Welcome to Solid State Guitar Amp Forum | DIY Guitar Amplifiers. Please login or sign up.

March 28, 2024, 04:48:19 PM

Login with username, password and session length

Recent Posts

 

Masco tube amp

Started by Peter Blair, January 22, 2015, 12:26:02 AM

Previous topic - Next topic

Peter Blair

OK I know this is a SS amp forum, but I'm hoping someone will have mercy on me.
Knowing almost nothing about electronics, I'm faced with the following problem. Working on a 1954 Masco model MU-17. (I've attached a schematic, but it is quite small)

The schematic says i should have 330v at the plates of the two 6L6GT power tubes. I have 468v.
I'm supposed to have 340v off the 5Y3GT rectifier. I have the same 468v.
Checked that with a new tube also. Same reading.

If I plug in either of the two old power tubes (one of which I KNOW is bad), into either of the two sockets, I get some kind of short as soon as the tube warms up, and my in-series 300w safety light starts to glow. Same thing with two new replacement tubes, again in either socket. I see no conspicuous faults in the circuitry or components. Everything is clean and appears to be in outstanding condition. No mess in the tube sockets or anything like that. If someone could have a look at the schematic and maybe tell me what to check. I've checked all the voltages from the power and the output transformers. Can't find anything that isn't just a few volts higher than what they should be. I sure would appreciate any suggestions. Note; All I have for test equipment is a multimeter.
Thank you in advance,
Pete

Roly

Hi Peter, welcome.


It's okay, some of us are valve-friendly.   8|

Man, that really is one out of the crypt, but it looks basically reasonable.  Nuthin' wrong with a circuit that is readable (and we've seen a lot worse).

Quote from: Peter Blairmy in-series 300w safety light

I do hope that is a typo and you mean 30 watts.   :o


Quote from: Peter BlairThe schematic says i should have 330v at the plates of the two 6L6GT power tubes. I have 468v.
I'm supposed to have 340v off the 5Y3GT rectifier. I have the same 468v.
Checked that with a new tube also. Same reading.

The voltages on the circuit will be the warm idle voltages.

The tranny is marked 330VAC, so the initial HT voltage before the output valves warm up (or are out of socket) will peak somewhere around;

Vac * root(2) = Vpk

330 * 1.414 = 466.62V

...close to your 468V, so that actually looks okay.


First thing I'd look at is the 50uF cap across the cathode bias resistor.  If this has shorted (and it might be older than me, and I'm a leaky crock) that that would give you those symptoms.

Try disconnecting the +ve (cathode) end.  We are looking for about 25V on the cathodes, and if it's much less than that both output valves will be trying to turn on.

If this still has the original waxed paper coupling caps those will also be (highly) suspect.

Plug in your known good 6L6's and clip your meter across the 200 ohm cathode resistor.  As they warm up this should rise to about 25 volts.  If not post what you do measure.

Could you also please post a well-lit and crisp pic of the underchassis.

HTH

If you say theory and practice don't agree you haven't applied enough theory.

Peter Blair

Hi Roly,
Well, my anxiety/frustration has already diminished some. Thank you so much for taking a look. I'll follow your advice/instructions and be back to you sometime later today or tonight.
Pete

Peter Blair

Hi Roly,
Back again. I tried my best to follow your instructions. I hope I got it all right. Here's what I did, hoping I didn't misinterpret.

Disconnected C5 (?) at cathode end. (Pin 8 ?) With the 6L6's out, zero voltage before AND after disconnecting C5.

Coupling caps (C11 and C12 ?) are different from each other in appearance, but of same value, as specified in schematic. One does look like you describe as waxed paper. The other is black plastic looking. I have to say though, that they both look original. You can see them right next to each other in one of the photos.

With C5 still disconnected, and getting a good reading on the cathode resistor (R27 ?) of 197ohms;
I plugged in the new 6L6's and clipped the meter on each end of R27. Turned on power. Voltage quickly climbed past 25 volts, to 50 volts and still climbing quickly, at which point I powered off.

Please let me know if the pictures are adequate, or if you're trying to see something specific. I'll get a better shot for you.

My safety light IS 300W. I included a picture for your amusement. Is there a reason a 300W bulb is not a good idea? I was told the heavier filament in a high wattage bulb would act more like a wire, and not light up so easily. Please advise if this is not recommended.

Peter Blair

Another update Roly.
I got to thinking about those coupling capacitors. I can see how if one was even partly shorted, and letting some of that 125vdc into pin 5 (grid ?) on the power tubes, that seems like it would spell big trouble. I don't know the proper way to test them but . . . My multimeter will put out and audible test signal. One of those caps seemed to be shorted, as the test signal would go right through it. It would not beep if I bridged any of the other caps I tried it on. I checked it several times and got a beep on that one cap (C11 or C12, not sure which one). Funny thing is, after I went to my salvage stash to look for a matching value cap, I tried it again and got no beep. I decided to try replacing that one cap anyway. (good luck in the salvage stash) After doing so, . . with both 6V6s plugged in, my test light would glow a little, but the amp was working!! I didn't want to leave it on long enough to test any voltage readings as I don't know what really should be checked. What do you think I should do?

Where should I go from here????????? Do you think it would be wise to just wholesale, replace all those paper caps?, or just all the coupling caps? Would that be C6, C7, C8, C9, C11, and C12? What do you think would be the safest bet? Or am I not necessarily on the right track at all?

Thank you again for checking this out for me. I really need some guidance, and I can't express how much I appreciate your time.

Pete

Peter Blair


Roly

Quote from: Peter BlairMy safety light IS 300W

AAAaarrrggghh!   :(

The point of a limiting lamp is to limit.  We only want to apply a limited amount of power so we can do some voltage faultfinding without cooking anything.  While valve amps require somewhat higher wattage limiting lamps than s.s. amps because of the heater power, we are still talking domestic wattage, 25, 40, 60, 75, 100 - not 500W or 1k theater lights.
{otherwise it looks great; and where did you dig up that 300W globe?}


Quote from: Peter BlairWhere should I go from here????????? Do you think it would be wise to just wholesale, replace all those paper caps?, or just all the coupling caps? Would that be C6, C7, C8, C9, C11, and C12? What do you think would be the safest bet? Or am I not necessarily on the right track at all?

Normally we disparage the common advice of "replace all the caps", but your amp is an exception.  When I was a teenager it was normal to replace waxed paper caps on sight because you could bet they were horribly leaky, more resistors than capacitors, and that's where the black one will have come in, already replaced decades ago.  That was fifty years ago and today we rip waxed paper out on sight without a second thought. 

You need a bunch of polyesters or similar with the correct values and suitable voltage ratings.

For reference, a way of testing a coupling cap for leakage is to lift the grid end and measure the voltage between the free end and ground.  A good cap will eventually charge up and read zero volts, or something very close, while a leaky cap will show a residual voltage (which in the case of some waxed paper caps may be close to HT), but I wouldn't even bother testing these.  Leaky coupling caps was the second possible cause of excessive stage current that I was coming to next.

Electrolytics are not quite so dramatic, but this bunch have served well above and beyond the call of duty and need to be updated.  You may not be able to get exact values, but they are not critical and as long as they have sufficient voltage rating they can be 50% or even 100% higher in value ('tho I wouldn't go over 25uF for the one directly on the 5Y3).

I would also go over all the resistors (which may mean unsoldering one end for accurate results) and replace anything that is more than 10% off value.

Clean all the pots with contact cleaner or metho, and replace any that are still scratchy, and the same treatment for all the sockets.

It's a sweet little vintage amp, and like the axe with three new heads and five new handles, once you have done all that it should be as good as new (as long as you don't have to replace any of the valves as well  ;) ).

This may sound over the top for a repair, and it is, because what you have to do with an amp of this vintage that is practically original is a restoration, renewal throughout wherever required.  Then it will be right for another fifty years.

Lots of people have done similar and it's always rewarding, "sounds better than ever".   :dbtu:


Something I've just noticed with is really curious is that the common cathode bias resistor, R27, is the silver braided thing with the red/black/brown bands connected to pin 8 of one 6L6 and ground on the other base.  I can't remember ever having seen a power resistor like this before.

If it still measures 200 ohms +/-10% then you can leave it, but I have no idea what its lifetime would be.  I suspect that it is resistance wire wound on something like a glass fiber core and should have a very long and stable life if not flexed too much, but that's guesswork.


It's my pleasure Peter, this is a sweet little combo amp and deserves a dose of renewal.   :dbtu:
If you say theory and practice don't agree you haven't applied enough theory.

g1

  In case you have a 300W bulb in your limiter because of the infamous video on youtube, please see Roly's discussion of limiter bulbs here:
http://www.ssguitar.com/index.php?topic=3636.msg28236#msg28236

Peter Blair

Hi again Roly, and G1,
OK, let's clear this issue up first. "Guilty" on the charge of viewing the YouTube video mentioned. This IS exactly where I got my information. G1, thank you for the link. I DID read Roly's article. Makes perfect sense to me. I have been converted, and educated. Thank you so much. (I'm learning as fast as I can!!)

Roly,
Now to the amp. I was hoping to retain as much of the original circuitry as possible. In this case, what you say does make perfect sense. Doesn't mean much to me to be original if it won't work. I WILL be replacing all the caps. I found these at "Just Radios". According to their product information these look (to the untrained eye) like they will fill the bill. They are available in the values that I need, and are very reasonably priced. Would you please render your opinion, and/or make me aware of other options or even a recommendation.

http://www.justradios.com/DMEcapacitors.html

By "electrolytics" are we talking about the large filter capacitors, four in one can? If so, I have found a new replacement with identical specs for somewhere just under $40. I also have an old, but very unused one that is 20, 20, 20, 20, 450 volt. Do you think it would be false economy to try using that. It's not an easy one to replace. (Very tight and lots of stuff soldered to it.) And I know caps go bad just from age. They don't really need to be used.

I will be following your specific advice on the resistors. I'm hoping at least SOME of them read ok, just so I don't have to replace SO much stuff!!

On the issue of being an "over the top" repair. I don't view it that way at all. If in the end, the result is a very nice sounding, somewhat rare, nice vintage tube amp, it's worth every minute of work, including all the research I've had to do to accomplish this. The learning part that I get out of it is the most important part of my reward, in addition to having the amp. It will take me some time to get all the parts in, and get all this done. I assure you I will post again on this topic when the repair is completed. In the meantime, I've included some pictures of the amp taken before disassembly. Hope you enjoy.

I can't wait to get this completed!!!!! Thank you Roly again. I think it probably would have taken me quite some time to stumble onto the solution, had it not been for you bringing those coupling caps to my attention. This is where experience comes in. What's totally baffling to a noob is so obvious to someone who's been around the block. Your knowledge and experience have been invaluable to me, and I don't take it for granted. God bless you for your generosity in time and advice.

Waiting to hear your comments on my post. Oh, and I got the 300W bulb at Home Depot. You wouldn't happen to know anyone that could use a 300W bulb would you?

Pete

Roly

The Internet is a great way to share information.

It turns out to be an even better way of sharing misinformation.



On the up side, it prompted me to write this;

http://www.ssguitar.com/index.php?topic=2093.msg28255#msg28255


These caps look like they will do fine, and their prices look reasonable.  Do not get sucked in to paying through the nose for "boo-teek" "tone" components - it's all cobblers.

No, do not use old electros, and do not bother with expensive special multi-units.  Leave the old ones that are visible in place for cosmetics, disconnect them below, and fit inexpensive brand new single units underchassis out of sight.  You may have to add a bit of tagstrip to support them (say by soldering it to the lugs of the old cap), but this is the way most restorers choose to go.


A working amp gets looked after, and there isn't a lot of call for amps in glass cases, so the trick here is to renovate the electronics underchassis as much as is needed to restore the amp to reliable working order, but try to maintain the original look, and don't do anything that can't be reversed (e.g. drilling holes).  Yes, this can sometimes mean replacing a lot of stuff, but if you are going to replace anything you have to replace everything that needs it in an amp of this age (or throw the towel in and buy a large glass case).

I expect that many of the resistors will be okay, but higher values in particular tend to drift high.  I wouldn't bother to replace any resistor that is physically intact and within 10% of its markings.


Quote from: Peter BlairIf in the end, the result is a very nice sounding, somewhat rare, nice vintage tube amp, it's worth every minute of work

I entirely agree, and I think it will turn out that way because, as I said, this is a rather sweet little example of a well built vintage amp, well worth the trouble.

Quote from: Peter BlairYou wouldn't happen to know anyone that could use a 300W bulb would you?

Buy a Diamond Python?



I had one of these fellas move in with me for a few years when I lived up north.  Not much on conversation but a great automatic mousetrap.    :dbtu:
If you say theory and practice don't agree you haven't applied enough theory.

g1

Quote from: Peter Blair on January 24, 2015, 03:08:51 PM
I found these at "Just Radios". According to their product information these look (to the untrained eye) like they will fill the bill. They are available in the values that I need, and are very reasonably priced. Would you please render your opinion, and/or make me aware of other options or even a recommendation.

http://www.justradios.com/DMEcapacitors.html

Just Radios are an excellent company and you won't go wrong with them.

Peter Blair

EYES OF EXPERIENCE PLEASE!!!!

Could someone confirm what I think I just found please!
Ok, so I think we've identified the likely suspect in this case as being one, or both of the coupling capacitors going to the grids on the power tubes. After replacing one of them with a replacement that I had in my salvage stash, most of the symptoms diminished markedly. So the plan was/is to replace all the caps, just because they're all so old. I do not disagree with this. Seem like cheap insurance to me. So while I'm waiting for the parts order to show up, I'm sitting here gazing alternately at the schematic and at the amp itself in an attempt to get better at reading one against the other. Most of the time I feel like I'm doing pretty good. Suddenly I notice something that seems totally wrong to me concerning the two coupling capacitors in question. It looks like their polarity is reversed!! If you look at photos 2 or 4, you can see them, one just above the other, just to the right of the input jacks. The lighter colored one (C11) on the top, and the black colored one (known replacement C12) just under it. I also notice that the lighter colored one MAY have been replaced also. I do notice that the legs on it are a lot longer than all the other legs in the amp. Most of which have been trimmed to a more compact length.

So please correct me if I'm wrong, but as I understand it, the end of the cap that has the solid line going around it is the negative end, and on the symbol for the caps in the schematic, the strait line represents the positive end, and the curved line the negative. How am I doing so far?

Ok, so what I think I see is this; (please reference schematic in first post)
C11 should have the + lead going to pin #1 on V3 (phase inverter), and - lead to pin #5 of V5 (6V6G).
C12 should have the + lead going to pin #6 on V3    (same tube),   and - lead to pin #5 of V4 (6V6G)

If this seems right to you, please look at the pictures again. I don't think you can make out the pin #s but you can see the negative end of both those caps is facing left, toward V3 (12AX7 inverter), and the positive end is facing right, toward the power tubes. What i see in the chassis is exactly opposite of what I've described above!
Negatives of cap are going to pin 1 and 6 (of 12AX7) and positives are going to pins 5 on each power tube.
Just for confirmation I traced the location of R24, 25, 26, and 28, just to make sure everything seemed to jibe. All these did agree with the schematic. For further confirmation I checked the polarity of several other caps as I interpret, and found them to be in agreement with the schematic.

Did I find the problem?, or am I all wet?
Dying to have someone look and render an opinion and or conclusion.
Thanks a million in advance.
Pete

Roly

Quote from: Peter BlairI do notice that the legs on it are a lot longer than all the other legs in the amp. Most of which have been trimmed to a more compact length.

This is exactly the sort of clue we look for when "reading the chassis" of a repair.  This indicates that this component wasn't fitted by the same person who fitted the others.  If it has a differing style or type this is another clue, as is the soldering.  I have opened up amplifiers and been able to identify repairs by four different technicians at different times using these observations.  This is valuable because it gives you a picture of the history of the equipment, and can be a clue to a recurrent problem (that others may have missed).


The brown waxed paper caps are not polarised like the red electrolytics.  It's generally a fair bet that any cap less than 1uF is not polarised, and more than will be.  If they are they will have a clear "-" minus and/or "+" plus marked on the case.

The black band indicates which lead is connected to the outer-most foil, and this end is normally connected to the part of the circuit that is closest to ground, or has the lowest impedance to ground, the driving end in the case of a coupling cap.  Caps these days don't have such markings; you have to resort to some trickery to find out which lead is connected to the outside foil.  Getting this wrong might make a slight difference to the background noise performance, but that is all; so the polarity of these coupling caps is of no matter.

Quote from: Peter BlairDid I find the problem?, or am I all wet?

Yep, you found one of no doubt several leaky caps (pulling the following grid positive and making the amp sound 'orrible).   :dbtu:

Quote from: RolyFor reference, a way of testing a coupling cap for leakage is to lift the grid end and measure the voltage between the free end and ground.  A good cap will eventually charge up and read zero volts, or something very close, while a leaky cap will show a residual voltage (which in the case of some waxed paper caps may be close to HT)
If you say theory and practice don't agree you haven't applied enough theory.

Peter Blair

#13
Thanks Roly.
Thanks for the education. So please clarify for me: In a perfect world the black banded end of the coupling cap (polarized or not) would be better facing the "driving end". By that would you mean signal end driving the grid of the power tube? Better but not necessary? The other end being driven by the signal coming from the plate of the 12AX7? Do I have that right? And by your explanation, even though the polarity of a cap under 1uF is inconsequential, the banded end COULD be considered the "-" end?

It is still my intention to replace, (and I have ordered) all the caps, including the filter caps (individual as per your suggestion). Which brings me to another question. These large, multiple capacitors in a can are on the top of the chassis for obvious reasons. But I've also noticed that a lot of the Fender amps place the individual filter caps on top of the chassis also, under a cover. Is there a reason for this?
Pete

while a leaky cap will show a residual voltage (which in the case of some waxed paper caps may be close to HT)
What is "HT"?

Roly

When these, and similar low value non-polarised caps, are manufactured they are made as a very long strip sandwich of two conductive layers with a layer of dielectric in between (in this case metal foil and waxed paper), and a backing layer.



This is then wound up into a cylinder to take up minimum space.  This results in one of the capacitor "plates" finishing up on the outside of the cylinder.  The other plate is shielded to the outside world by this outer plate, but the outer plate itself is then prone to stray capacitive coupling to anything nearby.

With a bypass cap this outer plate end is the one connected to ground, so the strays don't matter.

When one of these caps is used to couple two stages however this outer "plate" could be connected to the driving end, the anode of the previous stage, or the driven end, the grid of the following stage.

Normally the driving anode end will have a lower impedance to ground than the grid end, and so will be less prone to picking up surrounding signals via stray capacitance.

With modern caps a way of telling which is which is to simply hold the cap body between your fingers and touch each lead in turn to an active amp input.  The end that gives you more hum/noise is the outer (but we normally don't bother with guitar amp applications).

By contrast an electrolytic cap is an electrochemical cell and inherently polarised, much like a battery or diode, and may/will be damaged if connected in reverse polarity.


Quote from: Peter BlairI've also noticed that a lot of the Fender amps place the individual filter caps on top of the chassis also, under a cover. Is there a reason for this?

Robustness mainly.  You will notice that the chassis mounted cans are pretty secure, but later wire-ended types are a bit more difficult to tie down mechanically, so Leo apparently decided to put them all in a can shell above chassis.

Leo did a lot of things he didn't both to explain to anybody, so we have to do a lot of tech "channeling" to surmise what was in his mind.  I've had a Fender Twin on the bench that literally went cartwheeling down a dirt road - and still worked okay that evening, so I think that building his amps road-tough might have had a lot to do with it.


HT - High Tension, a.k.a. B+, high voltage, supply rail, etc.  Opposite of LT - Low Tension, typically the heater supply - British tech terminology.

If you say theory and practice don't agree you haven't applied enough theory.