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Location: Sunshine State, Australia | Look what someone gave me last week! :cool: :D |
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Location: Vernon CT | Good Luck with that Muz!! :eek: :) |
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Location: Utah | Looks like a major overhaul there. But that could be a fun project. |
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Location: San Bernardino, California | One thing to take note of: when transformers get rusty they become less effcient and generate heat causing them to burn up. Make sure to check for that. |
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Location: Blue Ridge Mountains | Best wishes...hope it works out... |
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Location: New South Wales, Australia | Good luck Muzz...it looks terrifying to me :D |
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Location: Colorado Rocky Mountains | Hat's off to you if this is something you're good at and interested in. If it was me, I'd send it to Al with a smile if he was interested in it (its lost, art and vintage, if anything ever was). If he passed, I'd toss it in the dumpster because projects like this would just sit around taking up valuable space on my workbench. |
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Location: NJ | wow |
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Location: Sunshine State, Australia | Update; Someone on one of the tube-head forums dates this sucker to possibly post-war 1940's. Who am I to argue?
Anyway, capacitors are toast. Resistors look dodgy.
Transformer (most expensive and difficult to source part) is fine. The rectifiers and power amp tubes work. The preamp tubes glow, but I've no way to test them.
The choke works.
I'm tracing the circuit now. New capacitors and resistors on the way after Easter. Complete re-build on the way. This could prove obsessive.
I'm in negotiations with a crew in Brisbane to get the speakers re-coned. (Seriously 'vintage') |
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Location: Utah | All you need is the transformer. Everything else is easily replaceable.
What are the tube types in there? |
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Location: Sunshine State, Australia | The 'mystery component' has since been identified as a vibrator/oscillator for dual power (AC/DC) and has been removed.
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Location: NW Washington State | "The preamp tubes glow..." Yikes! You powered it up in that condition? Maybe you used a Variac to bring it up slowly? Even better, and cheaper, is a Dim Bulb Tester .
6X5s have a reputation for shorting out and frying the power transformer. You might want to add a fuse or two: 6X5s
I'd offer to test the tubes, but it might cost more to ship them than to replace them!
-Steve W. |
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Location: Sunshine State, Australia | I used a variac. (I'm an Electrical teacher at the local technical college. But I know almost nothing about electronics, especially OLD electronics. Bit of a learning curve ahead.)
That 6X5 link you provided implies that the 6X5 GT's (glass tube) version had very few problems. Mine are GT's. |
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Location: NW Washington State | Didn't know you were an electrical instructor, I'm sure you know the fundamentals better than I do.
I haven't had problems with 6X5s, it was just something I remembered reading. Here's another 6X5 reference where there's a picture showing both types.
-Steve W. |
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August 2005
Posts: 3736
Location: Sunshine State, Australia | I've drawn up the circuit and uploaded the schematic on my \'blog\' so now I need some experts to look it over and give me some guidance.
Yes, I'm an electrical teacher, but this stuff is all new to me. |
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Posts: 4069
Location: Utah | I can help a little bit. But you'll find experts over on the DIY tube audio forum at diyaudio.com .
That unknown capacitor on the bottom of your schematic is a cathode bias bypass capacitor. Its job is to pass audio frequencies directly to ground but block DC. The resistor in parallel is the cathode bias resistor, whose job is to drop DC voltage, thus raising the DC voltage of the cathode. This allows the grid input to be lower in voltage by that amount, which biases the tube properly.
So 25uF would be a good guess since the other input tube is bypassed with that value. It makes an RC filter with the parallel resistor, and the value isn't critical, just somewhere low enough not to cause problems. 40hz, 10hz, 2hz. Whatever, as long as it is above DC and doesn't reduce your bass response.
The unknown resistors circled in green, at first blush, 5K sounds ok maybe. Those resistors bias the cathodes so that the input grids are in the sweet spot. Essentially, if your input signal is 2v p-p you would bias the cathode at -1v or -1.5v. So the value of that resistor is important. You have pentodes there, so it is a big balancing act. You'll see that your R7 plus the 'new' 470K make a 510k resistance to high voltage supply for tube A, while tube B has R6 + R2, for 350K to high voltage supply.
So these tubes are not biased exactly the same.
The datasheet for the 6sj7 is here . Looking at the curves on the bottom of page 3 of the original document, average plate characteristics, you're trying to find a bias point probably in the middle somewhere. The specs say plate voltage max is 300V, with a typical of 250 or 100. So we're stuck in the left half. Find a point on one of the solid curves and read the voltage, maybe -2.0V. Look to the left and read the current. About 5mA. So the resistor that gives that would be 2V/5mA, or about 400 ohms. Now the 5k sounds way too high. 5K would put us down at the bottom at the -4v or -5v curves which are near 1mA. It could work, but just seems a bit odd. But I don't think you'll burn anything up by putting in 5K resistors.
Tube C is a phase splitter, not a preamp though it may have some gain. If I read it right, it is a concertina phase splitter. The phase splitter creates two signals, 180 degrees out of phase. One goes to the upper output tube, the other goes to the lower output tube. The concertina is a good design for hi-fi.
The blue resistor that you moved in the schematic does seem to belong in the output stage and should be moved as you indicated. It biases the output tubes. If the tubes are well matched, you will get good fidelity. Be sure to get matched tubes when/if you replace them.
The capacitor, 0.05uF that is drawn next to it at the top of the schematic, should not move, I believe. It is a DC blocking capacitor. You'll see that pin 5 of the lower 6V6 tube also has a blocking capacitor if you track back near the tone pot. Your phase splitter is putting out an AC signal riding on a high DC voltage, probably near 250V of DC! You have to block that DC to make the signal usable by the 6V6 tubes.
What software did you use for drawing the schematic? It is time to upgrade my method of pencil and paper. |
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Posts: 3736
Location: Sunshine State, Australia | Thanks for your fantasic response, Flysig. Lot's to absorb and investigate there.
Software wise, because I use a Mac, I've adopted Vectorworks as my CAD app. It's a bit overkill, but a more than suitable replacement for AutoCad, which was the last software application that tied me to Windoze. Now I'm completely free to use my Mac 100%
I'm pretty sure Vectorworks is also available for the Windoze masses. |
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Location: Sunshine State, Australia | Updated schematic here. |
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Posts: 4069
Location: Utah | Muz, your link is kaput.
There are some great tutorials on the web for tube circuits. The Valve Wizard is a very practical approach. Another very good one is Aiken Amps , click on tech info then click advanced. |
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August 2005
Posts: 3736
Location: Sunshine State, Australia | Try the link now, Flysig. I had a capital in my filename. I changed it and it works now.
diyaudio won't let me post a thread. They ask me to do a search before posting. Done a dozen searches and it still won't play with me.
I'll investigate the other links tomorrow at work. Sunday evening here now. |
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October 2005
Posts: 4069
Location: Utah | Diyaudio doesn't make it easy at first. On the page where you enter your text for a new topic look for a little box on the left side towards the top labeled "Have you searched?". Click that box and it should work.
You're on moderated status for the first few posts, so it won't show up for a few hours until the mod reviews it. They must have had problems with spam or other inappropriate content at some point. But once you're in, it is as easy as any forum to use. |
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Posts: 4069
Location: Utah | Are you going to rebuild from the schematic or are you just trying to document what is there? In other words, how critical is it to be accurate?
I'm not sure about a couple of things in the circuit.
The 6.3v heater winding on your transformer looks a bit odd. The outer taps are grounded and the center tap is used to supply the heaters? It might be correct, it just looks odd.
The tone control also looks odd with only two terminals of the pot connected. The .005uF cap connected to the Or side of the pot is a bit of a mystery to me. It blocks DC, but is it part of the tone circuit? I guess I'm not at all understanding how the tone circuit is supposed to work. I would have expected a cap to ground somewhere, or a pot to ground. Maybe the tone pot bleeds high frequencies right into the signal going to pin 5 of the lower 6V6, thus boosting the treble?
The phase splitter should be looked at by someone who is familiar with pentode concertina designs. It looks ok but I only have a passing familiarity with triode concertinas.
When you rewire this thing, pay attention to how the original was laid out. For example, the grounding scheme. You correctly show on the schematic that the heaters are connected to ground, but you don't want the heater grounds to be anywhere near the signal grounds. |
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August 2005
Posts: 3736
Location: Sunshine State, Australia | Yes, my intention is to rebuild it to the schematic, which is why I need it to be accurate. I don't have enough (any) experience to experiment.
Regarding grounding, I have shown on the schematic that they are grounded right next to the component, but this was for drawing simplicity. In reality, there was a single continuous wire connecting all the grounds. |
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September 2006
Posts: 275
Location: Canberra, Australia | G'Day Muzz,
long time no speak...
long story short, moved to Canberra, life change,
playing the 05 Collectors lots now, etc etc
working on an original Mk I Boogie at moment..pure vintage sweet anger in that baby...
Re this thread, have a couple of guys who may be able to help your project - did U get it sorted?
Looks like the Oz connection is still alive and well here.. Cool!
Regs, Dave |
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August 2005
Posts: 3736
Location: Sunshine State, Australia | Dave! Welcome back.
Yes, I could still use some help on my 'project'. I've got an almost finished schematic but need someone to make a decision for me on some component values.
Meantime, have a look at the \'sister site\' and join the 'Downunder' group. It's still a part of the OFC, but you'll have to register again as it's a separate site. The OFC admin created it as a repository for photos, videos and sound clips, but there's also a few sub groups and blogs and member's pages.
A few of the boys visit Canberra semi regularly, so you may get some visitors. I prefer Queensland in July, personally.
Good to hear from you again. Hang around a little longer this time mate. |
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For the vintage valve lovers...
