Replacing a driver tube in an existing EL84 SE Amp

[corbato]

hi fdegrove, joel, 7N7
how safe or stupid is to replace a ECC82 with ECC81 or ECC83 ?

sorry..its got nothing to do with this thread but this was something that was agitating me for a while, and when I saw you gurus in one thread...

thnks

[fdegrove]

Hi,

While all 3 types are pin to pin compatible,and there are more,swapping one for the other is not a good idea.

The tubes will probably not get damaged by it and will likely work too,but how?

Not a good idea.

Cheers,

[7N7]

Originally posted by corbato
hi fdegrove, joel, 7N7
how safe or stupid is to replace a ECC82 with ECC81 or ECC83 ?

sorry..its got nothing to do with this thread but this was something that was agitating me for a while, and when I saw you gurus in one thread...

thnks

I'm completely with Frank on this; they are different devices intended for different applications.

I know that the guitarists do this, but it should not be done.

ECC82/12AU7 is designed to run a reasonable amount of current as is ECC81/12AT7; ECC83/12AX7 is not.

The one possible swap you might get away with is to replace a 12AX7 with a 12AY7 for reduced gain. These valves are designed to run at similar currents i.e. <4mA, so you will probably get away with it. 12AY7 is a good valve.

As I have implied elsewhere it is a good idea to replace ECC82 with almost anything (!) but the '81 (which is not a good valve either) is not an option.

If you want more gain, which I suspect is your reason for asking, there are no plug and play solutions realistically. Most of the higher gain alternatives have different pin-outs; many are quite expensive too. Additionally, they have different characteristics which will require an element of re-design. If I had something with an ECC82 in it and I needed more gain, I would be looking for something that had some driving ability, so for a modest increase I might try 6463 (but I would need more current) or for much more gain I would consider E88CC,. If there were only modest drive requirements, other types I would consider would be 6BQ7, 6BK7, 6BZ8, or even 5965/E180CC which has mu of 47, although this is still less than ECC81.

7N7

[corbato]

Hi fdegrove, 7N7. Thanks for the info.

I have an salvaged an old EL84 SE amp. It originally extracted from a Phillips Stereo radio called "Maestro" and assembled in a new chassis in the early 70's by my uncle. I don't think you would know about this radio model - it was made by Phillips in India around mid-60's.

The amp, as it reached me, has ECC82 in the driver stage. It perhaps could have had any other tube just as well.

I want to replace the ECC82 with eaither ECC81 or ECC83, simply because these are tubes I have with me at the momemt. Any mods that you might suggest?

Sorry for diverting the topic of this thread, but thanks so much for the help.

[fdegrove]

Hi,

Why do you want to do this?
Is the ECC82 dead?

Either way it requires a lot of work to modify for another valve and since I can't find a circuit diagram for it, it's a bit like working blindfolded.

Maybe you can trade some of the ECCs you have for a couple of ECC82s?

Cheers,

[fdegrove]

Hi,

Actually I forgot you were the person with the 13D3/6158 valves.

From what I see from your tubelist that one is the closest to the ECC82.

It is NOT the same valve however so it may not work satisfactorily in that amp.

Cheers,

[corbato]

Hi fdegrove..
indeed I'm the guy with those 6158's ..

In between postings, I was doing some search and came across EL84 Single-Ended - Bonavolta. It looks very similar to the thing I have at hand. Atleast, the caps are ..can this circuit work ok with a ECC81 or ECC83.

I am willing to change the radio amp circuit to the Bonavolta circuit.

thanks...and btw ECC82 is not available where I live.

Thanks for the help.

[fdegrove]

Hi,

I am willing to change the radio amp circuit to the Bonavolta circuit.

Depending on the HT rail you have available this may work.
You'll need to some measurements,I assume the EL84 and ECC82 shared the same B+.
Probably around 270 VDC.

I am willing to change the radio amp circuit to the Bonavolta circuit.

Then you would still be using the ECC82?


Cheers,

[7N7]

This is quite a problem.

270V is not much to run a driver stage with resistive loads. Of course I have no idea what you have available to you where you are so it is not easy to make a r4ecommendation. I really do not think that the ECCs that you have are suitable

As for 6158/13D3, this is an excellent valve, but sadly again not really suitable. I am running one a s a diff pair in my experimental 13D3/D3A/6CK4 amplifier. 13D3 is low distortion but sadly has quite a high anode resistance.

The other problem is the HT you have available. You could run it with a low value of anode load and take a chance.

As I have only 300V available, I have opted for active loads; these use MJE350 PNP power transistors. The only other components are a 6.2V Zener, a few resistors and a capacitor. This arrangement gives me an anode load of about 800k which is very good for distortionm and I get to operate at the anode voltage I want (185V)

Even if you do not want the active loads, in your position I would go for the 13D3 and see how you get on. I would rather use this one than either of the other two types you mentioned.

Best of luck

7N7

[fdegrove]

Hi,

That 270 VDC B+ was just my wild guess really since this a common anode voltage for the the output valve.

Let's wait and see,