Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: passaloutre on March 11, 2024, 01:12:42 pm
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I have picked up a cheap ~1960s "Crown GPA-25" PA amplifier. It has 2x6L6, 2x12A*7, and a 6CA4 rectifier. Based on the mounting and wiring of the power transformer, I have reason to believe it is not original, and based on the high voltages throughout the amplifier, I have reason to believe it may be unable to deliver the current required of 2 6L6s.
The transformer is labelled "Motorola 25B81385-C", date code "138739". I cannot find any documentation for this transformer, or any indication of what equipment it may originally have been installed in (maybe a television?).
With no tubes installed it put out 276 VAC from ground to each side of the secondary. With the rectifier only installed, the B+ was 359, and with the rest of the tubes installed, B+ went down to 291. Would you think these numbers are indicative of a faulty or overtaxed transformer?
Are there any tests I can perform on this transformer to determine how much current it can safely deliver?
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Hard to say what the specs are. Some nice hi-res pics might help for us to see what you are talking about though.
Dave
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One thing you might do that will give you some kind of idea as to whether the transformer is overtaxed... Not fool proof, but some indication.
Turn the amp on with a variac set to 117VAC. When the tubes get hot, check the voltage on the filaments. If it is below, say, 6.0 volts, it might be a not-very-scientific indication that something is not right.
Dave
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cathode biased or fixed bias??
does the amp work?
heat is one way to check, does it get real warm, sorta warm, HOT??
the B+ "drag" is in the ballpark for a pair of 6L6's
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I have picked up a cheap ~1960s "Crown GPA-25" PA amplifier. It has 2x6L6, 2x12A*7, and a 6CA4 rectifier. Based on the mounting and wiring of the power transformer, I have reason to believe it is not original, and based on the high voltages throughout the amplifier, I have reason to believe it may be unable to deliver the current required of 2 6L6s.
The transformer is labelled "Motorola 25B81385-C", date code "138739". I cannot find any documentation for this transformer, or any indication of what equipment it may originally have been installed in (maybe a television?).
With no tubes installed it put out 276 VAC from ground to each side of the secondary. With the rectifier only installed, the B+ was 359, and with the rest of the tubes installed, B+ went down to 291. Would you think these numbers are indicative of a faulty or overtaxed transformer?
Are there any tests I can perform on this transformer to determine how much current it can safely deliver?
Either it can’t make enough juice for the 6L6s or the rectifier tube is past its use-by date. If heater voltage is normal when the 6L6s are running, it’s more likely to be the rectifier.
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The amp does work. I replaced the filter caps. It is cathode biased with 25v across a 225 resistor. It doesn’t get very hot, no more than any of my other amps. I did notice that whoever installed the PT wired the 6CA4 rectifier to the 5V taps (I assume they’re 5v because they’re yellow; the rest of the tubes are wired to the green heater tap). I could rig up a quick diode rectifier to rule out the tube completely (I’d like to switch to a circuit with solid state rect anyways).
It has some ugly-but-cool distortion, which I attribute to the grid leak input stage and/or the cathodyne PI. It sags quite a bit on big chords, but I’m not very used to tube rectifiers, so that may be expected? It’s not *super* loud, but again I’m not used to running 6L6s at such low voltage.
Working on photos, I can’t figure out how to resize them from my phone…
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I could rig up a quick diode rectifier to rule out the tube completely (I don’t have any other noval rectifiers available).
Yes. Two 1N4007s. Connect one between pins 1 and 3 and the other between pins 7 and 3. Pull the tube and re-evaluate.
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can’t figure out how to resize them from my phone…
that's a "design" feature :icon_biggrin:
the datasheet I looked at says ~~~30 - 35Vac before you "overdrive" so probably is Pre/PI funky distortion.
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That rectifier takes 6.3vac on the heaters. If its hooked up to 5VAC, its wrong and could be part of the problem.
Dave
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Here is the transformer label
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And the bottom/back of the transformer. You can see that it was designed for a vertical mount, but someone has bent the tabs over to mount it laying over
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What is that connector at the bottom of the PT?
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That's something that I installed to power a remote pilot light. I should have said that I am in the middle of gutting and converting this to a guitar amplifier. The amp did work as is, but the sound was uninspiring and I was itching for a project. If this transformer isn't up to the task, I have others, or I can use 6V6s instead.
To answer your question more specifically, it is one of these: https://mm.digikey.com/Volume0/opasdata/d220001/medias/images/1148/PT02E-8-4S.jpg
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That's an expensive plug just for a remote pilot light.
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It sure is :laugh:
We have an odd assortment of surplus electronic components at work (a US gov't research lab), leftover from the days where people actually built and/or fixed the equipment.
I've been able to save some things from the dumpster, like these connectors, a variac, oscilloscope, a stash of Germanium transistors, lots of 50W resistors...
On the original topic, that date code with 138 implies the transformer was actually manufactured by Stancor, so I may be able to look up their 1947 or 1957 catalog to see if any specs match up...
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A shot in the dark, but here is the Stancor catalog power transformer page from 1955...
I wonder if a PC 8405 might be inside that Motorola shell: 270-0-270, with 5v and 6v filament windings, in which case the HT supply is 120mA
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Who knows? Trying to guess x-former specs from any old random source will not alleviate your uncertainty.
What is the heater VAC under load?
If that looks okay, then try the rectifier diodes. (Edit: if you've still got much-lower-than-expected (i.e more than about 10% lower) B+ voltage across the HT winding, it's a good bet that its an under-rated winding)
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from the field;
figure out what "operating" B+ you want, current you need, find a new transformer, say $100. Go to your favorite bouillon store, buy $100 in silver, when the current transformer finally dies, sell the bar, buy the new PT, I get 10% of the leftover cash :icon_biggrin:
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It was definitely the rectifier.
Replaced with diodes, the unloaded B+ was about 390 unloaded. With all the tubes installed, B+ came down to about 365. Filament voltage is a healthy 6.4.
Loud. Punchy. No sag.
Gotta work on my grounding to tame some noise.