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!!
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!!
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
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 😉.
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.
I said 20%. Have you seen more?
--------------------------------
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.
I do not understand your offense. The A53C is maybe the last good option left. If you find something else, please let us know.
Let's focus on the topic and leave personalities out of this.
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.
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.
Post #15 shows a field-sourced, owner-drawn, schematic for a Style III (Charlie Christian-era) EH-150, s/n in the 12xxx range before they went to EH-185. In post #15, the 6F5 & 6C5 stages are X-ed out in red...is this because it's not applicable to the earlier version first question? I don't know who provided that schematic, but think it appeared on one of the guitar amp sites, like EL34World or AmpGarage.I have never seen a Gibson-published schematic for the 7-tube Style III, so I am thankful someone drew their own. That schematic also shows two filter chokes, one possibly the speaker field coil. Mercury Magnetics sells an EH-150 12 H filter choke in addition to their PT and OT.
Also, there is a red question mark by the PI-IT primary. I drew one in my mind here also, because I was disturbed by the drawing of the primary being capacitor-coupled, and the 6C5 driving the primary being powered through a 100 k resistor instead of through the IT primary. There is a DC current 'rating' for such transformers when they pass plate current to the primary driver stage.
I have only a seen a couple other Gibson amps using PI-IT, and neither of them used a DC-blocked (cap-coupled) primary. 'Shooter' in Michigan on one of the guitar amp sites posted that he has build capacitor coupled transformer PI's. They do work.
Let's say I throw a dart and guess that the EH-150 IT has a primary inductance of 60 H and a couple thousand ohms DCR. 60 H and 0.05 uF has a resonant frequency of about 92 Hz. An LC circuit series resonance has an impedance minimum with the primary DC R keeping it from looking like a short circuit., and parallel resonance has an impedance maximum with the DC primary resistance. Neither LCR circuit (and the schematic shows series, anyway) with a resonance at 92 Hz seems like a good idea. So I think the 0.05 uF coupled to the IT primary deserves a question mark.
With the later versions, a bass/normal switch was added. It adds/removes a coupling capacitor to change the bass response. Another version uses a pot to vary the effect of the same larger capacitor in the switch-only tone control. For an amp that tried to offer 30's HIFI sound, having contradictory RC networks in a coupling role doesn't seem like a recipe or predictable frequency response.
I bought a 13xxx s/n Style III at a bargain, presumably because the previous owner had 'modernized' it...Fender/Jensen-Italy Special Design PM speaker, a Hammond PT with 750 VCT instead of 800VCT of the original, replaced the ugly wax-leaking cardboard box Aerovox filter caps, and has 7 electrolytics where the Aerovox ones were. The terminal/tag strips all the RC components are mounted on, were apparently loaded with parts on the top side and bottom, and I missed that inspecting the mods (they also replaced paper capacitors with 6PS Orange Drop style). Since there is no Gibson schematic for this 7-tube configuration, I think there is one error in the 12xxx owner-drawn schematic, and I have no idea (yet) what the prior owner of mine actually did, I think I have to draw my own. Mine has 4 iron core magnetic components (PT, OT, IT, choke), so I believe the previous person didn't need to add a a choke in place of the field coil that was eliminated.
Earlier discussion mentioned the question of who built the early EH-150 amps. I think I read Lyon & Healy in Chicago. But which versions?...and how many versions were there? I've read about 4 Styles, with the 4th being the randomly-marked EH-150 or EH-185, but there seem to be more than 4. Someone suggested they were built one at a time with available parts, and the war-era ones may not even have been authorized during the civilian product manufacturing shutdown period (war effort)...there might be undocumented versions.
The ES-150 guitar markings (if present) after WWII are a joke (1946-49). At least the EH-150 amps had s/n's.
As far as the builder teaching Fender some things, the double-sided horizontal terminal strips with tight point-to-point wiring is not at all service-friendly. Now that it's pushing 85-90 years old, is no longer original, and works/sounds great, I'm not in a hurry to rush to restore as original. The wire insulation is pretty fragile-looking.
Last thing: Mercury sells EH-150 transformers., claiming they are exact replicas. The IT-PI transformer is $108 in 2024, and they occasionally have a 10% off sale. That makes them the only still-alive supplier of PI transformer. The original transformer p/n's (T-xxxxx format) some people say are Thordarson's I cannot find in vintage catalogs. Thordarson usually had a letter (sometimes an S) in the middle of the p/n numerals. So I think the Gibson EH-150 transformer numbers (when a schematic exists with p/n's) are GIBSON p/n's not Thordarson.
$108 plus California postage is a lot more than the Tubes and More 1:3 transformer that people don't seem to find workable. For the ridiculous prices people are seeking for EH-150 amps, or wanting restoration to original, $108 is NOT a bad price for a claimed exact replica.
P.L.T. (Post-Last-Thing) EH-160 and EH-195 prices can be laughingly ridiculous too, especially since they are not even the same circuit, and most people refuse to work on them for safety reasons (if you see a '150' for sale with 13 tubes, run like hell...you can't 'modify' it into a '150' - the chassis layout is absolutely different, and there is no power transformer...if you need to ask for explanation, heed the well-intentioned 'no' advice from others). I feel more sorry for people who sell a 160 as a 150, than the person who buys one described inaccurately by seller...buyer has some recourse, but what an unpleasant mess to straighten out (double shipping, potential damage, isn't/wasn't usable before or after)..."Tested"...REALLY?
Murray
Also, there is a red question mark by the PI-IT primary. I drew one in my mind here also, because I was disturbed by the drawing of the primary being capacitor-coupled, and the 6C5 driving the primary being powered through a 100 k resistor instead of through the IT primary. There is a DC current 'rating' for such transformers when they pass plate current to the primary driver stage.
I have only a seen a couple other Gibson amps using PI-IT, and neither of them used a DC-blocked (cap-coupled) primary. 'Shooter' in Michigan on one of the guitar amp sites posted that he has build capacitor coupled transformer PI's. They do work.
Let's say I throw a dart and guess that the EH-150 IT has a primary inductance of 60 H and a couple thousand ohms DCR. 60 H and 0.05 uF has a resonant frequency of about 92 Hz. An LC circuit series resonance has an impedance minimum with the primary DC R keeping it from looking like a short circuit., and parallel resonance has an impedance maximum with the DC primary resistance. Neither LCR circuit (and the schematic shows series, anyway) with a resonance at 92 Hz seems like a good idea. So I think the 0.05 uF coupled to the IT primary deserves a question mark.
With the later versions, a bass/normal switch was added. It adds/removes a coupling capacitor to change the bass response. Another version uses a pot to vary the effect of the same larger capacitor in the switch-only tone control. For an amp that tried to offer 30's HIFI sound, having contradictory RC networks in a coupling role doesn't seem like a recipe or predictable frequency response.
I bought a 13xxx s/n Style III at a bargain, presumably because the previous owner had 'modernized' it...Fender/Jensen-Italy Special Design PM speaker, a Hammond PT with 750 VCT instead of 800VCT of the original, replaced the ugly wax-leaking cardboard box Aerovox filter caps, and has 7 electrolytics where the Aerovox ones were. The terminal/tag strips all the RC components are mounted on, were apparently loaded with parts on the top side and bottom, and I missed that inspecting the mods (they also replaced paper capacitors with 6PS Orange Drop style). Since there is no Gibson schematic for this 7-tube configuration, I think there is one error in the 12xxx owner-drawn schematic, and I have no idea (yet) what the prior owner of mine actually did, I think I have to draw my own. Mine has 4 iron core magnetic components (PT, OT, IT, choke), so I believe the previous person didn't need to add a a choke in place of the field coil that was eliminated.
Earlier discussion mentioned the question of who built the early EH-150 amps. I think I read Lyon & Healy in Chicago. But which versions?...and how many versions were there? I've read about 4 Styles, with the 4th being the randomly-marked EH-150 or EH-185, but there seem to be more than 4. Someone suggested they were built one at a time with available parts, and the war-era ones may not even have been authorized during the civilian product manufacturing shutdown period (war effort)...there might be undocumented versions.
The ES-150 guitar markings (if present) after WWII are a joke (1946-49). At least the EH-150 amps had s/n's.
As far as the builder teaching Fender some things, the double-sided horizontal terminal strips with tight point-to-point wiring is not at all service-friendly. Now that it's pushing 85-90 years old, is no longer original, and works/sounds great, I'm not in a hurry to rush to restore as original. The wire insulation is pretty fragile-looking.
Last thing: Mercury sells EH-150 transformers., claiming they are exact replicas. The IT-PI transformer is $108 in 2024, and they occasionally have a 10% off sale. That makes them the only still-alive supplier of PI transformer. The original transformer p/n's (T-xxxxx format) some people say are Thordarson's I cannot find in vintage catalogs. Thordarson usually had a letter (sometimes an S) in the middle of the p/n numerals. So I think the Gibson EH-150 transformer numbers (when a schematic exists with p/n's) are GIBSON p/n's not Thordarson.
$108 plus California postage is a lot more than the Tubes and More 1:3 transformer that people don't seem to find workable. For the ridiculous prices people are seeking for EH-150 amps, or wanting restoration to original, $108 is NOT a bad price for a claimed exact replica.
P.L.T. (Post-Last-Thing) EH-160 and EH-195 prices can be laughingly ridiculous too, especially since they are not even the same circuit, and most people refuse to work on them for safety reasons (if you see a '150' for sale with 13 tubes, run like hell...you can't 'modify' it into a '150' - the chassis layout is absolutely different, and there is no power transformer...if you need to ask for explanation, heed the well-intentioned 'no' advice from others). I feel more sorry for people who sell a 160 as a 150, than the person who buys one described inaccurately by seller...buyer has some recourse, but what an unpleasant mess to straighten out (double shipping, potential damage, isn't/wasn't usable before or after)..."Tested"...REALLY?
Murray
Mercury says the EH-150 PI IT came in two footprints (a large and a smaller), so there is more schematic and version-ology the living have never encountered. I think a version of an earlier EH-amp (-100 or -125) may have used the same transformer as the version(s?) of EH-150 that did, but I got tired of trying to find a schematic (for the sake or reference or argument) that showed one drawn and identified it...I already had a '150 schematic challenge and don't have a '100 or '125 amp anyway...I was looking for evidence of a 'wet' (DC current in primary) or 'dry' (AC-coupled, so no DC in primary) schematic with the PI IT.
Has anyone ever seen the Echo Speaker with 35 foot cord that was offered (or described) as an accessory for some Style #'s of EH-150? (The ones with an 8-4-0 OT secondary and a switching output jack on the back panel). I think it had to be a PM speaker, if located 35 feet away. If it was actually also a field coil type, it would have had its own 'exciter' power supply in the cabinet (unlikely).
Has anyone ever seen the Echo Speaker with 35 foot cord that was offered (or described) as an accessory for some Style #'s of EH-150? (The ones with an 8-4-0 OT secondary and a switching output jack on the back panel). I think it had to be a PM speaker, if located 35 feet away. If it was actually also a field coil type, it would have had its own 'exciter' power supply in the cabinet (unlikely).