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Old 29th May 2005, 01:33 AM   #1
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Default McMartin MA-50 Troubleshooting

Hi again!
As I've written in other threads, I have been enjoying a McMartin MA-50 tube amp. But being a relative newcomer to tube amps, I would welcome some assistance in troubleshooting a problem.

Lately this amp has started to develop significant audio pops and scratches in the main output section, and ultimately blows the 0.5A fuse in the bias circuit on the cathodes. I've checked the bias and it's still at 100mA per the spec. The clicks and pops are pretty loud, and happen even when the preamp gain and master gain are set to minimum. This leads me to believe it's in the output section.

I've posted the schematic at: http://frontiernet.net/~whitfield/Mc...MA50-w_cap.jpg

You will note a 47uF cap (marked in red) on the 470V output of the OD3regulator tube (and Grid2 of the 7027A output tubes). I installed this to remove some significant 60Hz hum.

All tubes test fine. I can't test the OD3 regulator on my tube tester, but I've swapped it with another with no change. Is my addition of a 47uF cap on regulator causing trouble? Is it's rated voltage (500V) too close to the operating voltage of 470V on the grids?

So where should I look to find the cause of the scratching/pops and the bias fuse on the cathode circuit? I welcome all ideas and help in fixing this. While I'm an EE, please keep this basic so I can learn more about this fascinating world of tubes.

Best regards,
Art
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Old 29th May 2005, 08:55 PM   #2
anatech is offline anatech  Canada
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Hi Art,
Lose the cap. When the regulator fires it has to charge the added cap. Probably not good for the regulator.

There must be another reason for the 60 Hz hum, like C17A and B. Possibly a bad rectifier on top of that (SR-9). A problem in the HT supply would give you a 120 Hz hum.

-Chris
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Old 30th May 2005, 05:42 AM   #3
PRR is offline PRR  United States
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> 47uF cap (marked in red) on the 470V output of the OD3 regulator tube

No! Gas tubes plus caps make oscillators. Look-up the OD3, there is a maximum capacitor that can be directly connected across it. (Yes, your red cap is not across the OD3, but the 16uFd and 47uFd are effectively in series across it, and ~12uFd is surely too much.)

> to remove some significant 60Hz hum

60 or 120?

In either case, I would assume the 16uFd main filter cap has gone sour. It isn't big enough to begin with, and if it has gotten weak it might still make 10-20 watts without buzz in the plate circuit.

Now wait. Ripple in the G2 feed should not cause buzz. So what you really did by putting 47uFd on the bottom of the OD3 is add 47uFd to the +610V line, except the ripple is cooking the OD3. If that fixed your hum/buzz, C16 is sick.

Fix C16. If you cut it open you will find it is really two 350V caps in series in one can; get two 33uFd 400V caps to stay authentic, or a couple 100uFd 450V caps because the designer probably wanted to use 40-50uFd and that 16uFd cap was a cost compromise. (Myself, I've used 2*470uFd in similar amps with good results, but some call that over-kill.)

Still, your symptoms sound to me like a tube with an intermittent short. Tubes can test good and still have troubles.

Or the -43V bias supply is failing. THAT would pop a 0.5A fuse (nothing much else could). If C17 A/B have intermittent shorts (they never short while you are checking, of course), 7027 current goes to max which would be over 500mA (and also cherry-red plates).

I would in general assume ALL the electrolyic caps in a 40 year old amp are bad, but the main filter cap is your buzz and a short in those bias-caps is real bad news. At least for first-aid, get good-brand (like Panasonic) caps in 105 degree rating (Digikey.com stocks them).
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Old 31st May 2005, 03:04 AM   #4
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Thanks to both of you who took the time to look at the schematic and post a reply. I'm starting to understand better how the OD3 may be compromized by my additional cap. Originally I didn't change C16 or C17A/B as testing by just adding a new cap in parallel didn't change the hum. I'm now supposing that when an old cap fails, just adding more (new) capacitance in parallel to the old won't necessarily fix the problem. Is that because the old cap is failing ohmic (shorted)?

Thanks especially for clarification on the main failure mode - the possibility of shorts in the 7027 power tubes causing pops and clicks. A replacement pair of tubes looks to be $45 or so (Parts Express, matched pair), which I can deal with.

Onward and upward with my education. I truly enjoy learning this stuff, and I appreciate your patience with my newbie questions and mis-guided repairs on the hum. I know the McMartin is not an audiophile amp, but it's easy to work on and fun to use for guitar. And that's the point - enjoy new things while learning.

Sincerely,
Art
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Old 31st May 2005, 10:48 AM   #5
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Why play with a commercial amplifier? McMartin wasn't known for their superior circuit design but rather as a ho-hum 70volt line driver of limited bandwidth at best.
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Old 1st June 2005, 01:03 AM   #6
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Why play with the McMartin? a) it was free and needed help, b) it has more than enough power for my son's guitar, c) the chassis is large enough to allow easy access to the wiring and components, and d) it was broken and I fixed it (initially). Since guitar is limited bandwidth, a PA amp like the McMartin certainly seems ok.

My primary audio amps are a pair of Ampex 2012 (6V6 output tubes) that I dearly love. A bit cramped inside to work on, but they sound great. So why risk messing up the Ampex as a learning tool when the McMartin was available. Please remember, I'm new to the world of tubes, so any tube amp is a new adventure. I don't have a basement full of tubes - just my Ampex (a gift from a friend), the McMartin, and a pair of 8W kits from Antique Electronics Supply (11MS8 output tubes). Sure I'd love a McIntosh or ST70 someday, and maybe sometime I'll build a pair of SE amps. But until I learn to maintain and repair the few I own, I probably can't justify something like a McIntosh.

Now if you want to discuss the complexities of various wind synthesizer controllers, I can get a deep as you'd like. But for tube amps, I'm just a novice.

Respectfully,
Art
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Old 1st June 2005, 02:20 AM   #7
anatech is offline anatech  Canada
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Hi Art,
I don't see a thing wrong with what you are doing. I fix things just 'cause.

-Chris
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Old 1st June 2005, 02:50 AM   #8
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But Chris, if he fixes a McMartin what will be next? A McGowan?, A Bogan, an Executone? My God where will it stop? A shelf full of commercial amps that only a mother could love? My God the horror of it all.
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Old 1st June 2005, 06:17 AM   #9
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> just adding a new cap in parallel didn't change the hum.

Odd, illogical, but not unknown. And in any case, old electros can BURST. Not usually dangerous, but messy. If left unattended: rots the steel chassis. I know it is hard work, but get them out of circuit.

> to use for guitar

That trick with the gas-tube is strange, and IMHO not suited for guitar-amp duty.

Keep C16 in the 16uFd-32uFd range (not my 470uFd HiFi/PA overkill). A gitar amp should "sag": pass the strum of the transient clearly, drop during the sustain, rising as the note fades. Heavy tube-heads favor bottle rectifiers for this reason, but 700V 200mA is a big bottle and a major change. (Also less power, but measured power isn't so significant.) Not over-sizing the cap keeps the "sag" time constants about right for guitar work.

Use good 7027: made in the 1960s-1970s by a major-brand US or Euro company. I don't know if anybody has "re-issued" the 7027, but the majority of new-made tubes will not take the extreme high voltages that this amp runs. We used to run 6L6GC a lot harder than 450V, but I hear many new "6L6GC"s won't take that for long. That's a shame: you can get a replacement 6L6GC on a Friday night.

The old EL34(?) is a guitar favorite and rated 800V, but isn't as beefy as 7027: it might work well if you can put 16 ohms on the 8 ohm tap and accept about half the watts (which isn't half as loud).

At this lower power, self-bias (cathode resistor) makes at least as much sense as fixed-bias: less to go wrong, possibly more euphonic sound (guitarist's tastes vary so much, both set-ups have fans.) Even in fixed-bias, some cathode resistance (in series with F2) is wise: 10Ω gives a handy place to check bias; ~200Ω gives enough self-bias to reduce run-away while still mostly working like fixed-bias.

6550 was always good for 600V, and ElectroHarmonix has contracted for new production. EH tubes are usually good, and organists say the EH6550 works well in their Leslies (which is tough duty). But 6550 needs more heater power, will warm your power transformer.

All these tubes have similar but not identical pin-outs. You have to check closely: G2 moves around, and "NC" (NO connection) on one tube may be "IC" (INTERNAL connection) on another tube: watch for "NC" pins being used as handy tie-points and be sure they don't connect on the other tube-type.

Replace the wacky OD3 with 20K 2Watt. Use about 20uFd from G2 to ground. While the OD3 does have a lovely glow, its function here was dubious. It may have met a particular test-spec at a particular price. They may have had a crate of OD3 left-over from another product. But using a gas-tube as a screen-dropper makes the amp over-sensitive to supply voltage variation, risks failure of the OD2 on audio transients (you can't get a replacement OD3 on Friday night), and maybe cascading failure of the output stage. I may be fretting too much; but I know a resistor-divider WILL work in guitar-service and the slight drop in test-power isn't a big deal.

Yes, by HiFi amp standards, this 20K||75K screen-divider is "too high resistance", and it won't make maximum sine test power. But with ~20uFd G2 cap, the G2 voltage will hold-up through guitar pluck transients, while letting G2 voltage dip under sustained overdrive to protect tubes and ears. Some playing-around will be called for.

A PA amp is not a guitar amp. It is too heavy, as I'm sure you know by now. You don't want to carry 50Hz iron for an 82Hz instrument through 120Hz speakers. OK, your son will carry it. But also, it has too much feedback for classic open-back guitar speakers. Raise R29 to 10K or 22K. Gain will increase; ignore that and listen to speaker bass resonance. High damping kills the "growl" of classic guitar speaker systems. Lack of damping gives blatty bass and screeching highs. The classic Fender formula was damping factor pertty close to unity, instead of the 5 or 6 this amp has (and >40 for HiFi). Calculations are amusing, but the true test is the tone with the specific speakers.

Guitar amps also usually have a rise: gain above 2KHz is 6dB higher than gain below 1KHz. You can't exactly duplicate this with those tone controls. Again many different tastes exist, and several very popular amps did use essentially this tone control scheme.

The concertina phase splitter is a clean reliable driver in HiFi. It is faultless up to and just past the edge of clipping. But when driven far into clipping (as guitarists play for effect) it goes asymmetrical, which is not the usual sound.

You might at some point consider stealing the plans for a classic Fender. Fender used a modified long-tail phase splitter, a strange dirty thing but its gross-overdrive waveforms are part of "the electric guitar sound". (OTOH, Ampeg sold a lot of boxes very like this one.) The Fender tone stack is also the gold standard of guitar work, and actually simpler than what you have.

Oh: your "MIC Inputs" (without any transformers) ARE guitar pickup inputs. (Except C20 has to be a typo. If it is a 50uFd 6V across R51, try with it, without it, and with 0.1uFd on the cathode resistor to lift the treble.)

You might try removing R19. Or reduce R13. This will skew the tone action, but also allow V2A to be driven into fuzz while keeping the Master control fairly low and soft.

"Program" input is for MUZAK (and burnedfingers complains about Executone?). It could take a CD player outout. Use two 10K resistors to mix the stereo.

> A Bogan, an Executone? My God where will it stop?

Right there. Bogens are excellent minimal amplifiers. Executones are %$#@!. I think this McMartin is a dull boatanchor, but good practice. It has usable iron (if you don't worship NFB), solid chassis, enough sockets: good mule for modifications. And nobody else in town plays through a rare vintage McMartin.
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Old 2nd June 2005, 12:33 AM   #10
anatech is offline anatech  Canada
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Hi Joe,
What's next is a McBogen. Stacks and stacks of McBogens.

PRR has made some excellent points. Get it working and then try his ideas. Never leave dead electrolytics in circuit either as they do like to leak. Sometimes they short as well.

-Chris
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