Could someone have a look at my schematic?

[muzza]

Hi

I originally posted about my old tube amp in the guitar amp section, not aware that there was a Harp Amp section (silly me)

Anyway, I think I'm almost ready to start re-building this old baby.

It is a Chandler gramophone amp, circa 1940s, made in Brisbane, Australia. I have stripped it out, thrown out the capacitors and resistors and kept all the iron and tubes. (Before you ask, I meggered the iron - I'm an electrician)

I made a few mistakes when I was tracing out the circuit and luckily, someone pointed me to an Epiphone Electar Zephyr amp, which is almost identical.

My final amalgamated drawing can be downloaded here (its a .pdf) I'd like a harp amp 'expert' to look over the schematic to see A) if it will work and B) tell me any way to 'tweak it' for blues harp. (There are some questions typed directly on the .pdf)

A few of the component values differ on both amps, so I'd also like some advice on which values to go with.

As if I haven't already asked too much of you, I'd like you to look over a second schematic and let me know which preamp circuit would be better for a harp amp. Like I said, the Zephyr and Chandler are almost identical, but the first stage is quite different on both. That schematic can be downloaded here. This drawing also shows an alternative tone / volume configuration which I found on another Zephyr schematic. The original shows the Tone first, then Volume - the alternative reverses this. I wonder how this would affect the amp?

Last questions (for now) (I promise)
I already have a 5 watt single ended amp loosely based (I believe) on a fender 5E3, I believe. It has a gain AND a volume. I find this handy for getting a dirty sound at low volume. Could I incorporate a master volume into my circuit? And how would I do it?

[MWJB]

I'd stick to one preamp stage prior to the PI tube, place the tone stack between the preamp stage & the PI, use the original Zephyr tone pot config, not the alternative (this will ramp up much quicker, not good for harp).

R1 - will depend on B+ voltage, if you have 330-380vdc at the plates use 250ohms/5W. Higher plate voltage go to 300/330ohms. Lower plate voltage try >200ohms.

R2 & R3 could be your master volume, use a dual 500K pot, wire as rheostats, mount them physically near to where they will be found in the circuit (e.g. not by the vol control/input jacks).

R10, try either. Ditto C5.

5E3 is a push pull amp, perhaps you meant 5E2/5F2A?

PI & preamp filter caps 8uf will work fine. 16uf either side of the choke will be fine.

[muzza]

Thanks for the advice.
Why only one preamp? The Chandler did have a gramophone input just after the mic volume and the tone pot WAS just before the phase inverter but I'm pretty sure I messed up tracing the circuit there. I'm torn between keeping this as close to original, or scrapping it and just make a known Fender type clone, just using the iron and 6V6's.

A colleague at work has suggested dropping the rectfier circuit and use SS. He also suggested dropping the 6SJ7's and use more modern 12AX7's. What do you think? (My colleague is a guitar man - not harp.)

[MWJB]

Only one preamp because you'll end up with more gain than you can easily use.

Which "known Fender" would you clone? There are wildly differing opinions on the suitability for harp of some of the smaller tweed Fenders.

I would see what kind of B+ voltage you get with the tube rectifier, & see how you like the sound. I usually prefer a SS rectifier for harp, in most amps except a Champ style amp.

When I set out to build anything I always go for 12A#7 type tubes because you get flexibility...5751, 12AT, 12AY, 12AU, 12DW can all be used, so I would say that there is some validity to that suggestion...but here's the thing, you don't seem to have listened to the amp yet & you're already looking to mod...you have an unknown quantity, that you want to rebuild as another unknown quantity. Who knows what mojo your 6SJ7, tube rectified amp might have, the only way to know is to build it.

[muzza]

Yes. I think I should just build it. Part of me agrees with going to a SS rectifier for simplicity. From what I've read, a lot of people on the 'net say there is an imperceptible difference in tone / sound between a tube and SS rectifier.

So to get the ball rolling, is my current schematic workable?

[MWJB]

"From what I've read, a lot of people on the 'net say there is an imperceptible difference in tone / sound between a tube and SS rectifier." That's poppycock, based on a misquote. SS rectifiers (if we're talking just diodes) have no internal resistance & greatly reduced envelope of sag/decay. SS rectifiers have no voltage drop, therefore your amp will sound different because of the change in B+ (there is usually no way to compare tube rectifiers & SS at the same B+ voltage in the typical guitar amp). Sometimes SS rectifiers sound better, other times worse...but always different to tube rectifiers.

Some SS rectifiers have built in resistance, to emulate a tube, like the Weber Copper Cap, they sound different again.

As to your schem, do you want to post a final version, without the extra bits that you won't be using? I'd perhaps be looking to slightly reduce cathode resistor values at R5 & R9 (say 2.2K to 2.7K), R8 might be a goer at 390K, see what voltages & tone you end up with, but it might not hurt to experiment with slightly lower values thare too.

[muzza]

Thanks for that. I've re-drawn the schematic and included your suggestions. Download the file here.

The original Chandler gramophone had 2 inputs - one for the mic and one for the gramophone. The gramophone only uses one preamp.

I also moved the tone control back to where it was originally.

[MWJB]

R13 could come down to 2.7K too. I'd keep the vol & tone controls together at the same stage after the input stage.

[muzza]

Thanks MWJB. I'll keep you informed. I'm sure I'll have more Q's once I get started.

Thanks for all your help so far.