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previous: rmike Caps: "To Form, or Not to Form?..." -- 4/23/2000 9:42 PM

Cap Forming, Standby Switching, Tube Basing

4/24/2000 3:15 PM
Ray Ivers	Cap Forming, Standby Switching, Tube Basing Mike, a) I always form new caps with a Variac. It's no big deal for me, since every time I work on an amp's power supply I power up the the Variac, I just do it slower when the work involves a cap change. I power up slow, over the course of about a minute. That's the technique I use - I know others power up MUCH more slowly than that, but I haven't had any problems so far. Better safe than sorry, in my opinion, especially with HV circuits. b) That sounds OK, although I much prefer the Marshall method of using a DPST switch to switch both sides of the HV secondary in and out - I believe the microscopic amount of time it takes for the rectifier to conduct saves the switch from arcing damage, at least to some extent. The Ampeg SVT method - two power transformers, one for fliaments and one for HV/bias/screens, standby switch on the HV transformer's primary - would be the best method except for the bias voltage. I've always wanted to do an amp this way, with a small Variac on the HV transformer, but... c) It only matters if you use another tube type... but I would still ground pin 1, unless you're planning to use 7027's. The 7AC basing scheme used by the 6V6, 6L6, 5881, etc. doesn't use pin 1. The 8ET basing used by the EL34 does use pin 1 (Traynors have the raw negative bias voltage here, to increase DC tube stability), so if you didn't ground pin 1, EL34's won't work (or maybe they will, but as floating-suppressor-grid tetrodes - anybody ever try this?) If you then plug in a 7027 (8HY base), you will fry your screen resistors. I know this is WAY more info than you needed to answer your question, but I'm doing it to make a point: you did the right thing in asking before you went ahead and grounded that pin. I've read many times that people have tried putting EL34's in Ampegs and they didn't like the sound - this may well be because in a 7027 amp the EL34 will have two screen grids, one really close to the plate and not designed to handle ANY current. I totally agree with you - this is a wonderful forum, I only wish I had found it sooner! Ray Ivers R.A.G.E. Electronics

4/24/2000 3:15 PM

Ray Ivers

Cap Forming, Standby Switching, Tube Basing

Mike,

a) I always form new caps with a Variac. It's no big deal for me, since every time I work on an amp's power supply I power up the the Variac, I just do it slower when the work involves a cap change. I power up slow, over the course of about a minute. That's the technique I use - I know others power up MUCH more slowly than that, but I haven't had any problems so far. Better safe than sorry, in my opinion, especially with HV circuits.

b) That sounds OK, although I much prefer the Marshall method of using a DPST switch to switch both sides of the HV secondary in and out - I believe the microscopic amount of time it takes for the rectifier to conduct saves the switch from arcing damage, at least to some extent. The Ampeg SVT method - two power transformers, one for fliaments and one for HV/bias/screens, standby switch on the HV transformer's primary - would be the best method except for the bias voltage. I've always wanted to do an amp this way, with a small Variac on the HV transformer, but...

c) It only matters if you use another tube type... but I would still ground pin 1, unless you're planning to use 7027's. The 7AC basing scheme used by the 6V6, 6L6, 5881, etc. doesn't use pin 1. The 8ET basing used by the EL34 does use pin 1 (Traynors have the raw negative bias voltage here, to increase DC tube stability), so if you didn't ground pin 1, EL34's won't work (or maybe they will, but as floating-suppressor-grid tetrodes - anybody ever try this?) If you then plug in a 7027 (8HY base), you will fry your screen resistors. I know this is WAY more info than you needed to answer your question, but I'm doing it to make a point: you did the right thing in asking before you went ahead and grounded that pin. I've read many times that people have tried putting EL34's in Ampegs and they didn't like the sound - this may well be because in a 7027 amp the EL34 will have two screen grids, one really close to the plate and not designed to handle ANY current.

I totally agree with you - this is a wonderful forum, I only wish I had found it sooner!

Ray Ivers
R.A.G.E. Electronics