Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Stuff4bikes on April 18, 2022, 08:34:11 pm

Title: Will this work?
Post by: Stuff4bikes on April 18, 2022, 08:34:11 pm

PP 6v6gt amp
5u4gb rectifier
6sc7 PI using half for the pre amp and half PI.
Channel 1 6sf5gt
Channel 2 6sj7.

Each channel will have seperate inputs, vol and tone controls.
I have 4 sj's  to find a quiet one, if not I will try sg's I have a half dozen.
I can use sn7's if the PI doesn't work out.....

Any schematics with this similar tube line up..?

Title: Re: Will this work?
Post by: sluckey on April 18, 2022, 09:05:39 pm

Quote

6sc7 PI using half for the pre amp and half PI.

This tube has cathodes internally connected so won't work.

Title: Re: Will this work?
Post by: Stuff4bikes on April 18, 2022, 09:12:36 pm

 can I parralel the two sides of the 6sc7 and run it at V1 and use the 6sf5 as the PI?
Man I have a bunch of questions but not sure they make sense.

Title: Re: Will this work?
Post by: Molwam on April 19, 2022, 03:26:52 am

You could use the 6sc7 as a LTP phase inverter. Even its data sheet states that is was designed to be a phase inverter (https://frank.pocnet.net/sheets/201/6/6SC7.pdf).

Title: Re: Will this work?
Post by: Stuff4bikes on April 19, 2022, 08:18:23 am

I have decided not to use the 6sc7 for this build.....pretty sure I am going to need another gain stage on one or both
Channels before the PI.

Title: Re: Will this work?
Post by: bmccowan on April 19, 2022, 12:13:13 pm

Quote

Any schematics with this similar tube line up..?
PP 6v6gt amp
5u4gb rectifier
6sc7 PI using half for the pre amp and half PI.
Channel 1 6sf5gt
Channel 2 6sj7.

Sure - there are many amps that use those tubes. Although an exception is the 6sf5, but I think that is basically equivalent to 1/2 of a 12AX7. Many, many all octal amps from the 40s and 50s used the 6sj7 as the V1 and some that used multiples. It often is followed by a 6N7, 6SN7, 6SL7, 6SC7, or additional 6SJ7s. I have a cool Danelectro that has 3 6SJ7s and 2x6v6. If you look through Valco, Gibson, Rickenbacher, Dan Electro, Flot-a-tone, etc. schematics from that era you will find many examples. What flavor are you looking to build?

Title: Re: Will this work?
Post by: Stuff4bikes on April 20, 2022, 12:03:40 am

Well the experiment has progressed.....

6sg7......the 6sj7's just screamed with any kind of volume.....
Into a 6sf5gt feeding a parallel set of 6v6gt's....
I only have about 265v on the plates of the 6v6's but just a small sound check seemed promising.....

I don't play guitar very well but my friend who is strictly a Marshall guy enjoys my creations.....many times
I will just make something and then he plays through it and then I disassemble it and start something else.
I just enjoy tinkering with electronics, it mirrors my job----except no tube equipment at work.

Title: Re: Will this work?
Post by: bmccowan on April 20, 2022, 06:51:57 am

Quote

the 6sj7's just screamed with any kind of volume

I assume you are using "screamed" as a negative term. You said that you had 4 of them to try. Unless they were all old pulls, I would suggest that you might not have had things set up right for that tube. Anyways keep us posted. BTW, the early Gibsonette and GA-9 amps are 6SJ7 feeding parallel 6V6s - no additional gain tube. Plate voltage in those amps is only slightly higher than what you have.

Title: Re: Will this work?
Post by: Stuff4bikes on April 20, 2022, 08:17:18 am

Sorry, screamed in a bad way. Honestly the 6sg7's seemed to work well...further testing will surely be needed...
First two sj's were noisy. Put in an sg7 and no noise so let's give it a shot..I may revisit the sj's, I think I have some more from a hammond pull.

####    looking at the Gibson ga 9 schematic you mentioned, my voltages are a little higher at V1 than gibsons.....maybe that's why I had problems with the sj7's.