....and Now for Something Completely Different!

[David Root]

At least for me, that is. Needed a break from Dumbles so I did a variant on Geezer's HoSo56 amp, which he developed over at the Hoffman forum, although he posts here too. My thanx to Tubenit at the Hoffman forum for providing the modified HoSo56 schematic.

The HoSo56 is a 2x6V6 with a 5879 pentode first stage. Mine is a single ended version using a single KT66, and with the 12AX7 post-TS LTP changed to another 5879 as a driver for the KT66. Output is "Ghetto UL" which is a PP OT used as a SE OT, but taking the screen B+ off the OT CT, so about 50% UL. No GNFB, other than the UL.

No idea what it will sound like. Not quite finished yet, but I just have to hook up the footswitch and associated wiring to the relay.

The schem is now up to date, the PS dropping stages are right and it does show the little 20H choke I put right behind the screen gridstopper. You can see that choke in the gutshot.

[renshen1957]

Hi David,

Keep up the good work!

Best regards,

Steve

[David Root]

Thanx Steve! Doing a build from a blank chassis is pretty wearing. At least I only do that in aluminum. No way I'd do it in steel, and I have a 16" drill press too.

[Blackburn]

Very interesting amp! Nice work!

[David Root]

Thanx!

The 5879 does not seem to have a shield like the EF86 does, but the RCA datasheet says to ground pins 2 & 6 to reduce heater hum. I have seen some advising not to bother with these pins on the 5879.

Any comments, anyone?

One thing I forgot to mention, the second 5879 (driver) is set up for maximum voltage out, which is what you want in a driver, whereas the first stage 5879 is of course set up for maximum gain. The datasheet gives a chart for each mode which is real helpful when you want to use the 5879 as a driver, which is not what you see it used as nor was it designed for. Yet that max. voltage chart is there....the Ancients knew why!

[renshen1957]

Thanx Steve! Doing a build from a blank chassis is pretty wearing. At least I only do that in aluminum. No way I'd do it in steel, and I have a 16" drill press too.

Hi David,

I use Greenlee Punches for the "tough stuff" on steel chassis, and Titanium Nitride coated drill bits for the easy holes. The Punches cost real money, but have been worth their weight in gold.

Best regards,

Steve

[David Root]

Way back when I started doing this I bought a 7/8" Greenlee to do a 7-pin Harvard in a steel tweed chassis. They work real nice!

Then I discovered aluminum!

[xtian]

I love the variety of components from the parts bin. I call these "beaks and feet" builds.

[David Root]

Very discerning of you, xtian!

Yes, most of this was done ex-stock. A veritable smorgasbord of parts!
I had to buy the 300R 10W Dale cathode resistor, it was bigger than I expected, it's good that I cut that board where I did (the main and PS boards were one piece from a recycled older build that didn't make the grade) so I had the space to fit it in.
Had to buy the 10uF Sprague cathode cap too, all else except the main PS choke, the C-7X 10H, which you can't see, was on hand, including the chassis. Even had a single button footswitch in stock too!

So only about $18 in new parts!

[M Fowler]

Looks like a fun built David.

When I built the TOS with a 5879 tube I didn't ground 2 & 6 and have no noise issues.

Mark

[David Root]

Thanx Mark, then I'll not bother with pins 2 & 6.

Here's a layout of the main board if anyone wants to do it. It's designed for turretboards. I cut up a long turretboard I reclaimed from a build that didn't make the grade and used it for the Main & PS boards.

Normally I use eyelet boards but it wouldn't be too difficult to redraw it for eyelet boards.

[es345]

David,

to prevent pop noise during switching bridge Relay 1A with e.g. 47K (high value compared to 1K5 cathode resistor of V2a). Then the cap 1u is loaded all the time.

Hans- Georg

[David Root]

Thanx for the tip! I wonder why it was not on the original HoSo56.

[amplifiednation]

Looks great David what does one of these sound like? Is there any hum from the SE power section?

You said you used a pp OT, so did you only use half of it? Did is UL does it stay really clean?

[Cliff Schecht]

The problem with using a push pull transformer like this is it isn't gapped at all, which means it is not meant to have any quiescent DC on the windings. There ideally should be no energy storage if you don't have a gap, or conversely stated the gap is used to store DC energy and without it the transformer will be saturated from the get go. You'll lose low and high end response at the least and if you push things too far, you could cause the OT to overheat. I don't think the latter will happen necessarily, but it's good to at least know the limitations that are inherently imposed when you do something like this.