Stromberg Carlson to Princeton Reverb Plate Resistance?

[jonathan_kea]

I am working on converting an old Stromberg Carlson console amplifier to a guitar amp. I've removed the 6AV6 and tone circuitry from the amp and want to replace with a Princeton style tone circuit and preamp by adding a 12AX7 and new tone controls.

I've preserved the 470K plate resistor with the 310V B+ that was originally going to the 6AV6. The Princeton circuit has a 100K plate resistor with 240V B+.

Will I be OK to keep the 470K for the plate resistor for the new 12AX7 or do I need to use 100K or something else?

I've attached partial schematics for both the SC and Princeton.

[bmccowan]

I've converted quite a few vintage PA, console, whatever amps to guitar amps. So I won't try to do any math here, just my experience. If keeping the power amp topography - and I usually do, just tweaking it as needed - I will find a proven preamp that I want to try. I try to get the B+ feeding the preamp down to the right ballpark by adjusting the dropping resisters and installing the proper amount of filtering. That allows the plate resistors to be in the right range, and allows matching the cathode and screen resistors accordingly. Posting the full SC schematic would help, but what I see here is a preamp B+ that is too high followed by a large plate resistor.
Additional thought - Sluckey has documented really well conversions that he has done: organ amps, a jukebox amp, a toaster oven! etc. Good idea to review his site - all the descriptions and graphics are very clear.

[sluckey]

Will I be OK to keep the 470K for the plate resistor for the new 12AX7 or do I need to use 100K or something else?

Get rid of the 470K and use a 100K just like the Princeton does. If you are concerned about the B+ being 310V rather than 240V as used in the PR, then adjust that with a power supply dropping resistor, not a plate load resistor.

If all you are gonna replace is the preamp don't worry about that 310V B+. The PR preamp is identical to the bigger AB763 preamps and will operate just fine on 310V or even much higher B+. The Super Reverb (same preamp) uses 410V!

[jonathan_kea]

Thanks, that's good to know!

Sluckey, you recommended this conversion several months ago and I am finally getting around to it. Posting the full schematic here in case you or bmccowan is interested.

[sluckey]

The attached schematic will make a nice little guitar amp...

[jonathan_kea]

THANK YOU!!!! I'll give it a shot. Might send you a DIYLC layout to review.

[sluckey]

THANK YOU!!!! I'll give it a shot. Might send you a DIYLC layout to review.

Better to just post some hi-rez pics of the amp chassis, inside and outside.

[jonathan_kea]

See attached photos and layout file. The photos are of the chassis with new pots and the 12AX7 installed. I am not sure if I am handling the B+ supply to the plates appropriately in the layout file.

I am also wanting to run a ground bus across the back of the pots, let me know if that is a good idea. I plan on adding in the bright switch once I have my next parts order put together.

[jonathan_kea]

DIYLC Layout

[bmccowan]

I won't jump in front of Sluckey commenting on your layout, but a few other comments. I would not solder a buss to the back of the pots. If you want a buss, I'd just solder it to the vol ground tab and the other end to a chassis ground - solder the preamp grounds to it - like you show it, but not soldered to the body of the pots. I suggest a strain relief of some sort on the power cord - change the wire from the fuse to the switch to black (white denotes neutral and best to stick to norms.) I guess the 3 filter caps installed that way might be ok, but it would make me nervous - know them loose and you have 300v exposed. Replace the dropping resisters - they look somewhat cooked. Hard to tell, but is something wrong at the IEC socket? And I would not trust that connection for the green safety ground - it should be really tight and that would likely crack the IEC socket. I also do not trust old rivets for ground connections - but it'll likely work. I would replace all of the resistors, but testing them is ok too. Looking forward to how this turns out.

[jonathan_kea]

Thank you for the advice on the ground bus, and the ground on the IEC socket. I can move that to a machine screw/nut directly on the chassis.

I have read that in star grounding schemes, it is best to separate the preamp ground(s) from others. There is a terminal strip right beside the new 12AX7 with a ground lug. Is that a good spot to use for a preamp ground?

Thank you for the comment on the filter caps! Definitely not ideal, and I might move them inside once I get the new preamp stage sorted.

[bmccowan]

Yes, its best practice to keep preamp section grounds separate from the power amp sections. And most folks here, me included, locate that preamp ground at or near the input jack. Having said that, I have seen Gibson,  Valco and others do every damn thing imaginable in terms of grounds. Sometime what we assume is poor practice works well.
The #8 terminal rings that Doug sells work great for chassis grounds.

[shooter]

what we assume is poor practice works well.

it's still poor practice, even if you don't get caught...this time 

[jonathan_kea]

In the 2nd photo of the chassis, there is a ground point on the terminal strip close to the input jack and the bass control. The input jack is also right there. That ground point is really crowded, so I'm thinking I'll send the cathode caps/resistors to the ground on the RCA input jack. Make sense?

Should I jumper the input jack to the more crowded ground point at the terminal strip, or just connect to one of the ground tabs at the base of the old tubes?

[bmccowan]

No big worries - no need to overthunk it IMO. My choice would be to ground near the input jack  with a terminal bolted to the chassis. next choice, just ground at the input jack itself, using a toothed washer on the jack. Easy to change that ground location if you end up chasing hum.

[jonathan_kea]

Just wanted to post the last revision to the layout file and see if anyone had any suggestions for improvements, or see any errors. The parts that are gray are existing, and those in color are new. Thanks in advance!

[jonathan_kea]

So I finished wiring up the new preamp and tone section and am getting NO SOUND. I think there is an issue in the heater circuit, as the new 12AX7 and downstream tubes are not lighting up. This amp uses only one side of the heater pair; one side goes to each tube, and the other goes to ground with the center tap. I think this is how heater circuits were done in early days.

PIN 9 of the phase inverter 12AX7 is grounded; do I need to ground PIN 9 of the new 12AX7 as well? Currently it is not connected to anything.

[sluckey]

PIN 9 of the phase inverter 12AX7 is grounded; do I need to ground PIN 9 of the new 12AX7 as well?

YES

[jonathan_kea]

That was the issue.

I've played through it for about 20 minutes and it's a totally different amp - lots louder and a vast improvement on tone. I am so happy with how it turned out. Picture of the amp in a cabinet I made for it over the summer is attached.

Can't thank you guys enough for all the help and guidance.

[bmccowan]

Very nice - I love the way you retained a portion of the Stromberg Carlson cabinet.