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#1 2012-04-01 18:53:59

New member
Registered: 2011-02-19
Posts: 1

VT-40 w/6CA7s

About a year ago, I got a 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 it to 6CA7s (Sovteks) from the original 7027As. I've been playing the amp almost daily since I got it, and it seems to work fine, and sounds good. Yesterday I noticed that the plates were starting to glow. Hadn't noticed any redplating before that. I shut it off 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.


2. Where would be the best place in this bias circuit for a pot, to make bias adjustments more convenient? (I was thinking I could remove one of the EXT. AMP jacks and put a bias pot in the hole.)

Thanks for any help on this...



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