Prewar 42

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Some time ago I restored an Aetna Radio and acquired an identical chassis for building its sister prewar guitar amp.
https://antiqueradios.com/forums/viewtopic.php?f=6&t=390773

The design I have landed on is cobbled from several prewar designs (www.prewaramps.org)
Using a Pentode Preamp, a Cathodyne PI (Only had one 75 triode and limited heater current) and Push Pull 42's
GOAL is to use as many Aetna radio parts in Prew42 and keep the design as prewar as possible (modern safety)
https://stillampd.com/prewar-42-guitar-amplifier

Any advice would be appreciated before I do any "cutting"

1 Any problem in using the #75 as a straight triode without using its two diodes?
Any thought for the two diode application in this circuit?

2 RobRob suggests adding Grid Stop of 470K to mellow the Blackface Princeton Cathodyne (Agree?).
BTW - any thoughts about using a prewar cathodyne here in place of the 60's Princeton? having trouble finding stuff that does not use transformer coupled PI or that is cathodyne that makes sense. Have been through prewaramp.com and google search.

3 What do you think about the placement of the Volume control? I've seen designs where it precedes preamp, Not sure if preceding the preamp would be better or worse

4 Many prewar PS use a choke. Do you think there would be advantage to adding this or not?

5 Is it ok to keep the #42's screen at same voltage as plate? Or drop it some?

[PRR]

Of course you need a coupling cap into the cathodyne grid.

And grid-leak resistors on the 42s!!

One pentode gain stage is not much for guitar. Similar to the first Champ, the one that nobody copies.

What was wrong with the original plan? Convert the IF to an audio preamp.

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Of course you need a coupling cap into the cathodyne grid.

And grid-leak resistors on the 42s!!

One pentode gain stage is not much for guitar. Similar to the first Champ, the one that nobody copies.

What was wrong with the original plan? Convert the IF to an audio preamp.

Thanks PRR

There is a .02uf coupling cap off the 6D6 plate into the 75 grid. Will that suffice?

The 42 push pull was copied from Volutone 5 which did not have grid leak but was fed from a transformer PI. Is that why they did not need to use grid leak? http://prewaramps.org/media/VT5%20Schematic.JPG

The 6D6 pentode gain should be about 91 with the 250k plate resistor (from another post). thinking the 6D6 is like a  6SJ7 (which is the Gibson BR-6F front end). And this is more of a deluxe design with the 42 in PP. Thinking the 42 is quite similar to 6V6.

Didn't quite get the convert IF to Audio comment

[sluckey]

There is a .02uf coupling cap off the 6D6 plate into the 75 grid. Will that suffice?

No. You still need a cap between the volume pot wiper and the junction of the 1M and 470K to block the high grid voltage from appearing on the volume pot. This cap also prevents the volume pot from messing with the bias voltage on the 75 tube.

The 42 push pull was copied from Volutone 5 which did not have grid leak but was fed from a transformer PI. Is that why they did not need to use grid leak?

Yes. The center tap of the transformer secondary gives the grids a ground reference.

Didn't quite get the convert IF to Audio comment

IF equals Intermediate Frequency. Make sense now?

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Thanks. The wiper coupling cap and the 42 grid leak will be added.

But I'm not sure I see how the HV gets on the pot with the .02 in place.
for example the 5C3 does not have a coupling cap off the wiper into the PI
EDIT - looks like there could be 50v or so from the grid at the wiper

In 5C3 there was no grid stop, but in the 5E3 grid stop was added,
Does grid stop make sense here?

In the radio, the 6D6 is the IF amp,
but in the guitar amp the 6D6 has been converted to a guitar preamp modelled off the BR-6F
I do not see what still is needed in preamp

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revised schematic per comments
EDIT - added tone stack from 5E3

[sluckey]

But I'm not sure I see how the HV gets on the pot with the .02 in place.

The voltage is not coming from the previous tube's plate. It is coming from the cathodyne cathode through that 1M resistor.

Your revised schematic looks doable.

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Finally getting back at it....

Used the inspiration from some of the recent speaker cabinet builds to make a first time head cabinet for Prew42 (electronics not done yet).
Cab is beech wood and miter corners will eventually get walnut splines for "decoration"
More here https://stillampd.com/prew42

What do you think about doing an oil finish?

What do you think about 90 year chassis patina? Keep? or shine up?

[TIMBO]

Chassis patina is where to MOJO is

[TIMBO]

Hey mate
Here is one I did...
1948 2A3 AMPLIFIER

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Exterior Mojo gone....  Will have to start again from scratch.
Good news is that inside mojo remains and of course the tube lineup
80 6C6 6D7 75 42 42

BUT this was a good experiment and it was quite successful.
thinking the clean head cabinet (my first) and dirty chassis might clash
I tried soap and water with a scrubby then to a sander, then to sanding pads.
Pics below are in process sanded to 400 and one coat Boiled Linseed
Gonna be nice when done. Might not even reinstall the old dead can caps and coils.
Might try some modern patina with a vinegar spray
Also thinking with all the holes might be interesting to have the pilot inside the chassis
Light from everywhere...

[shooter]

might be interesting to have the pilot inside the chassisLight from everywhere...

If it's AC, beware of it's location to sensitive wires!  I did one with LEDS inside, was anti-climatic 
the light wasn't very diffuse so you didn't get the visual effects the brain said you would.

[Platefire]

If your tubes are going to be visible, you can let your tube's glow be your pilot light. That's what I did on my Dukane with a perforated cover----works for me!

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This is pretty much how its going to look, front, back.
I like the tube glow as pilot, have added an example pilot in case

Now to fill the inside with mojo.

These tubes will be the oldest I have used...
Thus the name Prew42; Pre World War 2 with 42 for output tubes

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the old radio cap and coil cans while cool history
makes the chassis look busy
and causes tube peek-a-boo