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Sold 7027A as EL34 equivalent. Amp not working now

thomo57

thomo57

Super Member
l bought a couple of valves off an elderly gentleman who fixes valve radios.
He had shelves of hundreds of different valves.
I was after valves that maybe useful in some of my amps
He had 3 , 7027A tubes which have written on the boxes EL34/6CA7.
I bought 2
He explained to me that they are a direct swap for EL34's
When I arrived home l put them into my amp and turned it on.
Pop in the speakers and no power:wtf:.
Checked the fuses which were OK.
Replaced the original EL34 no power:mad:
Have since done some reading and find that to substitute these in there may need to be some circuit work done (image.jpeg slaps himself on the forehead)

Question is what sort of damage have l done and is it fixable
 
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you turned on your amp before connecting the speakers? that can be very very bad for your output transformers.
 
Duncan TDSL is your friend...

http://tdsl.duncanamps.com/tubesearch.php

7027A

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6CA7

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Looks to me that you connected the screen and suppressor together with the 7027A. What that did to your amp is beyond my ability to guess without a schematic
 
Probably blew the cathode resistor if I had to guess at it. Might have popped a fuse in the HV supply if it has one of those. Likely nothing majorly involved to fix, just needs a bit of investigation.
 
Note also that EL34 does not use pin 6, and so that is often used as a tie point in some amps (I do this in 6L6 amps all the time, connecting the screen supply to pin 6 and putting the screen stopper resistor between pin 4 and pin 4). But with a 7027, pin 6 is also connected to the control grid.

Anyway, my guess is that you either blew the HV fuse (hopefully!), blew a cathode resistor or some dropping resistor in the power supply (less hopefully, but not too awful), or killed your PT (sad trombone music)... on an EL34, pin 1 is connected to the suppressor, and is usually tied to pin 8 (the cathode) at the socket, which is then connected to ground through a resistor of some sort (larger value if cathode bias, or a small value for current sensing if fixed bias). But on a 7027, pin 1 and pin 4 are both tied to the screen... so you effectively shorted your screen supply to ground via whatever resistance is in the EL34 cathode circuit.
 
Yaqin MC10T

Dandy said:
What amp is it? The 7027 is equivalent to a 6L6, not an EL34/6CA7. http://www.triodeel.com/7027.htm

Regarding damage, you'll have to do some voltage measurements to see where the power disappears. The speakers are okay, aren't they?
Click to expand...

Yes speakers are OK thankfully.
I've taken the old fella on his word instead of doing my homework first.:crazy:
I'll contact him tomorrow as he maybe able to help with fixing it hopefully.
 
thorpej said:
.. on an EL34, pin 1 is connected to the suppressor, and is usually tied to pin 8 (the cathode) at the socket, which is then connected to ground through a resistor of some sort (larger value if cathode bias, or a small value for current sensing if fixed bias). But on a 7027, pin 1 and pin 4 are both tied to the screen... so you effectively shorted your screen supply to ground via whatever resistance is in the EL34 cathode circuit.
Click to expand...

Yup, that's what I see too.

The tubes would be loosely interchangeable if pin 1 of the socket wasn't connected to pin 8 of the socket (and pin 6 of the socket wasn't used for a non-related tie point).
 
Hi-Q said:
Tell him you want 6 PX25's as compensation :D
Click to expand...

I think you should not blame the ol man for your error. the 7027 is an "odd ball" in that it has all those internal connections to make it incompatible with the el34 pentode tubes. One should always check tube compatibility before making substitutions. Caveat emptor.
 
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whoaru99 said:
Yup, that's what I see too.

The tubes would be loosely interchangeable if pin 1 of the socket wasn't connected to pin 8 of the socket (and pin 6 of the socket wasn't used for a non-related tie point).
Click to expand...
Right, all you need to do is put in a switch that disconnects pin 1 from pin 8. I made an amp with a switch which allowed the amp to use el34 by making the connection via the switch. Then when you switched out you could use the 7027, 6550, 6l6gc, etc.
 
This is why I am very leery of claims that all manner of different tubes work perfectly well in an amplifier not specifically designed for them. As a guy once said, "Trust, but verify".
 
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