looking for schematic for prewar Gibson EH150

[DavidHilts]

David: does your tube shield say "Goat"?

Hi PRR, sadly the tube shield is MIA, I can see where the ground wire was attached to the chassis though.


That looks like 11 Watts in an 11W plate. I'd fret; except with Field Coil speaker the speaker sensitivity and damping changes with amplifier total power demand. As I do not think this will be a 40 hour a week amp, I'd leave it as it is.

I think it can only be a curiosity, or special-sound, amp, not something any modern player would use a lot. Gain is way low, apparently low even within a few years. We could "fix that" with another stage or a booster pedal. But that speaker was only strong enough to play clean (like a radio), not to survive OVERdrive. And hasn't gotten any stronger in 80 years.

 

[Prattacaster]

Hey David, dont know if you have this amp still. I am in need of some measurements on the interstage transformer. I have a Supro on the bench that is basically this exact amp. It has a half open secondary, causing one of the power tubes to run super hot due to no grid DC reference.
If you could be so kind, I need resistance verification on the primary and secondaries. You can measure in circuit. My primary measures about 1.7k, the secondary that is good measures about 3k to CT(ground). Do you get 3k to ground from each power tube grid?
I've subbed in an interstage tranny(124D) but I can tell it does not quite match. Any help would be amazing!!

 

[PRR]

I've subbed in an interstage tranny(124D) but I can tell it does not quite match.

That's like 1/10th the impedance of the original. BTW, the two sides should match in DCR within 20%. Same number of turns. Probably different average diameter.

The usual thing for this application is the Stancor A53-C or copy:
https://www.tubesandmore.com/products/transformer-audio-interstage-10-ma

 

[Prattacaster]

That's like 1/10th the impedance of the original. BTW, the two sides should match in DCR within 20%. Same number of turns. Probably different average diameter.

I know this, that's why I posted...the one side of the secondary is completely open, like over 20M. obviously the amp clips asymmetrically.

I also realize the Stancor and some others are the typical replacement, most likely because the options are few. The resistance measurement of the primary would lead me to believe I could do better than the Stancor or similar. Once again I measure 1.7k on the primary. 6A6 is a medium mu tube with around 24k plate resistance.

Also, my tube shield has "goat" on it .

 

[Enzo]

Resistances will be even only if the transformer is dual wound. If they wound the first half, connected a center tap, then continued the wind, the second half will have considerably higher resistance, but the turns ratio will remain the same.

 

[PRR]

the second half will have considerably higher resistance

I said 20%. Have you seen more?

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The 124D is clearly too low Z for this application. If you want to look for better than A53C, I wish you the best of luck.

 

[Prattacaster]

I said 20%. Have you seen more?

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The 124D is clearly too low Z for this application. If you want to look for better than A53C, I wish you the best of luck.

My goodness.....I'd say Im sorry for restarting this thread but I still hope for some useful reply.

 

[PRR]

I do not understand your offense. The A53C is maybe the last good option left. If you find something else, please let us know.

 

[Enzo]

I said 20%. Have you seen more?

Marginally, but 20% sounds substantial enough to me.

 

[Prattacaster]

Marginally, but 20% sounds substantial enough to me.

Do you guys have some sort of rivalry? This has added nothing.

 

[kevinkr]

Let's focus on the topic and leave personalities out of this.

 

[Enzo]

I didn't realize I was arguing. I apologize to all involved.

 

[Prattacaster]

I'll state again what I am hoping for: resistance measurement from each power tube grid to ground if you have what I believe is the oldest version of the EH150 or the 1930s National Dobro Amplifier. They were basically the same amp(power tube substitute). Also verifying the primary resistance of the interstage transformer would be appreciated. I realize this is an almost absurd long shot; anything I can do to preserve this amp is worth it.

 

[Enzo]

You might consider disconnecting the transformer from the circuit and applying a gentle voltage to it to see if its transformer action seems OK.

 

[Prattacaster]

You might consider disconnecting the transformer from the circuit and applying a gentle voltage to it to see if its transformer action seems OK.

I had done that Friday. 1 volt on the primary side gave me about 1.5v on the working secondary. The problem remains I'm not sure if the working secondary is partially shorted, open, healthy or not. Remember half the secondary is open. This is why I need a resistance confirmation so I know the turns ratio to spec out and then I can also calculate the impedance; or get close.

If half the secondary is factory spec, I guess I have a 1:3 ratio, which tracks with other designs from this age.

 

[Enzo]

Sorry, I did not remember one side was open.