Valve query.

[bikerhifinut]

Apologies for all the questions of late.
With 2 power amplifier projects on the go at the moment I am attempting to collate the valves and passives for both projects.
After much pondering and agonising over the best use of the chassis with octal valve sockets that I bought off Richard I have decided on a circuit based on the LEAK TL25+ which was based on 2 EL34 in push pull with LTP phase splitter and pentode preamp valve. I have researched the available octal valves and there's no real problems with the front end and phase splitters, I'll use 6SJ7 which as far as I can tell is an octal EF86 near as dammit and a 6SL7 for Phase splitter which research indicates will work in place of the ECC81 Phase splitter without having to alter resistor Values. I decided on this circuit as it has capacitor coupling between the preamp and phase splitter as i felt the 5-20 circuit should really only be attempted with the exact circuit values and valves specified by Mullard.
Now to the EL34 bit, the Leak amps were designed to work equally well with EL34/KT66/5881 as printed on the original chassis and circuit diagrams. I have been led to believe a 6L6 will also work in a circuit designed for EL34 types.
Is this true? I ask because they are readily available for not a lot of money and seem to be a highly regarded "Toob".
I won't be rushing in to any purchases for this amp as with luck, mrs Nuts health allowing, I will be at Tonbridge in october rummaging through the valves on sale, theres a few reputable dealers have stalls there. The cheap as chips eBay deals will be there for a while.
I admit to a slightly perverse streak also, in that a deviation from the EL34 route will give the amp a uniqueness perhaps.

A lot of information so apologies there.

Andy.

[Ardee]

Hi Andy,

I would say that the different output valves will all probably work, but some would be relying heavily on feedback to compensate for their not being run under optimum conditions. It should not be too difficult to alter the bias to suit the valve you are using.

The EL34 and 6L6/5881 are widely used in guitar amps, so reliable matched pairs are readily available. It's worth paying extra to someone like Watford Valves to get them properly tested and matched. The ones on ebay might be what is left when a batch has been picked over and sold on more than once.

Do you really need so much overall gain? You might get better sound with the same valve count by using a Williamson/GEC circuit.

[Alex Kitic]

Hi Andy,

I've got an advice for you on this issue:

If this was an original TL25 amp, we might consider whether one should alter the schematics in any manner. But since you are going to build something based on the TL25 schematics, and would like to be able to use various output tubes...

One thing to do would be to replace the cathode resistor/capacitor foreseen in the schematics with an LM317 and bypass capacitor for each of the output tubes. You need to set the current to a level that would be acceptable to your power supply, as well as the output tubes planned for use (in terms of anode dissipation/maximum current/voltage across the tube... etc.).

If you are going to stick to the original schematics:

this means each output tube would draw 68mA, which can be achieved with an 18 ohm resistor setting the current for the LM317.

Not only will you be able to use all the output tubes that interest you (EL34, 6L6 25+W dissipation types, KT66, etc.) but you will not need to pay for matched tubes - any tube inserted in the socket will draw exactly the same current, and DC in-balance in the primary of the output transformer will be minimized or inexistent. Of course, some difference will remain due to the difference in screen grid current draw for each tube (cathode current = anode current + screen grid current) but this in-balance will be minimal at probably less than 3mA.

If you play with the rectifier tube, you will probably be able to use even lower dissipation 6L6 tubes: using a 5R4 instead of GZ34 will yield much lower anode voltage, which combined with a constant current draw will result in lower anode dissipation in the output tubes...

[little eddy]

Originally Posted by bikerhifinut

I have been led to believe a 6L6 will also work in a circuit designed for EL34 types.
Is this true? I ask because they are readily available for not a lot of money and seem to be a highly regarded "Toob".

I think I am correct in saying that in the 6L6, g3 is internally connected to the cathode (8), so can be used in an amp that is designed for the EL34 where g3 is connected to pin 1.

So if you link pins 1 and 8 together, I think you can freely interchange the two, or use the KT66.

[Alex Kitic]

Originally Posted by little eddy

I think I am correct in saying that in the 6L6, g3 is internally connected to the cathode (8), so can be used in an amp that is designed for the EL34 where g3 is connected to pin 1.

So if you link pins 1 and 8 together, I think you can freely interchange the two, or use the KT66.

I understand the query as a theoretical question - can a 6L6 be used in circuits "designed for EL34" - can it withstand the voltages, etc.

Obviously, the 6L6 (all 25W+ anode dissipation versions) can be used in commercial EL34 circuits, since there is virtually no circuit that calls for 500V across the tube or similar exploits that the 6L6 might not follow.

As for the pins 1 and 9 issue, the fact that they might be already connected in an EL34 amp will not represent a problem to the 6L6 types - and if you use the EL34 in an amp designed for 6L6 the worst that can happen in that regard is tetrode operation (g3 disconnected). The latter is the core of the Hazen mod, a "mod scheme" that sounds different by disconnecting the g3 in pentodes (where this is possible) from any DC reference (by connecting g3 to ground via cap, i.e. no DC reference)... the authors of this "famous" mod regard this as an improvement, but the choice is all yours: if you like the tetrode kink (in the curves) this mod is going to give it to you

Jokes aside, the only relevant issue is bias voltage/cathode resistor/power dissipation. These are the things to watch carefully when attempting to use the 6L6 in EL34 amps... but the OP intends to build a new amp, so he can easily customize all details on time.