18 watt Superlite TMB Reverb with 6V6 Tubes

[fizzyone]

Hey everyone,
I'm Ralf, usually I'm more a reader than a poster in this forum, but now I'd need your help. To give you a little more info on me, I started building my own guitar pedals many years ago, also making single-layer pcb's myself, designing the boxes. I also built a couple guitars and about 2 years ago I bought a not working madamp (built from a madamp kit) and fixed it .... so I got into amps Following Uncle Doug, D-Lab and A/tech on youtube and Rob Robinette on his website gave me some idea about tube amps and how they work. There's stilll a whole lot to learn, I only have a rough skills in amp building. But how does one of my buddys say: "Learning by burning ...." So in that manner I want to start my first amp project.
Some weeks ago I bought a classic 18 Watt Combo (1974x), played it with my band and there was no chance to get through the sound (It was a pretty loud practise). I was also sure, that I'm not gonna need that really sweet tremolo - but a little more power an versatility from the amp. After reading in this forum and checking out the download section, my decission felt to the Superlite TMB, but I wanted it with 6V6 powertubes and additional reverb.

So I want to rebuilt the amp I have to the one of my choice.

In the last couple days I drew a layout and took the Superlite schematic and changed it (I hope that is ok to do so for only here in the forum, I'd like to respect the owners rights on their drawings) and now my question is, if you could confirm that schematic and layout. (The layout is mirrored horizontal, because thats the way the amp I bought was built.)

Beside the technical approvement I have some questions:

1. Are my drawings correct an make sense? Any errors that you'd see?

2. I checked out the changes between EL84 and 6V6 Tubes in this forum. All the needed and optional changes are numbered and explained in a box on the schematic. Can you confirm the needed changes? Do the optional changes make sense?

3. Do I need shielded cables for the fx loop if they don't go out at the back of the chassis and have a longer way through the chassis?

4. Do I need a shielded cable for the connection of the reverb pot to the RCA? I't quite a long way of cable through the chassis.

Thanks in advance, I hope I did it all right, you can help me and I used the right spot on the forum for this question.

Thanks

Ralf

[Bieworm]

Hey

I’m looking at this on my phone so I can’t determine whether the drawings are right. Will look into those tomorrow on my laptop.

But: the 6V6 SL TMB will doubtfully/hardly be louder than the EL84 amp you played through. You’ll benefit more from the EL34 Tremolo TMB if you want more headroom and nice tremolo.

The only wire on your diagram that sits between the reverb pot and the RCA is a ground wire. No need to have that one shielded. I usually do only long signal wires longer than 12cm with shielded wire. Lead dress will get you through noisiness.

[fizzyone]

Hey Bjorn,

thanks for the reply A friend of mine also has a professionally built regular 18 Watt TMB and this was a lot louder than my combo, so I suppose there was also sth wrong in the construction of the amp. If this built will get a little louder than his with some more headroom, I'd be happy If not I could go to EL34 ... but in this case I'd prefer KT66's. I drove mine with a 16 Ohm Speaker (efficency about 100 db) and his was driven at 8 Ohms, but we switches Speakers to doublecheck and the was still a huge difference. So hopefully, that "overhaul" will make a hearable difference

Thanks for the hint on the ground wire, so I switch the connections of pin 1 and 3 on the reverb pot, then it's more clear and upload the layout again.
First thought here was to ground the pot at the other pots and the RCA Connector to chassis ... but I'm afraid of producing a ground loop here ?


I just found another question to be answered:

5. The two 1 uF cathode bypass caps, one at the fat switch and the next one two at the cathode of V1b. Can they both be replaced with polarized electrolytic caps? It's hard, if not almost impossible to find 1 uF film caps in a usable size.


Looking forward for your check Thanks in advance

[Bieworm]

About grounding you would be doing yourself a favour to read the documents in Josh’s signature. Whatever you do, just don’t make ground connections on the back of the pots. You ‘could’ get lucky when an RCA jack grounded on the chassis doesn’t give a ground loop.. but it’s preferably grounded on the negative side of that circuit’s part cathode bypass cap.
About the 1uf bypass caps.. sure you can use electrolytic caps. Just make sure they’re oriented the right way. Neg lead to ground.

[fizzyone]

Yes, I read some posts about and that's the reason I was afraid of grounding to chassis. I also read that having the ground on the back of the pots is the worst idea.
But I will read the doc about grounding, thanks for the tip Didn't notice it yet in Josh's signature.

Did have have a chance to check the docs, if the amp is buildable like I drew it?

[fizzyone]

Some time and some thinking brought me to the point, that you're absolutely right with the 36 Watt TMB Tremolo Reverb ... it's perfect and I still have the sweet trem and also have some more power left in case I need it. As far as I see, I can keep the power transformer but need to change the output transformer. The PT has 300-0-300 V but only 126 mA, is that enough for the higher valued output tubes?
Soundwise I'd really prefer the kt66's, I heard some samples in the past and my favorite was the kt66's.
If I build the amp like shown and want to swap to kt66 in the future, can this easily be done or dies it need further changes to the amp?

[Bieworm]

The 126mA is kinda low, but the most important is the heater current. A KT66 or an EL34 needs a lot of heater current. You need 3A only for the power tubes already. And each 12AX7 will need 300mA. So with 4 preamp tubes an a pair of EL34/KT66 you need a power transformer that can supply over 4.2A of heater current.

[JMPGuitars]

You want your PT heater taps rated for 5A for an amp with a pair of EL34s.

Most manufacturers don't supply more than around 3A for the heater connections on 18W transformers.

The only exception I've seen to that is Mercury Magnetics P18RS which can be used for either 18W or 36W amps, and has 8A available on the heater taps. It also has 200mA available on the HT secondaries (though I don't know why they don't say that in the datasheet).

Thanks,
Josh

[fizzyone]

Hey guys,

thanks for your help, I think I found the right PT and they also sell the according OT at this shop.

One more question for your experience: I had the 18 Watt 1974x Clone with tremolo. The normal channel had a fat/bright switch like the superlite tmb and I really liked that sound. It was open, sparkly with a lot of body ... but the trem channel was dark and a bit muddy. If both channels are cascaded, like in the 36 Watt Trem tmb version, can I expect that more muddy sound, if yes, is there a chance to sparkle the sound up?

[wmwelch55]

I have never seen a long tail pair phase inverter that has a plate resistor of 470Ω. Typical values I have seen are 100K on both or 82K on one side and 100K on the other. I think 470Ω in that position would not work well.

Mike

[JMPGuitars]

I have never seen a long tail pair phase inverter that has a plate resistor of 470Ω. Typical values I have seen are 100K on both or 82K on one side and 100K on the other. I think 470Ω in that position would not work well.

I don't know where he got 470Ω for the plate resistors, but that doesn't really have anything to do with the LTP type. It's simply too low for the PI tube regardless of the PI type.

Thanks,
Josh

[Bieworm]

That 36W PT from Tubetown is an excellent piece of iron. I use it a lot, even for my Rockin’bird or my Goliath amps.

[fizzyone]

Thanks Bjorn, then I will order a PT and OT for my KT66 Version

[fizzyone]

Hey Mike and Josh,

I checked the schematic PI and Output stage and recognized that I obviously collected to much data for the 6V6 values from different sources and mixed some of them up as novices might do Sorry for the confusion, I try to learn .....
So I went back on my search and concentrated on Phils threads here in the forum. He adviced to lower the value of the PI's tail resistor to drive the 6V6's harder, he further changed the screen resistors to 470R instead of 1K.
The 470R on the plate resistors where wrong, these stay at 100K.

I posted the "rebuilt" pic of just PI and Output stage, and checked as good as I could. Can you agree with what you see?

[JMPGuitars]

You don't need to make all those changes. All you need to change is the cathode bias resistor value for the power tubes, and the cathode and tail resistors for the PI. Everything else is unnecessary and not specific to the power tube change (those are circuit variants that are unrelated).

Thanks,
Josh

[fizzyone]

Thanks Josh,

I will fix schematic and layout and upload it again. Then I will start building the amp like this, before I build the 36 Watt Trem Reverb. I already bought all the parts for this build and will report on it.

Thanks

Ralf

[JMPGuitars]

Don't forget to correct the pin numbers for the switch to an octal base pentode.

[fizzyone]

Yes, I did that. Had a reference sheet/schematic from dockeryamps that has a conversion list of pin numbers from el84 to 6V6.
But good tip, I will double check it with tube specs and other schematics.

Will upload the schematic with list here, also has some good explanations on schematics and circuits that helped me understanding the circuit. Maybe it could help s.o. else

[fizzyone]

Hey everybody,

and happy easter .... finished the built, works after a little tweaking here and there, but my voltages are a little different as the voltage sheet. On preamp and powertubes they are higher than given, on PI they're lower. So the PT also gives out 308 Volts (308/0/308), it's supposed to give out 300 and the calculation is based on 290/0/290.

So are these voltages acceptable or do I need to adjust here? I know, biasing is not acceptable, I'm running 40 mA on the 6V6 what is supposed to be 30 mA. But if I have to change the other voltages on the board, biasing might change too. So then I would first change values for the rest before I change the biasing resistor.

Another question is the powertubes, is the positioning acceptable like it is, or do I have to make the distance bigger? I used the EL84 holes and made the wider, but I have another hole left, that I could use to make the gap bigger, if needed.

Thanks in advance

Ralf

[fizzyone]

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