EL34 vs 7027

[AHeck]

I just picked up an Ampeg V4.  I was told it powered up, so I plugged it up and checked to see if it would make guitar noises, and it did for about two minutes.  Then it started red plating.  Dug  in aNd found EL34s in the power tube positions instead of the 7027s that should be living there.  Would switching pin 1 from cathode to screen (right?) still make noise and then start to fail, or has it been modded?  I can look deeper, I just wanted y’all to chime in.  Thanks for your time and consideration.

[sluckey]

Pin 8 is the cathode for both tubes.

Pin 1 is a suppressor grid in an EL34. It is most usually connected to pin 8 (cathode) or sometimes tied to the negative bias.

Pin 1 is the screen grid in the 7027. It must be connected to a big positive voltage.

The socket must have been rewired to operate with EL34s. Once rewired you cannot run 7027s. I don't associate Ampeg and EL34s. I would wire the sockets for 7027s and rock the Ampeg sound.

[AHeck]

Agreed, I plan to do just that.  Thanks for the feedback.

[PRR]

If this will be more than a hobby-amp (if you pay roadies to move it):

The "Professional" tubes for this series were 6550. (My VT-40 even had a note.)

I did devise a way to use EL34 7027 6L6 6550 in ignorance and nothing smoked; ah, but that was at reduced B+. (Which is another really good idea if you are not needing to fill large gymnasiums without PA.)

This decade I'd use EL34 (7027 is too $$$), until we proved the amp was going to do real WORK, and then fit with 6550.

[AHeck]

If this will be more than a hobby-amp (if you pay roadies to move it):

The "Professional" tubes for this series were 6550. (My VT-40 even had a note.)

I did devise a way to use EL34 7027 6L6 6550 in ignorance and nothing smoked; ah, but that was at reduced B+. (Which is another really good idea if you are not needing to fill large gymnasiums without PA.)

This decade I'd use EL34 (7027 is too $$$), until we proved the amp was going to do real WORK, and then fit with 6550.

This amp is gonna do some non-union Scale bass “work” on the road and mainly in the studio.  That’s why imma go back to the 7072s.   Now to figout the red plating.  Figure I’ll start with the bias circuit.

[sluckey]

No need to be concerned about EL34s red plating. Just return the amp to it's original configuration and set the bias for the 7027s.

[Willabe]

If your playing bass, 6550's are great fore bass, most bass players would prefer 6550's to 7027A.

Here's a link to some info on 7027A tubes and Ampeg amps, it's from the Triode web site.

http://www.triodeel.com/7027.htm

[j_bruce]

If your playing bass, 6550's are great fore bass, most bass players would prefer 6550's to 7027A.

Here's a link to some info on 7027A tubes and Ampeg amps, it's from the Triode web site.

http://www.triodeel.com/7027.htm

From your link:

". . .  if you want to take the Hard Knock University's Amp Repair 101 course."

 

[66Strat]

Ampeg says it's more than OK to swap 6550s for 7027s. "For improved performance the 7027 output tubes can be replaced with 6550 output tubes." See note 4 on attached schematic.
https://el34world.com/charts/Schematics/files/Ampeg/Ampeg_v2_vt40_v4_vt22.pdf

Given the limited availability 7027s, I would seriously consider this option.

[AHeck]

Thanks everybody,  I'm gonna replace the two 10uf electrolytics in the bias circuit along with the 75K resistor, everything else comes in good there.  I'll also put in new filter caps and increase the power handling of the 470R screen resistors to 5W  These just seem like good housekeeping on a 40 year old amp, right?
 While I will probably end up with 6550's, I'm gonna start with some JJ's 6L6GCs on account of a good price on a quad and still not sure its gonna be completely stable, after I'm satisfied by it's performance and good behavior, I'll reward us both with the heat.  Two of the EL34s were skinny little tubes, and pins 8 and 1 were never tied together on the sockets.  So there's that.  Thanks for all the feedback.

[PRR]

> start with some JJ's 6L6GCs

6L6GC is nominally 500V max. Ampeg V-amps were nominally 595V for 120V wall voltage.

While we know the 6L6GC's guts are same-as a high voltage TV tube, the TV tube has a top-cap and a 6L6GC does not necessarily have the extra-tough base material. (The original 6L6 claimed 400V but had to be walked-back to 360V due to base breakdown.)

Yes, I might try it.

[tubeswell]

807s might also work okay at around 600V

The Russkie 6P7S has an octal base https://eandc.ru/catalog/detail.php?ID=2581

Some cheap ones going on flea bay https://www.ebay.com/itm/6P7S-Russian-Vacuum-Beam-Tetrode-Tube-NOS-in-Box-G-807-6BG6G-6BG6-/254124568735

Plus they've got that whole 'spark-plug' look going on :-)


(Pete Millet did one running 4 x toobs at 500V http://www.pmillett.com/807pp_amp.htm )

[PRR]

I think we are getting off-topic for just fixing an Ampeg V-amp. They did do top-cap tubes for one run of SVT (and 6146 is a beast), but that's not Ampeg's style. Putting Ruskie tubes in a good American amplifier.... well, maybe you had to be there.