What project for an old PA?

[OldPup]

I picked up an old PA with the intent of making it into an amp for a turntable. I don't know anything about building home audio, but I've put together a couple of amps, and with the generous help of people on TDPRI I've repaired a few old Fenders. I figured I could find some schematics/example diagrams online and make something happen. I'm also considering making it into a head for guitar.

The faceplate is old and kind of quirky looking, which I like and don't want to alter. The leftmost knob on the control plate is a switch with six positions, which would be neat to use to tweak NFB, or who knows what else. The tube complement is three 12ax7s, two 6V6s, and a 5Y3. The iron looks good (???), but transformers are an aspect of amps I've not done a deep dive into. The PT looks odd from inside the chassis. A lot of the capacitors are rated for 500 or 600 V, so I've taken the liberty of assuming it's putting out 350 - 450 V, which seems good for a guitar amp at least.

This is my first project amp. I'm wide open to possibilities for it. I like a challenge and am in no extreme rush to get it done. I would like to do something a little different than what I already have. I have a 5f2a and 5E3, which I built. And I have a Vibro Champ, Princeton Reverb, and Deluxe Reverb. I've inadvertently worked my way into a Fender rut. Is there some version of an 18-watter I could do in the current chassis? Is home audio the way to go?

It seems the website is rotating my pictures. They are oriented appropriately on my laptop and go haywire below. I apologize!

 

[Wally]

That appears to be a mono amp, correct? You will need two of them for a stereo. You might do better to build a stereo hi-if kit.
you have the makings of a variety of guitar circuits there, though.…depending on actual voltages.

 

[Drew617]

This is timely, I just picked up the same, or a very similar, Bogen amp. IIRC the DB-110 schematic rates that PT 350-0-350 @ 80 ma, so a constraint there. Note also the OT's 11K pri impedance. There's multi impedance taps though, so can get to 5.5K reflected easily enough.

The basic plan for mine is to recap and hack any guitar preamp into it, and see if that's accidentally magic. Then when it isn't, rebuild as a low-power bassman or other Fender.

I'm aware of 6V6 18W builds. Seems possible to get in that neighborhood, though may have to design around the transformers.

 

[printer2]

The transformer has terminals rather than wires coming out. Ask me what I think of old iron where the wires are crumbling. Or not. Otherwise you can make a 6V6 or EL84 with the amp. Or even a Plexi or a 2204 styled amp with three preamp tubes.

 

[OldPup]

Thanks, @Wally, now that I've cleaned all the rodent droppings out of the chassis I will plug it in and see what voltages are.

@Drew617, that is timely! I am very interested in hearing what you decide to do and how it turns out. You said the PT is a constraint being 350-0-350 at 80 mA. Do you mind elaborating on that? I see that mojotone's Bassman build has a PT that's 355V @ 300 mA. That seems like a huge difference. Can it be overcome? I don't care if it's not loud, honestly. I got bigger amps and dirt pedals for band practice. Breaking up at low volumes would be great for me. Again, I know very little about transformers. Might be time to read through that Merlin Blencowe book. I have to admit I'm apparently not a natural based on how reading the first few chapters went.

@printer2 I will ask you about old transformers with crumbling wires! What make model amps is one liable to find those in? Also, nice suggestion with the 2204.

 

[printer2]

Thanks, @Wally, now that I've cleaned all the rodent droppings out of the chassis I will plug it in and see what voltages are.

@Drew617, that is timely! I am very interested in hearing what you decide to do and how it turns out. You said the PT is a constraint being 350-0-350 at 80 mA. Do you mind elaborating on that? I see that mojotone's Bassman build has a PT that's 355V @ 300 mA. That seems like a huge difference. Can it be overcome? I don't care if it's not loud, honestly. I got bigger amps and dirt pedals for band practice. Breaking up at low volumes would be great for me. Again, I know very little about transformers. Might be time to read through that Merlin Blencowe book. I have to admit I'm apparently not a natural based on how reading the first few chapters went.

@printer2 I will ask you about old transformers with crumbling wires! What make model amps is one liable to find those in? Also, nice suggestion with the 2204.

Real rough math, 350V x 0.08A = 28W. Knock than in half for losses and you have a 14W amp. This is right in line with the datasheet of a 6V6.

http://www.r-type.org/pdfs/6v6gt.pdf

Push-pull, Class AB 285V, 14W

https://worldradiohistory.com/hd2/IDX-Audio/Archive-High-Fidelity-IDX/IDX/50s/High-Fidelity-1955-Apr-IDX-95.pdf

Distortion:
0.125%, 0.25%, and 0.65% harmonic at 1, 5, and 12 watts respectively; 14 watts peak.

Guitar amps are roughly rated at 5% distortion, not the 0.65% of a hifi amp. Also there is a lot more power eaten up reproducing the lows.

A 5F6a Bassman transformer

https://www.hammfg.com/files/parts/pdf/290D2X.pdf

355V 0-355V x 300mA = 106W, knock that in half and you get 50W, sounds like a Bassman to me.

What makes and models with crumbling wires? Ones that had the chassis run too hot. I am talking about reusing iron from hifi stuff. The insulation can be fine but it is old and you can not bend it all that much. Some manufacturers did some sharp bends coming out of the bell and through the chassis. To bend them so they get through the hole if you are not using it in the chassis can sometimes damage the wires. You could always try heating up the wires with a hair dryer but sometimes it will be a losing proposition.

About firing it up, I hope you are removing the rectifier tube and measuring the AC voltage. While you are at it measure the resistance of the secondary also.

 

[Drew617]

Thanks, @Wally, now that I've cleaned all the rodent droppings out of the chassis I will plug it in and see what voltages are.

@Drew617, that is timely! I am very interested in hearing what you decide to do and how it turns out. You said the PT is a constraint being 350-0-350 at 80 mA. Do you mind elaborating on that? I see that mojotone's Bassman build has a PT that's 355V @ 300 mA. That seems like a huge difference. Can it be overcome? I don't care if it's not loud, honestly. I got bigger amps and dirt pedals for band practice. Breaking up at low volumes would be great for me. Again, I know very little about transformers. Might be time to read through that Merlin Blencowe book. I have to admit I'm apparently not a natural based on how reading the first few chapters went.

@printer2 I will ask you about old transformers with crumbling wires! What make model amps is one liable to find those in? Also, nice suggestion with the 2204.

I say constraint because 80 mA is lesser rating than that average new transformer I might pick up for a 6x6V6 build, so probably less flexibility here than with some. 80 mA is almost exactly the draw I estimate in that configuration, and that spec is from the Sams photofact sheet... I suppose there's some chance they are reporting current requirement rather than max.

Overcome, not really - Mojo's 5F6 is a 40W, 2x6L6 amp. This is I think a 12W-rated 2x6V6 amp, so as-is I take that basic configuration for granted and mean "bassman" here in terms of preamp and topology. Between schematic and evidence - the chassis has the right number of holes - we know it'll support 2 6V6 and 3 12AX7 at reasonable voltages. Should be able to get into the neighborhood.

 

[printer2]

Yes, 6V6 Bassman, not going to magically become a 6L6 amp. They have 375V (I am guessing 10V drop in the OT winding) and 25V across the cathode resistor, so 350V across the tube. Bump that up to 375V (fixed bias) and a few more volts replacing the 5Y3 with a rectifier with less voltage drop, or SS diodes and a dropping resistor if needed then 400V across the 6V6's, current goes down to keep the tubes happy. Or you could leave it cathode biased, whatever you want.

 

[bebopbrain]

The output transformer is larger than the power transformer, usually a good thing.

I vote single channel 6G3 with 4 knobs (volume, tone, speed, and intensity).

 

[Peegoo]

This is my first project amp. I'm wide open to possibilities for it. I like a challenge and am in no extreme rush to get it done. I would like to do something a little different than what I already have.

Read THIS ARTICLE ( <--click on that )...it provides plenty of info to turn that Bogen into a killer little amplificator.

 

[AlfaNovember]

I see it's got a mic input. That's a high-gain, high-impedance input, you can plug a geetar straight into it and get a taste of what it can do. With a little luck, some smaller coupling caps, and a reworked tone stack, it might could work more or less as-is.

Skip Simmons on the Truth About Vintage Amps podcast has a lot to say about converting PA amps to guitar use, give him a listen.

@OldPup mentioned reading the Blencowe book; also have a look at Morgan Jones' "Valve Amplifiers" book. It is more hi-fi oriented, but quite readable and the design principals are all the same.

 

[Chicago Matt]

Dang, I thought this thread was going to be about project ideas for someone like me. I'm 73 and have four grown up adult "kids".