EL34B 6N9P 5Z3P amplifier kit - any thoughts on schematic?

[sharpi31]

I bought a cheap amplifier kit from Hong Kong and have been sent the schematic in advance of the hardware.

I'd be very grateful if the more knowledgable inmates could offer any feedback on the design or possible improvements that could be made.

I plan on upgrading the coupling caps (larger teflon or PIO) and possibly changing to LED cathode bias (this worked a treat in my Hawthorne Spud).

[mashaffer]

Not sure I understand their approach. Fancy VAS/driver stage feeding a pentode mode EL34 with no feedback?

[DF96]

The SRPP first stage is probably just a fashion statement. The pentode EL34 output will make it sound like a rather loud but well-mannered 1950's wireless set. Perhaps they originally intended to run it in triode mode, but then found it had too little output?

Sorry, but you did ask for our opinion!

[sharpi31]

That's very helpful - please criticise all you like!

I'm keen to rejig the circuit as required to give best sound quality with these tubes and transformers. Power output is not important (I already have a 2-3W spud amp, and high efficiency speakers to match).

Any advice would be much appreciated :-)

[Eli Duttman]

As the other posters have indicated, it's a POS.

The easiest way to make something out of that dog pile is to disconnect the EL34 screen grids from B+ and tie them, via 100 Ω resistors, to their respective anodes. The result is triode mode, which will have tolerable (hopefully acceptable) characteristics in the distortion and damping factor departments. Power O/P will be 4 or so WPC.

[chrisb]

Originally Posted by Eli Duttman

As the other posters have indicated, it's a POS.

The easiest way to make something out of that dog pile is to disconnect the EL34 screen grids from B+ and tie them, via 100 Ω resistors, to their respective anodes. The result is triode mode, which will have tolerable (hopefully acceptable) characteristics in the distortion and damping factor departments. Power O/P will be 4 or so WPC.

Reasonably well executed, a 4 watt triode connected EL34 P/P is not to be sniffed at, but as originally configured this "kit" might well be considered as organ donor for a complete redesign.

Eli - would a CCS loaded version of the 12AT7 LTP driver stage from your and Jim's El Cheapo design drive a pair of EL34? What I know for sure (courtesy of Eddie Vaughn) works "well enough" is a WE417 / Raytheon 5842

[DF96]

What about pseudo-UL? Put two resistors across the OPT primary, with their junction taken to g2. Could this match the existing transformer better than full triode mode?

[Eli Duttman]

Eli - would a CCS loaded version of the 12AT7 LTP driver stage from your and Jim's El Cheapo design drive a pair of EL34?

There's enough gain to drive no NFB triode wired EL34s. For an EL34 circuit with NFB, gain is insufficient.

What about pseudo-UL?

That requires a buffering voltage follower of some kind. You be sure that the Chinese guano is on a PCB and not P2P wired.

[sharpi31]

Thanks guys. The 'amp kit' is basically a set of tubes, drilled chassis and transformers with schematic and token cheap components. As it's point to point, I'm completely happy to go with a completely different scheme (whatever is recommended by you chaps).

Looks like it might be quite easy to build a version of the SRPP Eli & Ezequiel recommend in this thread: EL34 SE amp schematic

What would I need to do to accomodate the 6N9P/6SL7 instead of the 12AU7?

Here are some photos of the kit:

[rmyauck]

I was looking for that one as I had printed it out! You may have to stick with 9 pin 6FQ7 or Octal 6SN7 as recommended if you don't want to change a lot.