Author Topic: Amp Build From a Vintage Radio (Read 41246 times)

blown240 · « **on:** September 15, 2011, 09:42:50 am »

I picked up a vintage Philco radio from a Garage sale a few days ago. It doesnt work, all the tubes are gone and the the speaker is shot. Originally my wife just wanted to use it as decoration, but I figure why not do something cool with it.

SO, I want to build it into a simple guitar tube amp. I was leaning towards a champ, but I am open to suggestions. There are a few specific things I want from this build.

1. I dont want to modify the original radio cabinet.
2. Since its got 4 knobs originally, I can only have 4 knobs/switches, inputs total. So probably a power switch, input jack, volume, and either gain or tone.
3. It will only take a 6" speaker, so power will have to be fairly low. Maybe 5-10 watts.
4. I would like to use the original radio output transformer, but I realize that I am asking alot with this one.

I hope to not spend too much $$$, and I have a box of misc amp parts from a build that I did about 10 years ago.

Does anyone have a suggestion/schematic for what I should build? I am a real newbie with amp building and have only ever built a preamp, so it needs to be pretty straight forward.

Thanks!

Here is a pic of the radio. I will get pics of the rear later.

labb · « **Reply #1 on:** September 15, 2011, 10:34:14 am »

I would just restore the radio.

blown240 · « **Reply #2 on:** September 15, 2011, 10:40:13 am »

The radio isn't worth much restored. And it takes 7 tubes that are hard to find. It wouldn't be cost effective to restore it. Plus I have a couple other vintage radios that already work.

eleventeen · « **Reply #3 on:** September 15, 2011, 11:09:54 am »

>>IF<< you wish to go the restore route and need/want tubes, PM me, I probably have them, yours for the shipping.

The obvious choice is to build a Champ-type affair and not to get too fancy. A 6" speaker imposes quite the limitation on how good the thing might sound, IMO, unless you buy a high quality one.

blown240 · « **Reply #4 on:** September 15, 2011, 11:21:05 am »

Quote from: eleventeen on September 15, 2011, 11:09:54 am

>>IF<< you wish to go the restore route and need/want tubes, PM me, I probably have them, yours for the shipping.

The obvious choice is to build a Champ-type affair and not to get too fancy. A 6" speaker imposes quite the limitation on how good the thing might sound, IMO, unless you buy a high quality one.

Thanks for the tube offer. I will get a list to see if you have them, though I will probably make it an amp.

I agree on the speaker limitations. I have a 12' Jensen I would like to use, but its just TOO big. I also need to find the specs on the stock output transformer to see if it will at all work. I have a Bassman preamp that I built, I may see if I can mate it to the power stage of the original radio. That would either be awesome, or a total pile of junk.

The Champ is definatly the way to go for a ground up build, I am not aware of one with 2 knobs though. Just the single volume knob. Is there one with a tone or gain knob as well?

sluckey · « **Reply #5 on:** September 15, 2011, 11:31:53 am »

18Watt Lite II may be a candidate. It's a small, simple 2 knob amp. May sound kinda cheap and tinny with a 6" speaker, but that's probably gonna be an issue with any amp.

tubenit · « **Reply #6 on:** September 15, 2011, 12:02:08 pm »

I love that radio cabinet! Beautiful wood. Craftsmanship back then was incredible. It should have something nice done with it. Looks like maybe a 8" speaker would work or a 10"?

With respect, Tubenit

blown240 · « **Reply #7 on:** September 15, 2011, 12:24:57 pm »

I have an 8" car sub and a 10" car sub, and a 12" jensen guitar speaker, they will at least let me experiment with speaker size. Once I figure out if the output transformer is good and how big it is, that will help me decide on what to build. Its pretty heavy, so the transformer may have some grunt behind it, or its just REALLY old.

Willabe · « **Reply #8 on:** September 15, 2011, 01:28:11 pm »

Vibro champ has vol. and tone, just leave out the vibro part.

Brad

sluckey · « **Reply #9 on:** September 15, 2011, 02:12:07 pm »

I believe a Vibro Champ has volume, treble, and bass.

ThermionicEngine · « **Reply #10 on:** September 15, 2011, 03:30:20 pm »

If you can get a 10" in there you might consider a Fender Pro Junior- two controls and a switch. You could use switched pot as well. Simple circuit with push-pull EL 84 power amp. Or go way back to a Champion 600/800- I think they only had volume and tone controls. You can also try one or more Denelectro guitar case amps, no power tranformer. Weird tubes there, though. Looks like a fun project, good luck.

Willabe · « **Reply #11 on:** September 15, 2011, 03:42:23 pm »

Quote from: sluckey on September 15, 2011, 02:12:07 pm

I believe a Vibro Champ has volume, treble, and bass.

Doh!

Well how about a 5E3 tone control? Just 1 knob.

Brad

stingray_65 · « **Reply #12 on:** September 15, 2011, 03:46:36 pm »

Must be a perspective thing, but that looks bigger than a 6" speaker.

what is the tube compliment?

does it have a 6.3 and 5V tap? or will you not salvage the PT?

That sure is some pretty veneer!

It was likely finished in shellac or varnish but nitro is not out of the question either.

Do you plan a refinish? or leave the patina?

Cool project!

labb · « **Reply #13 on:** September 15, 2011, 04:06:49 pm »

The Fender Princeton 5F2 and 5F2-A are two knob. Volume and Tone. Basically a Champ with tone control.

blown240 · « **Reply #14 on:** September 15, 2011, 06:01:35 pm »

Thanks for the replies and suggestions guys!!

I am definatly going to keep the patina. the only thing I will replace is the grille cloth since it ripped. I am going to get some pics of the inside of it and the other parts I have and will post them when I have a chance.

billcreller · « **Reply #15 on:** September 15, 2011, 08:22:40 pm »

If you want the original type grille cloth, grillecloth.com may have that color & pattern.

HotBluePlates · « **Reply #16 on:** September 15, 2011, 08:36:29 pm »

Recall that tweed Champs and Princetons used a 1M volume pot which had a switch on the back, which is used to switch on the line cord power. The switch engages when you turn the volume above zero.

As a result, the part is still available. Doug has them. That will save you a control position.

blown240 · « **Reply #17 on:** September 15, 2011, 08:40:15 pm »

PRR · « **Reply #18 on:** September 15, 2011, 09:39:58 pm »

It really should be repaired.

It would be a sweet guitar amp.

Recall that Tweed Champ and Princeton _ARE_ radio parts and circuit minus tuner. The parts Leo got in late 1940s are cheap generics of what Philco used in 1935.

'75 is a half-12AX7.

'42 is a 6F6 on a 6-pin base. 6F6 is a lower-gain precursor to the 6V6.

'80 is 5Y3 on 4-pin base.

A '75 is only $13. '42 can be had for $23 or less. '80s go for $70!

If you drill the socket rivets you can probably drop octal sockets right in, use 6F6 directly or 6V6 with mods.

The only non-Tweed bit is the SE power stage and interstage transformer, and the final stage working in AB2 Triode mode. Might not be raucous enough for a tavern, might be fine for the parlor.

Challenges: the grid bias is back-bias, needs ALL the radio current including tuner plus the field-coil current. The wiring will probably make you insane.

There's something wired to the top of the volume control. Cut it. Feed an iPod here, it will work fine (after repair). Tone self-adjusts at lower volumes, you also have four switch settings. For guitar you would need one more stage.

The PT, choke, and OT can be used directly in any modest two-6V6 plan.

3-pin cord of course.

PRR · « **Reply #19 on:** September 15, 2011, 09:43:29 pm »

The Phico OT is surely 3-4 ohms. Your 16 ohm monster will be a very poor match.

I like the look, but a parlor radio should be 6" or 8", maybe 10" if the power stage is weak. A 12" is really too big IMO.

blown240 · « **Reply #20 on:** September 15, 2011, 10:14:55 pm »

PRR- Thanks for taking the time to type all that. There is really some good info there.

I can get a 4 ohm 12" if I need to. Or I have the 4 ohm 8" in the crate amp. What would a modest two-6V6 plan be? And should I lean towards that instead of a champ? I wish I knew more about electronics and building amps, but at this point about all I can do is follow a schematic. Its funny, I used to be an electrician. I can wire a mansion by myself, but cant wire an amp without a guide.

Also, I like the idea of using a volume now withe the on/off built in. Now I have room for a 3 knob setup!

blown240 · « **Reply #21 on:** September 15, 2011, 10:18:40 pm »

And what would happen if I used the Philco output transformer in a champ build? Is it a problem to use a transformer that is too big?

blown240 · « **Reply #22 on:** September 16, 2011, 10:06:52 am »

OK, well after spending half the night up looking up info and staring at schematics I think I have figured out where I am going with this.

It will basically be a Champ AA764 with a switched volume. That way I have volume, bass and trebel control. I will use the 4ohm 8" speaker from my little crate amp. And I will use the transformers from the original radio. This is assuming that I can use the output transformer with a single 6V6GT without any problems, and assuming the original transformers work. I think the only thing I will have to buy are a couple tubes, and the switched volume pot.

If anyone has input, I would greatly appreciate it!

Other option is a Champ 5F1 with an added tone stack.

ThermionicEngine · « **Reply #23 on:** September 16, 2011, 12:25:20 pm »

Output Transformer- All you need is the proper impedance ratio and the wattage capacity. It sounds like the OT is big enough to handle an SE 6V6. You get impedance for a given load, your 4 Ohm speaker, by multiplying the square of the turns ratio by theactual load impedance. To determine turns ratio, put a convenient small AC voltage on the primary, like 10 VAC from your Variac. Measure the secondary voltage right around .27 VAC- This would give you a turns ratio of 37:1. Square that for 1369. 1369 x 4 =5476 Ohms. Pretty good for a 6V6 class A nominal load of 5500. If you don't have a variac, see if you have a wall wart transformer that has an AC output, like 9VAC. Use the same formulas substituting 9VAC. Allbest, Larry

blown240 · « **Reply #24 on:** September 16, 2011, 12:48:32 pm »

Quote from: ThermionicEngine on September 16, 2011, 12:25:20 pm

Output Transformer- All you need is the proper impedance ratio and the wattage capacity. It sounds like the OT is big enough to handle an SE 6V6. You get impedance for a given load, your 4 Ohm speaker, by multiplying the square of the turns ratio by theactual load impedance. To determine turns ratio, put a convenient small AC voltage on the primary, like 10 VAC from your Variac. Measure the secondary voltage right around .27 VAC- This would give you a turns ratio of 37:1. Square that for 1369. 1369 x 4 =5476 Ohms. Pretty good for a 6V6 class A nominal load of 5500. If you don't have a variac, see if you have a wall wart transformer that has an AC output, like 9VAC. Use the same formulas substituting 9VAC. Allbest, Larry

Great info. Thanks.

labb · « **Reply #25 on:** September 16, 2011, 01:09:31 pm »

From the "For what it is worth" section..I built a Fender 5B3 earlier this year using the volume pot with the switch built in to switch the power on and off. I got quiet a bit of hum from it. Had to go to a regular switch. Those old Fenders did not use a stand-by switch so that saves you one position.

blown240 · « **Reply #26 on:** September 16, 2011, 01:41:55 pm »

Cool thanks. I can always do a rear toggle switch for power. That will keep the 3 year old from turning it on too!

PRR · « **Reply #27 on:** September 16, 2011, 10:22:58 pm »

Push-pull OT is a terrible SE OT.

I think it should be restored nearly as it was designed. However this would be an incredibly tedious job. Really not sure what you should do.

Short-term, get a little chip-amp and tuck it inside the Philco case.

blown240 · « **Reply #28 on:** September 16, 2011, 10:29:28 pm »

Restoring it really isn't an option. I simply don't have the cash or knowledge to do it. However, it seems that I have most of what I need to make it an amp. I thought the PT, choke, and OT can be used directly in any modest two-6V6 plan. Is there a difference with a champ? And if so, whats a good 2 6v6 plan?

Thanks again for the input. I am hoping to get out to check the transformer tonight. I guess worst case, I can buy a Champ OT, they are only like $30.

blown240 · « **Reply #29 on:** September 17, 2011, 12:26:09 am »

blown240 · « **Reply #30 on:** September 17, 2011, 12:33:43 am »

Since it needs a total rewire, and tubes just to start, restoring it is out of the question. I may just completely gut it and use the chassis and enclosure to build a champ.

I am trying to make a list of parts that can be reused.

1. Chassis
2. Enclosure
3. Power Transformer ??
4. Choke ??
5. Output Transformer ?????
6. Recifier tube socket ??

Is it worth reusing any of the old caps and resistors to get a vintage sound? Or will that just cause more problems?

The speaker is definatly an 8", so I will use the speaker out of my little Crate amp.

I have a Hammond 262F6 transformer in another build. Will that do me any good here?

Any input is appreciated!

PRR · « **Reply #31 on:** September 17, 2011, 12:37:50 am »

> are these labeled correctly?

No. The OT is on the speaker. What you marked is the driver transformer. Innerestingly the DT cost more than the OT. (In part this is SE/PP difference.)

PM me an email address where you can take a 2MB PDF file, I'll give you full schematic and excellent parts layout diagram. Philco documented this stuff well.

I spotted one odd thing. The schematic shows "1.11 ohm" next to the speaker voice coil. This may actually include the OT winding. Can you break a connection from OT (which is on the speaker!) to voice coil, and read the voice coil alone? If it is 3 or 6 ohms, then it is wound for 4 or 8 ohm close-enuf. If the VC is really 1.11 ohm DC, then it is a "1.3 ohm nominal" speaker and that's very awkward.

The main filter-caps are the round cans inside "TV-shape" shields, a very odd thing. The service data confirms they are electrolytic (thus now scrap) and you can get new ones for $1.10-$1.80 (if this were the 1930s all over again!).

Otherwise....

Have parts been removed other than tubes? Has the cloth rotted off the wires real bad? Have you seen seriously burnt/melted parts?

If not, then it really is less work, the way I see it, to replace the rotten and obsolete audio bits than to rip and ravage. (I'm NOT talking about fixing the tuner; it likely just needs tubes, and is the best AM you will ever hear, but the AM band is crap and better SW radios are cheap.)

replace these caps:
C55 0.02u
C63 0.05u
C59 0.25u
C77 10u 75V + 8u 450V (100u 50V + 22u 450V)
C78 8u 450V (22u)
C71 3u (22u 400V)

three '42 sockets change to octal H H K G1 G2 P

verify 9-11 ohms at "10 ohm" tap on R66
short live end of R66 to center-tap, adjust for 200 ohms to ground

replace field-coil with 1K 10W resistor

two 1N4007 and two 250r 10W resistors on '80 socket (genuine high-loss 5Y3 hard to get)

three 6V6

smoke-test, quickly verify -1.0V at 10 ohm tap

squirt volume pot, tone switch

snip top of volume pot, feed iPod

build 1-stage guitar preamp

blown240 · « **Reply #32 on:** September 17, 2011, 12:48:25 am »

none of the wires are totally bare. But if you move them too much they come apart. In my pics there is a little square box that looks like the bottom has melted, and there is a cap that may have leaked.

I have the bassman preamp that I posted pics of earlier. I can use that as a preamp I guess?

I need to see if the OT comes off of the speaker. I will let you know tommorrow.

When you say snip top of volume pot, feed iPod. Do you mean to make it an iPod player, or is that where I would feed a preamp into?

Thanks!

blown240 · « **Reply #33 on:** September 17, 2011, 01:02:39 am »

The OT is riveted to the speaker cage, so it can come off.

It has 4 wires in and 2 out to the speaker.

http://i268.photobucket.com/albums/jj26/thebattens/Philco/IMG_5616.jpg

http://i268.photobucket.com/albums/jj26/thebattens/Philco/IMG_5615.jpg

http://i268.photobucket.com/albums/jj26/thebattens/Philco/IMG_5614.jpg

PRR · « **Reply #34 on:** September 17, 2011, 01:06:15 am »

That speaker is repairable with rubber cement and tissue.

me> build 1-stage guitar preamp

Got it.

RF stage is type '78, sells for $8. Pentode is fine preamp. Most wires may be left as-is.

Replace R32 with 0.68u (standard Marshall cathode cap).

Break plate lead. Add 33K from plate to a B+ source. Add 0.01u from plate to top of volume control.

For added fun: snip some wires on AM/SW switch, use that to select guitar preamp or line (iPod) input.

> The OT ...has 4 wires in and 2 out to the speaker.

The field-coil is also in there. One end is high B+ and OT CT. The other end comes back to power the low B+ loads.

blown240 · « **Reply #35 on:** September 17, 2011, 01:23:54 am »

ok Cool.

I understand MOST of what you have told me. I will get started on it ASAP and will have many questions I am sure!

"Pentode is fine preamp." Is that what my Bassman preamp is? I just followed a schematic when I built it.

blown240 · « **Reply #36 on:** September 17, 2011, 04:15:11 pm »

I met a local guy today that has vintage tubes and radio parts stacked floor to ceiling in his house. I took the radio over to him and he ran a few tests on it. Its pretty bad. He gave me a lot of information that was way over my head but basically the inside of this radio has alot of bad caps and burnt resistors that I didnt notice.

He is a radio restoration guy, and he suggested that I gut it and make it into an amp. He did have some of the original tubes I would need, but I dont want to spend extra $$$ on it until i figure it all out.

So now I am really confused, and am leaning towards gutting it and building a champ.

blown240 · « **Reply #37 on:** September 17, 2011, 10:50:07 pm »

I spoke some more to the local guy this afternoon, and now I am looking for a simple push-pull 6V6 schematic with 1 or 2 12AX7 preamp tubes. This should allow me a decent little amp and still allow me to use my transformers.

Does anyone have a good idea for a simple push-pull 6V6 schematic with 1 or 2 12AX7 preamp tubes?

ThermionicEngine · « **Reply #38 on:** September 17, 2011, 11:44:59 pm »

How about this one?

A 5E3 Deluxe- you can modify the volume controls to one concentric stacked pot maybe.

http://www.webphix.com/schematic%20heaven/www.schematicheaven.com/fenderamps/deluxe_5e3_schem.pdf

Have fun, Larry

blown240 · « **Reply #39 on:** September 18, 2011, 01:12:06 am »

Ya that may be the way to go. THANKS

I just found some interesting info:

1. The Power Transformer is 110v 25hz, which if I am not mistaken will be fine to use, but may give me a higher B+
2. The output transformer is a part number 32-7078. Its a push pull. The Secondary impedance = 1.25 ohms and the Primary impedance = 5,580 ohms. From what I understand I would need a 1.25 ohm speaker. Thats not really going to do the trick!

kagliostro · « **Reply #40 on:** September 18, 2011, 01:25:19 am »

You can use a 2ohm speaker and the reflected impedance will rise a bit

or you can use 2 paralleled 2ohm speaker (like if it is a 1ohm speaker) and the reflected impedance will drop a bit

1.25ohm and 2ohm or 1ohm are not so different

the impedance of ~~an OT~~ a Speaker is not constant referring to frequency

Kagliostro

blown240 · « **Reply #41 on:** September 18, 2011, 01:30:04 am »

Thanks again for the speaker info.

I may be wrong, but it seems that it would be cheaper to get another OT then to get a 2ohm speaker. I will need to look into that though...

blown240 · « **Reply #42 on:** September 19, 2011, 11:07:58 pm »

I took my radio to a local amp builder that I know. He is a super cool guy and he helped me get together a game plan. I also got my tubes, sockets, and a nice used 20watt output transformer that has 4, 8, and 16 ohm output. He also gave me some grille cloth.

So, Heres the plan.

Use the stock power trans, and choke.
5y3 Rectifier
1 12ax7 preamp for now, but possible add 2 more preamp tubes later.
2 6v6 power stage
20 watt, selectable impedance out put trans.

When I got home, I stripped the chassis, got the enture thing to bare metal, and painted the outside metallic gold.

Here are some cell phone pics:

Willabe · « **Reply #43 on:** September 19, 2011, 11:27:43 pm »

Quote from: blown240 on September 19, 2011, 11:07:58 pm

When I got home, I stripped the chassis, got the enture thing to bare metal, and painted the outside metallic gold.

OK, come on now, how did you strip it/get it to look like that???

Looks great!

Brad

blown240 · « **Reply #44 on:** September 20, 2011, 12:54:45 am »

I used a flap disc in a 4" grinder. I am also building a 51 Chevy and have been doing a lot of custom body work. So for me the metal work is the easy part.

blown240 · « **Reply #45 on:** September 25, 2011, 12:29:31 am »

I got the layout for the chassis figured out today. Here is a a couple pics of where I am at so far, If you see any problems with this layout, please let me know:

Here is the inside, nice and roomy:

I had a few extra tube sockets, so I used them to fill in some of the holes. There are 2 extra octal and 2 extra small 9 pin sockets. I would like to use them and was wondering if I should plan a second channel or something while I am at this stage. What do you think?

Here is how my power tranny read out:

There are 9 wires total. I marked the primary before I took it out of the original setup, so that was easy.

There are 2 wires that are black, they are 120v each or 6v together. This must be for the Filaments

Thera are 2 brown wires that are 74v and 75v each or 5v together. These must be for the rectifier

And then there are 3 tan colored wires: Here lies the question.

The separate voltages to ground are 33v, 252v, and 109v

The 33 and 109 together = 410v
The 252 and 109 together = 410v
And the 33 and 252 together = more than my meter can read.

This leads me to think that the 109 wire is the center tap, but I may be wrong. Also I THINK that originally the 109 wire went off by itself somewhere...

Does this make sense?

PRR · « **Reply #46 on:** September 26, 2011, 12:38:37 am »

When ALL winding ends/taps are lifted from ground, the "to ground" voltage readings are practically bogus.

I wish you had noted goes-to-what before you (expertly) chopped. It hasn't changed much. Two HV ends go to rectifier plates. The CT often goes to ground, though Philco took it a few hundred ohms "below ground" to get bias.

Resistance readings should suggest which is CT.

blown240 · « **Reply #47 on:** September 26, 2011, 05:33:43 am »

I checked the resistance readings and from what I found the CT is what I thought it was. Yesterday I got all the heater wires and the rectifier done. I am getting 368v on the B+ coming off the rectifier (using a 5Y3GT).

PRR · « **Reply #48 on:** September 26, 2011, 04:02:00 pm »

> I am getting 368v on the B+ coming off the rectifier

With or without a filter-cap?

blown240 · « **Reply #49 on:** September 26, 2011, 08:51:41 pm »

Without a filter cap. That is straight off the tube.

WHY? is that bad?

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Author Topic: Amp Build From a Vintage Radio (Read 41246 times)