Metropoulos Forum

I'm going to be converting my amp from two 6L6 tubes to two EL34 tubes. I understand EL34 tubes require greater filiment voltage (approx. 1.5 amps each) than the 6L6 tubes (0.9 amps each).

How can I determine if switching to EL34 tubes will require more current than my existing power transformer can safely give me?

Thanks,
Scott

What amp is it?
I know many guys will do this with two-6L6 Fenders, and despite warnings of filament current, it seems to work fine. I don't know where you can get specs on old transformers like that if you needed them though.

EL34s run on the same amount of heater voltage as 6L6s(6.3V). But yes the CURRENT draw of a 6L6 is 0.9A while the EL34's current draw is 1.6A.

You can also get a seperate 6.3V transformer to run the heaters of just the EL34s. Get one that can supply at least 6.5/7 amps and you'll be fine. Then you will seperate the heater string between the preamp and power amp tube sockets. However, you'll need to run wires from the stock power tranny's heater voltage wires to the preamp tubes.

Hope this helps.

Jon

Better yet, can some knowledgable soul please let the rest of us know what the voltage and current ratings (power transformers) of the more popular Marshalls?

Thanks for the replies guys, and for pointing out the correct issue flamingras - current instead of voltage.

Of course, one question leads to another question (or more) so...

1) the amp is a Laney Pro-Tube 30 running the two 6L6 and four 12AX7s (one for reverb). How can I determine if the present power transformer will supply sufficient current to run the EL34 heaters?

2) if I determine the present transformer will not work and I opted to get a second power transformer to run the heaters of the EL34s only, where do I find such a thing (in my mind I'm envisioning a smaller power transformer in addition to the current one already in there), and where do I mount it on the chassis?

3) would the smart move for the novice such as myself be to order the appropriate Heyboer or MM transformer so that I can be confident it will work if installed correctly?

Scott

1) not sure. You could pop a pair of EL34s into the amp and see if your voltage drops on the B+. If it does, then you'll need to supply fillament. However, I think the Laney uses some pretty decent Iron in the Pro-Tubes, so you could be just fine.

2) George could special order a transformer. Also WeberVST or Mojo could also supply one. Any convenient spot as far from the input jack as is pratical will work for placement.

3) Sure. Getting the "right" power transformer would work. I'd still be tempted to just try the existing one. I once owned a Laney LC-50, which had 6L6s and I popped a pair of EL-34s with no ill effects.

Good luck

Thanks Richard. I'll just go ahead and see if I can make the present transformer work. If it doesn't cut it, I'll order up a new one.

Other than swapping the tubes, did you need to make other modifications to the LC50 to accomodate the EL34s?

Scott

No - Fortunately the bias pot had enough range. You should be able to too. If you can't get the bias in range, then you'll need to change the bias resistor in line with the bias pot to a diferent value. Go up or down 5-10k and you should be able to get it in range.

Richard, do you know if my amp has a bias pot? I bought it used with no instructions and I haven't located a bias pot, but I'm wondering if that is simply because I do not know what I am looking for.

The amp chassis has been removed from the cabinet, so if its there I should be able to find it.

Scott

You might want to email Laney and see if they have a pdf of your amps schematic for download. If not they might mail one to you if you ask. They have a manual of the AOR series for download on their website http://www.laney.co.uk Your amp is included in that manual. Laney apparently used the EL34's in the 50 and 100 watt versions. The manual only give you some troubleshooting tips and has no internal images or schematics.

This is somewhat embarrassing, but I do have the schematic for the amp. (I purchased a set from England on cd-rom; if anyone needs old Laney schematics let me know!) The only problem is, I'm just not very good at reading it, so I do not know what to look for.

Well, a potentiometer should look like this:


And the actual bias pot might look generally like this (this is what's in a Marshall Superlead):



Probably on the pcb/p2p board, and probably the only one of it's kind in the amp.

I saw what appears to be a variable resistor on the circuit board, a little tiny circular thing about 1/2" in diameter with a rotating top and "22K" printed on it.

Looks pretty Mickey Mouse to me, nothing like the pot in the picture. But it was the only thing which appeared to be a variable resistor on the circuit board.

Question - (I do have the schematic for the amp) in what section of the amp should I be looking to locate the bias pot?

Is the bias pot in the "presence" section?

What you describe sounds like another type of variable resistor and it is most likely the bias pot.

If you don't know how to bias an amp, stop now and take it to a tech. An amp contains lethal voltages that CAN KILL YOU.