Buckeye Model B-23 & other fleamarket specials...

[Frankenamp]

I went down to a radio flea market this morning to see what I could see... and I saw about half a parking lot full of old jun-I-mean-VINTAGE parts and such. As you can probably imagine, there is the usual mass of card tables set up end to end, and tarps on the ground. I quickly spotted an old RCA record player that looked promising ("its $40, but I'll let it go for $30...") Two thirds of the way across the lot was a guy with several tables of Schtuff... including a Stromberg Carlson PA chassis (2-6L6) and moderately rusty iron; and a Heathkit PA amp with a couple 'Cannon' balanced input jacks and a couple internal knobs to 'balance' the inputs, and sockets marked 7591 less rust in the chassis. He wanted 40 each, and wouldn't take less that $60 for the pair (very interested). Also had an old Webcor wire recorder. Almost at the other end of the lot was a guy with a lot of tubes and other paraphenalia: an old Eico amp ($$$) and a little old Bogen amp with a PT rated for 25 Hz (hence the $$$$ price tag). In the vicinity was an old "Buckeye B-23" PA amp that had been restored, had a paper with pins and voltages scribbled on it with part of a schematic... The multicap can was removed and replaced with a Sprague 40uF 450V, and a Nichicon 22uF 450V cap underneath; the iron was in decent condition... The tag said "$60 w/o 6L6's, 5U4; $80 with." Phono input, 2 microphones two octal speaker out jacks, and a third octal with a pin to match impedances from 2.5 to 500 ohms. Decisions, Decisions. Coupla beat up chassis or a nicely restored mystery amp... After some back & forth, wheeling & dealing I bought it for $65 cash. The guy I bought it from said he couldn't find anything on that amp... I looked around and can't find much either anyone heard of such an animal? Tube compliment: 5U4G (Shoulder Tube), 6L6G (Shoulder Tubes), 6SN7 (splitter), 6SF5 (phono), 7B4 Loctals (x2 Microphones). Three volumes, one tone knob (off-white 'chicken head' knobs). Any info & Schematics would be appreciated. My friend the radio nut eyeballed the PT and said it looks like a 200 ma transformer...

Once I got that loaded in my friends car, I went back and looked up that old RCA "orthophonic" record player tag on the bottom said 2 6V6 tubes... I beat him down to $20 and lugged it to the car trying to ignore the 'throw in a free black widow' jokes Once home I did get out the vacuum cleaner and make sure there were no unwanted hitchhikers! Again there seems to be a void of info on that particular record player. RCA Victor Model SHF7; Ser. RVG 052953 Tube compliment is a 5Y3 rectifier, a PP pair of 6V6's 6CG7 (2nd AF stage & PI), and a 6AV6 (1st AF stage). The speakers are an 8" in between a pair of 4" tweeters. and a switch to aparrently cut in an outboard speaker jack for "stereophonic" sound (from a single channel). It looks like a typical crystal cartridge to a triode front end. The layout is a rats nest, and I suspect that the three extra 40uF Sprague 'Lytics are there because the multicap went south... suspicion is a bit stronger now that I found one of the lugs of the multicap nipped off and tucked under the Sprague cap (not taped) feel much better about beating the price down now. Whaddaya think I should do? Fix it up as is- if I can get a schematic (with the single triode front end) and see how it likes getting whacked with a P90 or two; or pull the 6AV6 and replace it with a 12a*7? As it is, the volume control is between the input and the triode. Why the diode/triode and not a pentode or dual triode. Should I bother returning it to "stock" at all? Should I make it into something "Princeton-ish" What to do, what to do...

[kagliostro]

I love these tales of hunting, the pursuit of prey, of his tracks and then go home with the trophy in the trunk of the car



Kagliostro

[eleventeen]

When I started diddling with this stuff, the idea that you'd someday pay money for a dead RCA record player you could fish out the garbage is like the idea that you'd pay money for half a used grapefruit. (or, a used half-grapefruit) Nowadays, with ebay, everybody knows the price of everything, don't they? Oh well; the stuff *is* much harder to find and people need the $$.

[Frankenamp]

Yep Eleventeen, I quite agree. I could go in the back of the local TV (repair) shop and find carcasses galore*. Now, granted at the time, about all I knew was to be careful around the big multicaps which could be lethal... and the glass when I was dissecting the occasional tube- (lookin back: "you bastard! That was an RCA BLACKPLATE!") What did I know... and then the neighbor down the street giving away old anodized aluminum racks of pull-out components stuffed with 12ax7's and other miniature tubes- probably avionics military surplus from Beale AFB. I lived just a few miles away as a kid- it was nowhere; in my 20's I realized it was probably #1 on the Rooskies 'places to nuke list' nowadays, I look back and imagine the streets paved with VT-107's & 5992's

*Old man likely tolerated hauling the odd Toyota trunkfull of junk because it kept me away from his beloved Capehart am fm sw console & record player complete with field coil 12" speaker. (found out at a young age that crossover capacitors really explode when you connect them wrong- like hooking up a Radio Shack compression tweeter to the field coil terminals by mistake, goo & foil everywhere!

Ah Mssr Kagliostro: life is one big adventure, or nothing at all :)
It's far far better to boast about the one shining conquest, than every other time we go out an' git skunked!


I did some digging around and found out that the Buckeye is from about 1949 (still wonder why the two mikes are 7B4's and the Phono is a 6SF5- supposedly the same tube in Octal or Loktal... Found a Sams Photofact for 3.95 and a dollar for shipping. The newspaper ad claimed 34 watts "ideal for singers, orchestras, and M.C.'s"... in the March 19 1949 pp 64 issue of Billboard magazine for the princely sum of $89.50 complete with tubes. Antique Radio site seems to love the RCA SHF-7 but haven't found a schematic ...yet. Anyone know why they used a 6AV6 instead of a 6au6 or a 12A*7 in the front end?

[PRR]

> why they used a 6AV6 instead of

Campbell's Soup and RCA were neighbors in Camden NJ.

Campbell's bought a LOT of tomatoes. The streets ran red.

RCA probably bought (or made) even MORE 6AV6 than Campbell's tomatoes.

With the needles of the day... pentode first-stage would hiss a little and cost more than a triode. Two triodes give more gain and hiss less than a pentode. It also leaves a place for bass/treble knobs (perhaps lacking on your version). Yes, for the pushie-pulley model this means three triode... but RCA had SO MANY 6AV6 that it didn't matter.

[Frankenamp]

Ah, the designers were being practical then. I was blessed with a photofact, and looking through it quickly, I noticed that the two diode plates were tied to the cathode and then the 2200Rk, and the 50uF bypass cap. This amp seems to have been made in the third month of 1958 making it a couple months shy of two years older than my carcass. This was the heyday of tube television and I'm sure that they were using what they had the most and cheapest of. Same reasoning for the 6CM7? seems to be some disagreement whether it is a dual triode or a dual dissimilar triode? One side was the second AF stage, and the other half is the PI to drive the 6V6's. I'm gonna have to recap it at least. Do the two stages have enough gain for guitar as is, the better question should be should I just leave it alone other than caps, and learn how the topology works as is- or plan on playing with it a bit...?

[eleventeen]

"RCA probably bought (or made) even MORE 6AV6 than Campbell's tomatoes."

Not to mention bazillions of WW2-era 7-pin tube sockets which almost went completely out of use in the late 50's/early 60's.

6X4 rectifiers went almost completely to solid state. 6C4 single triodes (got a zillion of those) got combined into dual-use tubes, 12AX7's, 12AU7s

[PRR]

> diode plates were tied to the cathode

If you don't need the diodes, it is Good Practice to tie them to cathode to keep them out of trouble. OTOH, it works fine if you don't. Wiring convenience/speed/costs may be the deciding point.

> 6CM7? seems to be some disagreement whether it is a dual triode or a dual dissimilar triode?

Why be disagreeable?

http://www.mif.pg.gda.pl/homepages/frank/vs.html
http://www.mif.pg.gda.pl/homepages/frank/sheets/093/6/6CM7.pdf

Clearly says "dissimilar".

And TV V-sweep.

BUT... they are not VERY dis-similar. If you go back in history: the first small-screen TVs could use generic twin-triode such as 12AU7. The last big-screen tube TVs needed DIS-similar duo triodes such as 6EM7 (second side is 10 Watts!). Evidently there was a middle generation with a slightly-beefed output side. And very likely it went out of style the year before your recordplayer was made.

BUT... this is the first time you have mentioned any 6CM7.... where did it come into the story?

[Frankenamp]

"
BUT... this is the first time you have mentioned any 6CM7.... where did it come into the story? "

I'll blame in on Mister Howard Sams! (and not enuf coffee)... I was lookin' at the schematic that says in bold block letters: "6CM7" and it did not penetrate my overly thick cranium that the tube chart on the chassis, the diagram on the bottom of the set, and the tube chart all said "6CG7" Who am I gonna believe- the schematic- or the numbers on that lyin' tube, and the charts, and the pictures? 

The CG lists both plates as 3.5watts [5watts combined]
The CM lists plate 1 as 1.25 watts, and plate 2 as 5.5 watts.

12A*7 have similar but not the same basing (looks like pins 2&8 are switched). I,m wondering if it's better to keep it somewhat stock or geetarize it?

[PRR]

> Who am I gonna believe-

Some days, it's hard to say.

> 5.5 watts

Yeah, but I bet it is a case of a 4x4 to hold up a shelf 'cuz no smaller wood was handy (or cheaper). I forget what the plan is, but I'd suspect it is a volt-amp and a cathodyne, neither one flowing over 2mA, no more than 150V across the tube. You need a 0.3W tube. You got the 2W and 3W tubes in regular inventory, and this oddball 1W/5W tube in excess unsold inventory, and if your phonograph costs 5 cents more than brand W you will lose sales, and you like your job... use what's cheapest. Allowing for pinout, any dual lowish-Mu triode will do the job without complaint.

> stock or geetarize it?

Some days, it's hard to say.

[DummyLoad]

flip a coin...

 

[Frankenamp]

Here's a schematic, hope it opens...

OK, second verse, same as the first:

[DummyLoad]

it's incomplete - try rotating the image before you save.

[Frankenamp]

OK, attachment fixed (gotta love Open Office's hidden page setup)

[PRR]

Remove all the needle-load and loudness-compensation krap off the input. We don't know what it is, but we know we don't need it.

Disable the NFB around the mellow output stage by mostly-bypassing V2A cathode, for more gain and brashness.

With the higher gain you may want to put the volume control _after_ the first stage. With volume pot de-wired, snip the treb pot wiper and insert vol-pot. Fix the input. A 33K grid resistor is not urgent but may someday save the gig.

Depending on speaker, the 0.05u+470K (7Hz!) coupling to 6V6 grids may let too much bass through. 0.01u here? Maybe also drop grid resistors to 270K with 5K-50K right at the grid.

The "tone controls" are cut-only. They may have been selected for a boomy woof and shrill tweets to get the effect of boost/cut; or it may just be to increase knob-count. We can't have any loss here since even the modded gain is not ample. So I simply shifted one resistor value for less drastic action for a more discerning user.

That should play guitar OK. No special magic but no annoyance.

If gain is low, or you want fancy or mid-scoop tone network, V2 has to be changed for more gain, and then you may as well 90% steal a Fender output stage (12AX7, 56K in cathodyne, etc).

C1C (preamp B+ filter) may want to be bigger than 10uFd; 40u is reasonable. Since the can-cap is old and likely doing poorly, get three new 40uFd 450+V and one 10u-50u 50+V cap.

[Frankenamp]

Remove all the needle-load and loudness-compensation krap off the input.

Agreed. 33K-68K front end resistor sounds like cheap insurance. Is the 6AV6 drastically different than a 12A*7ish triode? AFAIK it's a single triode with a couple diodes instead of the second triode?

Disable the NFB around the mellow output stage by mostly-bypassing V2A cathode, for more gain and brashness.

Put a switch on it?

...the 0.05u+470K (7Hz!) coupling to 6V6 grids may let too much bass through. 0.01u here?

Sounds like a plan!

Maybe also drop grid resistors to 270K with 5K-50K right at the grid.

As in hanging the 50K right off the grid pins? What does reducing the 470K resistors do? Allow more voltage to hit the grids? Seems like it'd be the other way around? (showing my ignorance)*


The "tone controls" are cut-only...So I simply shifted one resistor value for less drastic action for a more discerning user.

Sounds reasonable. Or, How close to the 5F10 Harvard can we get here?

three new 40uFd 450+V and one 10u-50u 50+V cap.

Was already planning on it. Someone soldered three Sprague Atoms 40u/450V from the cap can terminals to the frame (?) one terminal had broken off, the other two were still solid to the can. not much room inside the chassis, I might restuff the can if I can make some new caps fit... and save some real estate inside. Nevertheless- out with the old, in with the new (caps that is).

Thanks for the suggestions!

*Noticed that the Harvard (fixed bias) has the bias connected to the center of two 220K resistors, and looks to be 1.5K resistors hanging off the grids How different is this?

[Frankenamp]

Well I ordered a few caps and stuff like a 1 Meg pot with switch for use as a possible Volume Control to make the operation a bit safer. When they come in, I'll see how your suggestions work out. Thank you very much for the ideas!

On to the B-23:
2- 7B4's (glass Loctal version of the 6SF5) as microphone preamp tubes, and a 6SF5 (metal Hi-Mu triode w/gain of ~100+/-)
1- 6SN7 (Glass medium Mu dual triode with a gain of ~20 +/-) and two nice 6L6 GB's with old-fashioned shoulders.

The circuit looks simple enough- seemed like Bell wanted to go for stability instead of wringing every watt out of their amps.

The tone control is (I assume) just a hi-cut with a .005uF tied to the wiper of a 500K pot. It is fed by three 270K resistors coming off the wipers of the volume controls. How does this function? Whats the approximate knee frequency?

The two mike inputs have two gain stages, but the phono has only one, straight into the 6SN7. Signal goes through a .02uF cap and by a 10Meg resistor to ground (grid leak?) instead of w 30-68K resistor that rolls off radio freqs. Does the cap have a similar function in reducing RF or does it mere block DC from the input?

How difficult would it be to put the two 7B4's in series (or parallel) for more gain? How difficult would it be to create a more sophisticated tone circuit?

Have a look and see if you see any mojeaux!

The chassis is in surprisingly good condition, for 1949 vintage- looks like new caps in the PS...

Any comments or suggestions are more than welcome.

[PRR]

> .005uF tied to the wiper of a 500K pot. It is fed by three 270K resistors

And a 100K to ground.

> How does thins function- whats the approximate knee frequency?

Reduce it. Parallel resistors can be combined.

R4 is 500K, so when full-up it has hardly any effect on a 50K node. When full-down it is a simple R-C high-cut. In between it cuts about the same point then shelves-off at some lower gain.

I used "300K" to make the math easy. It is really 270K +plus+ the zero to 250K of a 1Meg pot plus a V1 plate system impedance (perhaps 100k).

[Frankenamp]

Another endorsement of Doug's turbo-service: I ordered some parts Thurs at Oh-Dark-thirty. Didn't bother to check the mail Saturday, didn't think there'd be anything till Monday earliest, finally checked Sunday morning and it was already there! Bravo-Zulu!