TMB with EL34s

[insanecopilot]

I have gutted out a old crate Blue voodoo 50 head and want to build a 50 watt version of a 18 watt TMB. I think i can get away with the stock pt, But the OT i don't know about. It has 1k6 primary's measured from my vom. I know the el34 likes different impedance so if i need to replace it what should i use?
Couple more questions..... what bias resistor should i use, and does the long tail resistor need to be changed to a different value?
thanks.

[krx]

Some day those Crates will be worth a fortune and you'll regret it

Are you sure you're measuring the OT correctly? 1.6k sounds really low for a pair of EL34s. Bias resistor I would use 150R/10W. The PI tail should be okay as is. You will also want 1k/5W resistors going to each EL34 screen.

Also, cathode-biased you're going to get more like 30W. If you want 50W, you'll need to go fixed bias.

[zaphod_phil]

You can't measure AC impedance using an ohmmeter. What was that OT used with before? If it was for a pair of EL34s then you should be OK. Otherwise you have to inject an AC signal into one side of the OT, and measure the primary and secondary voltages to get the turns ratio.

You may be better using separate power tube cathode resistors. If you use a single shared one, it will need to be able to dissipate quite a lot of power. I would actually use around 15k for the PI tail resistor, and 470 ohms for the PI cathode resistor to increase the PI drive to more suitable levels for EL34s.

The other thing you will probably need to change is the value of the resistor which reduces the B+ rail voltage feeding the preamp, since with a 50W EL34 PT will probably give voltages too high for an 18W style preamp.

[insanecopilot]

Thanks for the replies. The pcb in this amp was a mess from several repairs before so it was ripe for a rebuild.
This amp came with 6l6's but was touted to run with el34's which were in it when i got it, So i think i will try reusing the transformers then. I am sure to have to adjust the rails....

So a 150r 10 watt is fine for the cathode? my experience with el34's is in single ended amps where 330 to 360r is used? If i bias them separately could i use 2x 360r 5watt resistors? Thanks again.

[krx]

Thanks for the replies. The pcb in this amp was a mess from several repairs before so it was ripe for a rebuild.
This amp came with 6l6's but was touted to run with el34's which were in it when i got it, So i think i will try reusing the transformers then. I am sure to have to adjust the rails....

So a 150r 10 watt is fine for the cathode? my experience with el34's is in single ended amps where 330 to 360r is used? If i bias them separately could i use 2x 360r 5watt resistors? Thanks again.

The Matchless Chieftan and Clubman used cathode-biased EL34s. They used individual 270R resistors on each tube, which is probably a little on the hot side. In my opinion, a shared 150R/10W is fine (measure your bias, of course, to be sure), but you could use individual 10W resistors if you want some extra safety margin.

I'm wondering, if this is a TMB, you change the phase inverter, and you change the power section, how exactly is it even related to an 18w anymore? It's just a cathode-biased plexi. Maybe you would be better off building a plexi?

[sub]

I built a "18W" with EL34s.
To separate power tube cathode resistors (as Phil mentioned) is a good idea.
IMO 150ohm (or 2x300ohm) is a way too low value (it depends on your B+ of course).
I had a 395V B+ and i used 2x 330ohm 25W resistors.... i almost burnt out two sets of perfectly matched SED EL34 (tubes are still working, but these drifted out heavily in few minutes). I don't understand how Matchless can use such a low value
In the end 2x470ohm cathode resistor was seem to be good.
But maybe it's just my case...

[krx]

I built a "18W" with EL34s.
To separate power tube cathode resistors (as Phil mentioned) is a good idea.
IMO 150ohm (or 2x300ohm) is a way too low value (it depends on your B+ of course).
I had a 395V B+ and i used 2x 330ohm 25W resistors.... i almost burnt out two sets of perfectly matched SED EL34 (tubes are still working, but these drifted out heavily in few minutes). I don't understand how Matchless can use such a low value
In the end 2x470ohm cathode resistor was seem to be good.
But maybe it's just my case...

I'd agree that the Matchless values look a little low, but it depends on the B+ as you said. 25W is definitely overkill on the resistor power rating, but I guess it doesn't hurt.

I don't know if cathode-biased EL34s is really that great of an idea in the first place though. You lose a lot of power and the feel of a bigger amp, which to me is the whole point of using EL34s in the first place.

[zaphod_phil]

The Matchless Chieftan and Clubman used cathode-biased EL34s. They used individual 270R resistors on each tube, which is probably a little on the hot side.

IIRC that was quite a lot on the hot side!

I'm wondering, if this is a TMB, you change the phase inverter, and you change the power section, how exactly is it even related to an 18w anymore? It's just a cathode-biased plexi.

Not really - it's still the same basic 18W non-NFB phase invertor with the 100k/100k load resistors, which make it slightly imbalanced, and the 470k grid resistors, Just a couple of small resistor changes to compensate for the lower gain of the power tubes, along the same lines as with a 6V6 18W. The power stage is still the simple 18W type power stage, with a change in cathode resistor value(s). The changes are pretty small and trivial. Quite a few EL34 based 36watters have already been built by various people here, along the same lines.

I don't know if cathode-biased EL34s is really that great of an idea in the first place though. You lose a lot of power and the feel of a bigger amp, which to me is the whole point of using EL34s in the first place.

You get some big tone, with a beefier low end and more headroom than a regular 18W. EL34s sound great in an 18W style amp.

[insanecopilot]

Well i was looking at the old pcb and it was using 2x 470r 5watt bias resistors, and i remember checking the B1 at 425v before i took it apart. I just requested a schematic from Crate or "Loud" or whatever they call themselves now. At least this way i can get an idea of what rail resistors to start with. This might take me a while, Got the chassis holes drilled and then painted. Now its on to the board.......

[Freddy]

I built something like this about 8-10 months ago...but it was not exactly a TMB type front end. More like Mark Huss Plexi 6v6 but with cathode biased el34s. Used two 430 ohm 10 watt resistors...one on each cathode. Plate voltage was in the 450 volt range. This put it the bias in the 70% dissipation range and added a 1000 uf 100 volt cap to each cathode to tighten up the sound a little bit. It may not put out 50 watts but it is plenty loud. I used Sovtek EL34s in this one. Ended up putting a 2K 5 watt resistor on each screen to get the voltage about 10-12 volts below the plate voltage and to get rid of a little glow I saw on the screens.

[Freddy]

You might be better off waiting for the schematic from Crate. Those 470 ohm resistors may be the screen resistors. I think that amp as it ships from the factory is fixed bias with a grounded cathode. Do you plan to rip out the PCB and put a turretboard in there? Those double sided PCBs are not the best platform for doing a lot of modifications.

[insanecopilot]

Oh i already ripped out the pcb. You might be right on the screen resistors. There is a lot of work to be done still so i will get as much done as i can first.

[krx]

I don't know if cathode-biased EL34s is really that great of an idea in the first place though. You lose a lot of power and the feel of a bigger amp, which to me is the whole point of using EL34s in the first place.

You get some big tone, with a beefier low end and more headroom than a regular 18W. EL34s sound great in an 18W style amp.

Eh, I disagree. If I build a high-powered amp, I want it to sound like a high-powered amp. To me, that means fixed bias. What's the point of building a loose, touchy amp that you'll rarely be able to play in the sweet spot? Besides, cathode bias big bottles in push-pull AB, and you burn up so much energy in those cathode resistors. The inefficiency really bugs me

Also, for what it's worth, the Mullard data sheet recommends plate and screens at 375V, screen grid resistor of 470R (shared, but I would do the 1k individuals as mentioned before), and cathode resistor of 130R shared. I would bump that 130 up to 150 to cool things off a little. If you're running an appropriate B+ (under 400V), that is definitely not too hot. Load impedance should be about 3 or 4k.

[krx]

I built a "18W" with EL34s.
To separate power tube cathode resistors (as Phil mentioned) is a good idea.
IMO 150ohm (or 2x300ohm) is a way too low value (it depends on your B+ of course).
I had a 395V B+ and i used 2x 330ohm 25W resistors.... i almost burnt out two sets of perfectly matched SED EL34 (tubes are still working, but these drifted out heavily in few minutes). I don't understand how Matchless can use such a low value
In the end 2x470ohm cathode resistor was seem to be good.
But maybe it's just my case...

330R should have been just fine at that B+. 470R sounds really high... Did you use 1k screen resistors? That's crucial with EL34s. Otherwise they WILL melt in a guitar amp.

[krx]

Used two 430 ohm 10 watt resistors...one on each cathode. Plate voltage was in the 450 volt range. This put it the bias in the 70% dissipation range and added a 1000 uf 100 volt cap to each cathode to tighten up the sound a little bit.

That sounds about right to me for that high of a B+. If you're doing the huge cap, it makes sense to bias a little colder.

Copilot -- I would look at dropping your B+ a little from 425 with a sag resistor or zeners. Something like 100R/10W would give you the feel of a GZ34 and probably get you in a touch under 400V. (and I just ranted about not liking inefficiency )

Those 470 ohm resistors may be the screen resistors.

I'm certain they are. I don't have the schematic, but I would be shocked if a Blue Voodoo was cathode biased.

[sub]

330R should have been just fine at that B+. 470R sounds really high... Did you use 1k screen resistors?

Yes i did! 1K 7W screen grid on both EL34...
With 330R my tubes was hot like h*ll, my chassis was also way too hot
With 470R everything looks right to me, but i don't like this amp... I'm gonna rebuild this to a low voltage 50W plexi or something similar...

I don't know if cathode-biased EL34s is really that great of an idea in the first place though

I agree with you... My EL34 amp only a little louder and stiffer than my 18W. My EL34 amp sound is very average compare to 18W wich is much sweeter...

[zaphod_phil]

If I build a high-powered amp, I want it to sound like a high-powered amp.....What's the point of building a loose, touchy amp that you'll rarely be able to play in the sweet spot?

And by the same logic, if I build a 35W amp I want it to sound like a 35W amp. Naturally, it will play in its sweet spot just as nicely as a regular 36W with four EL84s.

To me, that means fixed bias.

To you maybe it does. Which is fair enough. But not to the rest of all the folks who've successfully built nice EL34-powered 18W style amps. I'm afraid you're swimming against the flow here.

Besides, cathode bias big bottles in push-pull AB, and you burn up so much energy in those cathode resistors. The inefficiency really bugs me

Now that I understand.

Did you use 1k screen resistors? That's crucial with EL34s. Otherwise they WILL melt in a guitar amp.

1k screen resistors are a good idea for EL84s as well. I'm really amazed how they seem to survive in traditional style 18W amps with just a shared 100 ohm resistor.

[insanecopilot]

Copilot -- I would look at dropping your B+ a little from 425 with a sag resistor or zeners. Something like 100R/10W would give you the feel of a GZ34 and probably get you in a touch under 400V. (and I just ranted about not liking inefficiency icon_lol.gif)

I was thinking that too with a 100r, I will also start with 470r cathode and go from there.

Lots of good info here guys: thumbsup: I've got another Crate amp that would make a good candidate for a 36watt But that's another story.

[krx]

I was thinking that too with a 100r, I will also start with 470r cathode and go from there.

Lots of good info here guys: thumbsup: I've got another Crate amp that would make a good candidate for a 36watt But that's another story.

If you go with the 470R cathodes, you'll probably end up with the bias around 70%. If you do that, I would try the big cap trick, something like 1000uF @ 50V (you will need separate ones for each tube if you're using separate resistors) -- should give it more of a "big amp" feel.

[Freddy]

With a 1000 uf cap on each cathode resistor, SS rectification and an oversized power transformer, I definitely got the feel of a bigger stiffer amp. If your screen wires glow like mine were doing, try a bigger screen resistor on each tube.