Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Bub on October 06, 2010, 12:57:41 pm

Title: Build #2 5F6-A PT and OT options
Post by: Bub on October 06, 2010, 12:57:41 pm

Hi Gents,

It's been a long time since I built my first amp so I'm starting all over again with the theory. I'm stuck in bed for the next two months so I figure it's a great time to order the parts for build #2. I'm looking for suggestions on what I need to change to build what I want.
I want to build Doug's 5F6-A with 6550s. I'm able to get a free Hammond potted OT and PT but don't know if the specs are close enough to what I need. I think I have a choice of two OTs and PTs.
372JXP, 243VA, 600V C.T. @ 288ma, 50V C.T. bias, 5V C.T. @ 4A, 6.3V @ 8A
374BXP, 200VA, 750V C.T. @ 201ma, 50V C.T. bias, 5V C.T. @ 3A, 6.3V @6A

1650PP, 6600 ohms, 60 watts
1650RP, 5000 ohms, 100 watts
I'de love to use the potted tranformers with the 5F6-A board so I'm looking for any ideas.
Thanks
Rob

Title: Re: Build #2 5F6-A PT and OT options
Post by: PRR on October 08, 2010, 11:11:24 pm

The 5F6A is a classic amp which maybe should not be tampered too much just to use some specific too-good iron.

But whatever.

To be sorta-like 5F6A into overload you need to keep the small-tube voltages very similar to the Original. That means about 48V peak grid drive on the output tubes.

> 374BXP, 200VA, 750V C.T. @ 201ma

375V per side in a perfect (or Silicon) rectifier is 530V DC. This is higher than a 5F6A, but you also propose bigger power bottles.

5F6A with 5881/6L6GC is loaded with 4K. You can instead work 6550 at 500-550V plate and screen into 3K-4K.

You don't list any 4K OTs.

6L6 6550 and 7027 are all fairly similar except ratings and pinouts. On the 7027 datasheet (http://www.mif.pg.gda.pl/homepages/frank/sheets/049/7/7027A.pdf) we find a plan for 540V plate 400V screen and 6.5K load. This is close-enough to 1650PP spec.

The grid swing is 38V. When you change the screen voltage from 400V suggested to 530V actual you will have to grid bias and swing to around 48V. 6550 will work very close to these values.

Increase dropping resistor from screen power node to driver power node to get near stock 5F6A supply voltage on the driver.

Result is around 70W-80W. The "60W" 1650PP will easily and safely handle this much over the guitar band.

You do know the 1650_P series has "odd" secondary wiring?

Title: Re: Build #2 5F6-A PT and OT options
Post by: Shrapnel on October 09, 2010, 03:11:47 am

Quote from: PRR on October 08, 2010, 11:11:24 pm

You do know the 1650_P series has "odd" secondary wiring?

Ahhhhh, the proprietary switch required model if you don't just hardwire it for one specific impedance. I personally didn't remember the model numbers, but just remembered seeing them on the market some years back.

Title: Re: Build #2 5F6-A PT and OT options
Post by: Bub on October 12, 2010, 08:17:56 am

Thanks PRR and Bill,

I think I'll go with the 1650k, 50 watt, 3400 C.T.

Bob