50Watt Vintage tube PA Tubes?

[James R]

I have been reviving my old gear and so far with great success, in part through much help from the members of this forum.
Many years ago I brought my little PA amp with me from UK along with my other gear. I don't have any identification on it but it did work quite well. I haven't run it for quite some time.
So general cleaning and inspection is what I am doing. I discovered that there appears to be one strange tube in it and this raises a question.The first tube is what I expect, ECC83, The rectifier is EBC90 or 6AQ5. The next preamp is an ECC85. The power tubes are 4 EL84s. My question is what is an ECC85 doing in there. This is a VHF double triode. Can anyone enlighten me please.

[g-one]

This is a little confusing. You are calling one of the tubes a rectifier. Why? Is it wired up as such? A 6AQ5 is a small power tube (similar to 6BQ5). An EBC90 is a triode with 2 diodes, so nothing like a 6AQ5. Neither of these is a rectifier.
An ECC85 (6AT6) is a double triode. Just because it can be used for VHF doesn't mean it has to be.
If this is a really old amp, they may have used tubes that were common at the time. Or it could have been a useable sub for some tube that is no longer available. Without a schematic it is pretty much guesswork.

[Jazz P Bass]

I would advise that you compare the datasheets.
The ECC85 appears (as the RF label indicates) to have lower interelectrode capacitance.
Anode current, transconductance, amplification factor & heater current are different as well.
But in the end, they are both dual triodes.
The same pinout applies.
Without an indepth knowledge of the circuit that they are installed in, who is to say "why" it's there.
-That's what the designer wanted.
-That is what was available at the time.
-Somebody, somewhere along the way, put in the wrong tube.
I wouldn't worry about it.
After all, it is labeled as intended for RF & AF amplifiers.
Then again, installing a ECC83 there may tell something of the tube itself & the circuit.

[James R]

Thanks guys. I really bollocks that lot up didn't I. EBC90 is of course a 6AT6. I made an assumption that it is being used as the rectifier. Tube rectifiers were common back then. A little more searching revealed two fat diodes hidden under a bunch of other stuff.
As you say who knows why there is an ECC85 stuck in there. I will endeavor to draw the schematic of the whole thing then maybe I can see what it's doing.
I will pop an ECC83 in and see what happens. Would there be any special reason why the ECC85 would be preferred.
Thanks a bunch.

[Steve Conner]

The ECC85 has a slightly different pinout to the ECC83. The anodes, grids and cathodes are the same, but the heaters are different. If you put one into a socket intended for the other, the heater won't light up, or it'll not get hot enough.

The ECC85 pinout is the same as the ECC88/6DJ8 and its Russian relative the 6N1P.

[J M Fahey]

If I had a lot of "RF/TV/whatever" tubes I would *definitely* design with them and turn them into $$$$$.
In fact, I bought 1000 ECC/PCC189 tubes for peanuts and am using them in hybrid MosFet power amps with great success.
My power transformers even have both filament windings (6.3 / 7.5 V) to accomodate both, because I bought them mixed from a (long time ago) failed BW TV manufacturer's widow.
She just wanted his room empty.

PS: and yes, these too have pins 4/5 filaments and use pin 9 as a shield.

[Steve Conner]

I've got about a dozen ECC88s, and the thought had crossed my mind!

How are you using them in your hybrid amps? Is it the usual White cathode follower circuit?

[J M Fahey]

I´m a minimalist.
Also hate configurations which make tubes work in very different ways than what we expect in guitar amps, so no cascodes (ugh !) , White thingies, constant current loads and stuff like that.
So, I use honest-to-God triodes, loaded with plain old resistors and biased accordingly.
Choose the bias points so waveform *is* assymmetrical and when clipping, shifts duty cycle.
None of that happens with "instrumentation type" topologies.
I´m trying to add "tubeyness" to my "cold sterile SS projects" , not the other way round.