Harmony H205

[madkatb]

Four or five years ago I bought a 1953 Harmony H205 amplifier at a flea market. It always seemed to be lacking in power but sounded not too bad. I never had any luck finding much information or a schematic. I recapped it, changed the power cord and changed the input tube from a loctal to an octal pentode (base was cracked) but never did much more to it. It did the circuit of friends and family. It recently returned home from my brother-in-law.
My 21 year old son had been talking about the "sound" he was looking for and it seemed he could use something like the Harmony. Back to the Internet to find more info. Turns out the circuit and amp are identical to a Silvertone 1342:
1951 Silvertone 1342 Amplifier [Build] | World, Magnified
The Harmony is tweed with blue vinyl accents and a straight cab rather than wedge shape. I changed the preamplifier tube to a 6sl7 and re-wired the volume controls to after V1a and after V1b. Now it only has one input. Power issue was a bad output tube. Tests into a dummy load at about 10 W at onset of clipping. Sounds great and I think it's quickly becoming my son's number one amp.
My question is: What kind of phase inverter is that and how exactly does it work? 36 ohm resistor in series with a 10 uF cap on the cathode of V2a?
(I hope I haven't offended anyone by modding this amp. I can change it back to stock at any time but would prefer to see it in a useable form for my son.)

[Jazz P Bass]

Silvertone 1342 Schematic

Here is the schematic.
It is the best that I could do.
I cannot seem to find this one anywhere else

[Bruce / Mission Amps]

ANother thing I have found is that after retubing and recapping, sometimes just removing the cathode bypass cap on the power tube of these old 50's SE amps seems to make them sound better at louder levels too.
Then again, if it doesn't have one, a decent 4.7uF to 22uF 50v-100v cap can go a long way too.

[madkatb]

No takers on the phase inverter circuit? I've looked all over and can't find another one like it.

[pdf64]

It's a paraphase type, see The Valve Wizard -Paraphase
Also used in the early Fender tweed amps, the 5B6 Bassman is quite similar.
http://www.webphix.com/schematic%20h..._5b6_schem.pdf
I think that the 10uF & 36 ohm resistor is part of the global NFB around the power amp, the 1k from the speaker being the other component of this.
The cathode dc ground return of V2a is via the 1k feedback resistor and OT secondary.
Again, this is similar (but slightly different) to the 5B6.
Pete.

[madkatb]

Ah, I see. I was looking at the schematic as the 36 ohm and the cap as being the bias and ignoring the 1k. Glad you pointed that out as I was contemplating removing or switching in/out the feedback resistor as a boost. Might have had interesting results on the bias of V2a. I guess I'd have to switch out the 1k NFB, the 36 ohm and the cap and switch in a 1k to ground (or just say "Good enough" and leave it the way it was designed).

[pdf64]

Yes, to make the global NFB switchable would need a 2 pole switch, pole A to open the 10uF ground return, pole B to switch the 1k feedback resistor either directly to ground or via the OT secondary.
I would give it a go, it was designed in an age when distortion was perceived to be a bad thing; open loop amps can seem to have a smoother transition into distortion.
Pete.

[Jazz P Bass]

Here is a repost of the schematic to resolve the 'dead link'.

1342.zip

[eetechTom]

Thank you, the adventure begins Pics to follow

[eetechTom]

Pictures

Just a few pics. I bought a few resistors, and a CE can cap that need to be replaced. Haven’t had a chance to install and bring it up on the variac yet. All tubes tested good on a Hickok 539C.