentially doesn't exist)

4:39 bridge pickup switched to single coil

Bright/Thick Switch:
5:04 = thick
5:26 = off
5:42 = bright

https://youtu.be/PeRZXThV54s

Thanks,
Josh

[geoff 1965]

well done Josh!
good range of tones & breakup,i like the thick/bright switch,simple but effective and looks easy to install.

[crgfrench]

Real nice! Sweet harmonics.

[geoff 1965]

Josh
what's the purpose of the .047uf film cap in parallel with 47uf electrolytic? i have'nt seen that anywhere!

[crgfrench]

High frequency stability cap.

[colossal]

Josh
what's the purpose of the .047uf film cap in parallel with 47uf electrolytic? i have'nt seen that anywhere!

Film caps have much lower ESR and ESL than electrolytics. Electrolytics often have significant reactance so adding a film cap in parallel at 0.01 or 0.001x the electrolytic's value can ensure improved high frequency bypass. This is commonly done in hi-fi power supplies to clean up unwanted noise and came to be known as the Jedi Cap Trick on another tube amp forum back in the day. The film cap should be rated for at least the same voltage as the electrolytic.

[gretsch61]

what would be the recommandation for the PT & the OT?

cheers, gretsch61

[crgfrench]

You could use the GDS/Heyboer 36W transformer set
http://gdsamps.com/parts/

[JMPGuitars]

You could use the GDS/Heyboer 36W transformer set
http://gdsamps.com/parts/

That's exactly what I used. GDS is my favorite version of the Heyboers. They sound great, and they use solder lugs on the PT instead of arbitrary lead lengths.

[geoff 1965]

just tried your superlite 1K5/680R on the triodes and 470K series on my cascade mod Josh! awesome!

[crgfrench]

Josh how did you decide on the 1kuF value of the output cathode bypass cap?

[crgfrench]

Josh you might want to consider changing your plate load resistor to pin 1 of the PI to 82k.
I believe the one at pin 6 should stay at 100k but if you use 100k at pin 1 the PI will have unbalanced outputs.

I'm no expert on LTPs, I only build cathodyne, but I am pretty sure you want lower voltage at pin 6 than on pin 1. Because at pin 6 the signal has already been amplified by the first triode. Using 82k will raise the voltage at pin 1 (I believe it should be 5V to 15V higher) which will result in balanced signals from the PI.

I'm not sure why Trinity uses equal 100k load resistors there (Richie Hall's 18W designs also suffer from this error but his 36WTMB is correct. Ian's reference schematic has it wrong too. All of the Wattkins LTPs have this problem too), but Marshall, Soldano, Egnater, Aiken, Huss, Hoffman and Robinette all use 82k & 100k at the PI.

You could also use 68k at pin 1 and 82k at pin 6 to run it hotter but still balanced.

This might all stem from the original schematic derived from Colin Macfarlane's amp, which shows twin 100k resistors here. That could be a mistake.

[Daviedawg]

Twin 100ks are sometimes deliberately specified to get more harmonic response from the power stage by creating a slightly out of phase signal. There have been discussions in the past about what if any benefits accrue from this. I don't recall ever seeing a side by side audio comparison.

It is another of those discussions which are never resolved. Like the benefits of having a standby switch.

Dd

[JMPGuitars]

What Dd said. You can look at theory all day long based on other amps, but that doesn't tell you anything. Every 18 watter I've built has had 100K PI plates. 99% of the variants do too. If it wasn't a great tone, somebody would have changed that by now.

Here's my actual PI voltages:
1: 177
2: 25.5
3: 34.3
6: 175
7: 25.5
8: 34.3

You can unbalance it or balance it based on the tube itself. An unbalanced tube can give a significant voltage difference. If that's what you want.

If you think that idea is a good one, then build an amp with an LTP pair. Record a demo with both resistor values in the same amp, and see what people think.

Thanks,
Josh

[crgfrench]

Yep it seems like both ways are common -- I noticed the WEM dominator and the Matchless amps use unbalanced PIs as well. This would make for a cool A/B experiment...

Actually an extra resistor and a switch is all one would need to A/B it in the same exact amp.