Ampeg VT-40 w/6CA7s

[Chris333]

About a year ago, I got an Ampeg VT40 (early Magnavox-era, no distortion or master-volume knobs). The guy who sold it to me said his tech had replaced some caps and converted the amp to 6CA7s (Sovteks) from the original 7027As. Yesterday I noticed that the plates were starting to glow. Hadn't noticed any redplating before yesteday. I shut the amp off and unplugged the power cord, and took the back panel off to have a look...

I have (hopefully) posted a photo of the schematic to show the bias circuit. R49,normally 10k, had been replaced with a 17K5. I was expecting to find pin 1 of the power tubes connected to pin 8 so I was surprised to find no connection there. I'm not sure how authentic of a 6CA7 the Sovteks are (whether they have beam-forming plates, or a suppressor grid), but it seems like either way, pin 1 should connect to pin 8 or ground.

So... I have two questions:

1. What are the consequences of no connection to the beam-forming plates/suppressor grids? Most of the amps I've built or messed around with use 6L6s, so I've never given any thought to not having a connection there.

and,

2. Where would be the best place in the bias circuit for a pot, to make adjustments more convenient? (There are two preamp-out jacks on the back panel, and I was thinking I could remove one and put a bias pot in the hole.)

Thanks in advance for any help in this...

[Jazz P Bass]

The 7027, being a true 'beam' power tube, has the supressor tied to the cathode, internally.
Not so on a pentode.(6CA7/ EL34)
You could double check with a meter.
Pin 1 & pin 8.
If the supressor grid on your amp is not physically tied to the cathode, that is incorrect.
The consequences of that are the tube is now a tetrode.

[Gtr_tech]

Real Sylvania 6CA7s are beam tubes as well. I think of them as a 6L6 variant that has EL34 bias requirements.....