Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Ocean7 on February 24, 2015, 07:17:20 am

Title: 6V6 plexi transformers question
Post by: Ocean7 on February 24, 2015, 07:17:20 am

Howdy,

I'm very happy I joined the forums. Looks like a cool place with lots of great folks  :occasion14:

I'm new to guitar amps but I have a background in electricity and I built a couple hybrid tube headphone amps that turned out great!

So I want to build the 6V6 plexi. I have the following transformers on hand.
- 0-190-275V power transformer (no center tap)
- 8k into 8ohm PP output transformer

They come from Ampmaker and I am wandering if they would be suitable for the 6V6 plexi.

The PT will give me more or less 360V rectified so my B+ will be lower than the stock 400V on the schematic. I assume I will have to tweak the fixed bias circuit a bit and bias the 6V6's accordingly? Will this work? Also should I tweak R39, R40 and R41 to get close to the stock voltages of 276V, 238V and 223V as show on the schematic? Or just live with the lower voltages?

From what I read the 6V6's are happy with an 8k OT so mine will be fine?

Thanks a lot for your replies!

Title: Re: 6V6 plexi transformers question
Post by: Willabe on February 24, 2015, 08:02:40 am

Welcome!    :icon_biggrin:

How many mA's is the PT B+ wind rated for?

And does it have a 6.3 heater wind? If so, what's it's rating?


                   Brad    :icon_biggrin:

Title: Re: 6V6 plexi transformers question
Post by: Ocean7 on February 24, 2015, 08:05:03 am

Thanks Brad.

275V winding is rated at 160mA.

Yes it has heater windings :) 6.3V @ 3A with center tap.

Cheers,

Philippe.

Title: Re: 6V6 plexi transformers question
Post by: sluckey on February 24, 2015, 09:04:08 am

Your PT and OT (if it's rated for about 20W) will be fine. You may want to experiment with the B+ dropping resistors but I'm pretty sure those "stock" numbers were just as built measurements and are not some magical numbers.

You will have to use a bias circuit that is suitable for a PT that uses a bridge rectifier for B+. Here's the circuit you need. The pot is 50K. See page 5 of this pdf...

     http://home.comcast.net/~seluckey/amps/misc/Amp_Scrapbook.pdf (http://home.comcast.net/~seluckey/amps/misc/Amp_Scrapbook.pdf)

Title: Re: 6V6 plexi transformers question
Post by: Ocean7 on February 24, 2015, 09:20:56 am

Thanks so much for that PDF S. That is GOLD! (and added to my collection).

So beside that full wave rectifier circuit do you think that I can build the amp as per Doug schematics even if my power transformer is a bit underrated?

Title: Re: 6V6 plexi transformers question
Post by: sluckey on February 24, 2015, 09:34:37 am

Your PT will be fine.

There's another version of this amp that puts the pots on the opposite side of the chassis from the tubes. It's a more traditional "Marshall" style. Take a look and go with the layout you like better. They both work very well.

     http://home.comcast.net/~seluckey/amps/6v6plexi/6v6plexi.htm (http://home.comcast.net/~seluckey/amps/6v6plexi/6v6plexi.htm)

Title: Re: 6V6 plexi transformers question
Post by: Ocean7 on February 24, 2015, 09:41:47 am

Thanks again Sluckey.

In fact that is awesome since that is exactly what I wanted to do! (pots opposite the tubes)

You definitely made my day here  :smiley: