Gibson BR-3 interstage transformer?

[Jerry garrcia]

Posted this on a lot of forums and need help. Stuck.

Planning on an all metal PP 6V6 amp. Leaning toward a Gibson BR-3 (only 79 made). Instead of the loctal 7B4 I’m thinking of the octal equivalent 6SF5’s.
My main concerns, due to lack of knowledge, is the PI and not to use a field coil speaker.
As I understand of the schematic the amp is designed to be run with or without a field coil. A 5z4 rectifier will be used so no fiel coil for me but an ancient alnico speaker.
My main concern is the interstage transformer Phase inverter. Never seen it before and don’t really understand it and am unable to find the specs for it. Winding ratio and so on. I have all the tubes, PT, OT and have this IT from an old ampro projector. Will be able to get the winding ratio from that but it doesn’t answer if it is the same specs as the BR-3 one. I could also get an Hammond IT but which? Read in a lot of old Hi-Fi forums that they usually are 1:1 up to 1:3. But their function as a PI? And a way to find out the right one without paying a lot for a mercury one (with no specs included). Really like the sound of the amp and want to get me my own.

Any help is appreciated.

[martin manning]

I think you could get it to work without too much trouble, but who knows if it will sound like an original. The load on the 6J5 is the 100k (2x 50k) across the secondary times the transformers impedance ratio, so you could adjust that for your transformer. Since it's a triode, it won't be too critical. The power stage is cathode biased, so the transformer's secondary center tap, and the grids, are ground-referenced.

[Phil_S]

I believe Mercury Magnetics makes a replacement for the interstage transformer. Yup. Here:
https://www.mercurymagnetics.com/produc ... ransformer
They are pricey and not specifically for the BR-3. I suspect the one for the EH-150 is the right one. I know about this topic a little because I have a much later (c. 1965) GA20-RVT and managed to blow the interstage transformer. The generic Hammond didn't work right. I was about ready to buy it when a full transformer set came up on eBay for cheap money. As hard as I tried, I could not understand how to find the math that allows you to calculate the correct winding specs. I get the concept but not the engineering science. I know you didn't ask, but, if you ask me, I'd tell you just use an ordinary tube driven PI. I suspect the typical LTP will do a nice job.

I'd offer to pull the amp chassis and see what I can find out about turns ratio, but it's in storage. Anyway, turns ratio is just scratching the surface. I can tell you, however, that it is a small transformer. I'd guess the core is about 1 cubic inch.

[Jerry garrcia]

I believe Mercury Magnetics makes a replacement for the interstage transformer. Yup. Here:
https://www.mercurymagnetics.com/produc ... ransformer
They are pricey and not specifically for the BR-3. I suspect the one for the EH-150 is the right one. I know about this topic a little because I have a much later (c. 1965) GA20-RVT and managed to blow the interstage transformer. The generic Hammond didn't work right. I was about ready to buy it when a full transformer set came up on eBay for cheap money. As hard as I tried, I could not understand how to find the math that allows you to calculate the correct winding specs. I get the concept but not the engineering science. I know you didn't ask, but, if you ask me, I'd tell you just use an ordinary tube driven PI. I suspect the typical LTP will do a nice job.

I'd offer to pull the amp chassis and see what I can find out about turns ratio, but it's in storage. Anyway, turns ratio is just scratching the surface. I can tell you, however, that it is a small transformer. I'd guess the core is about 1 cubic inch.

The mercury route is the expensive one and hoped to be able to avoid it. Especially since they keep their specs for them self.
The ampro one I have is also a tiny one with about size as yours. I’ll measure it up this weekend (maybe today depending on work). The best is that if it would work since then I’d be able to just utilize old parts, except for some caps.
Since it’s quite irritating that the parts list and specs seldom are published for the ols Gibson stuff, especially the rare jewels. As Gibson previously worked the parts might be the same as another part used in an amp from the same time period.
I was initially planning on a tube pi but was told by users that much of the magic of the BR-3 is due to the IT and mot as much the field coil.

If you have such an amp it would be highly appreciated if you took the time to check the winding ratio. It’s really easy if you have a 12V transformer or a variac with thwo DMM’s. Uncle Doug has a good description video on YouTube about on how to do it with OT’s. Same principle.
Also if you pulls out yours could you please take some high res pictures of the layout and part. Suspecting a true rats nest in there.

[Stevem]

I have worked on a ton of SS amps made from the mid 60s to the 1971 era that made use of a driver and PI in the same case which is the same for tube ones.

In checking out the sound details between practically the same amps with or without them what I can tell you with certainty is that there only plus is that coupling caps are not needed and the trouble they can make when driven hard and with the effects of charging and discharging.

Other then that they limit the overall frequency response of the amp in terms of both low and top end, but if you want this degraded function you can get it far cheaper with a tube.

My tests in regards to the SS amps that have used these shows that they really start to roll off the top end starting at 5.2K, which is likely a tad higher then where a tube type one would carry to.

I guess if your looking for a mellow Jazz guitar tone of the late 30s then around that frequency range Is where you want to play around with rolling off the highs if your just using a tube PI/ driver.

Also don’t forget that just the low power supply voltages that the preamp tubes ran on in these amps makes for a mellow low gain sound.

My 1954 Gibson GA 75 uses a 6J5 and when I had the chance to A/B it with a BR-3 thru the same speaker it was far more useful for different things.

I am not trying to steer you one way or the other, just providing empirical results.

[Phil_S]

If you have such an amp it would be highly appreciated if you took the time to check the winding ratio...

The problem is that I can't get to it without a major effort. However, I have some good news. I managed to find my newsgroup post (alt.guitar.amps 10/29/08) that documented what I could determine at the time:

"I'm posting here for the record, as I am not the only who has asked the question. I have a working pull from a GA15-RVT Explorer. This is the exact same transformer that was used in the GA20-RVT Minuteman. I know this because I've got a bad one (blown primary) from a GA20-RVT and they carry the same part and mfg #, and the secondary DCR's are very close. The part number is T-1001-D. The EIA manufacturer's code is 757 or Grand Transformers.

The laminations measure approx. 1 5/8" wide, 1 1/4" high, and 5/16" thick (measuring across the lams on the bottom).

The bell dimensions are approx 1 1/4" across, 3/4" high, 1 1/8" wide (going across from right to left when looking at the end bells).

Primary DCR (red to blue) is 724 ohms. Secondary DCR is 3436 ohms (green to green/white), 1834 ohms (green/white to yellow CT), 1603 ohms (green to yellow CT).

Finally, the turns ratio: 5.43vac input to the primary gives 12.92vac across the outer legs of the secondary. Let's call this 1:2.4 turns.

I don't have an LC meter, so I can't tell you the H on each side, sorry about that. This should, however, serve as a permanent reference for those who want to know. "

The lam dimensions are provided. You can derive the core size from that information and from that, a decent hint at other specs including the core, which I'd say is slightly larger that 1 cu in.

[FUCHSAUDIO]

Pretty sure someone said Heyboer made something like this....

[Jerry garrcia]

If you have such an amp it would be highly appreciated if you took the time to check the winding ratio...

The problem is that I can't get to it without a major effort. However, I have some good news. I managed to find my newsgroup post (alt.guitar.amps 10/29/08) that documented what I could determine at the time:

"I'm posting here for the record, as I am not the only who has asked the question. I have a working pull from a GA15-RVT Explorer. This is the exact same transformer that was used in the GA20-RVT Minuteman. I know this because I've got a bad one (blown primary) from a GA20-RVT and they carry the same part and mfg #, and the secondary DCR's are very close. The part number is T-1001-D. The EIA manufacturer's code is 757 or Grand Transformers.

The laminations measure approx. 1 5/8" wide, 1 1/4" high, and 5/16" thick (measuring across the lams on the bottom).

The bell dimensions are approx 1 1/4" across, 3/4" high, 1 1/8" wide (going across from right to left when looking at the end bells).

Primary DCR (red to blue) is 724 ohms. Secondary DCR is 3436 ohms (green to green/white), 1834 ohms (green/white to yellow CT), 1603 ohms (green to yellow CT).

Finally, the turns ratio: 5.43vac input to the primary gives 12.92vac across the outer legs of the secondary. Let's call this 1:2.4 turns.

I don't have an LC meter, so I can't tell you the H on each side, sorry about that. This should, however, serve as a permanent reference for those who want to know. "

The lam dimensions are provided. You can derive the core size from that information and from that, a decent hint at other specs including the core, which I'd say is slightly larger that 1 cu in.

[Phil_S]

That's very kind of you. I want to be sure you understand that transformer may or may not work in the BR-3. Good luck.

[Jerry garrcia]

That's very kind of you. I want to be sure you understand that transformer may or may not work in the BR-3. Good luck.

Just got the info. It’s a 1:3 winding ratio. The crazy thi with this amp are the voltages. It’s a PP 6V6 amp. PT 375-0-375, B+ 464, Vp 450V, screens 167V and cathode 12V.
I will have to reconsider using old radio stuff for this one since none of them can produce that voltage.
Still can’t find my IT so will go with the Hammond 124A

[Stevem]

It’s the mainly the screen voltage that sets the current peak, so that low 167 it what keeps that 450 from harming the tubes.

[Jerry garrcia]

It’s the mainly the screen voltage that sets the current peak, so that low 167 it what keeps that 450 from harming the tubes.

But would you benefit from that regarding tonality? what would be “better” with that setup compared to a more commonly used Vp of 350V and screens at about 315V?

[_Steve]

1:3 is on the high side for a IT in a guitar amp. Keep in mind that because the PI tube is inductor-loaded it will swing well above the B+, so you run the risk of slamming the power tubes even when the preamp section is very low. IT PIs are usually fed a lower B+ to compensate, usually by taking it from a more 'downstream' node closer to the input node.

Also, the key specs you need are primary inductance, primary DCR and max current, along the winding ratio.

I'll also warn you that its incredibly hard to get hard, factual information about their operating characteristics and implementation. Most people, like me, understand the basic principles, but as soon as you ask serious questions like how to approximate it's loadline or frequency response you get silence.

[Roe]

mojotone offers this interstage based on the fender music master: https://www.mojotone.com/Mojo-Interstage-Transformer

[Murrayatuptown]

I am looking at Gibson amp schematics that use the T-8790 interstage transformer phase splitter.

I see it in an EH-150 'Style III', an EH-100, and once in an EH-125, which I cannot find tonight. I found a BR- amp (probably 3) amp with a IT, but this post shows T-1001D for the BR- amp. Or was it for the GA19 or 20..and which one?

There is a user-drawn (could it be by Colossal?) EH-150 Style III (6F5 - 3x 6C5 - IT-PI - 2x 6L6) schematic that has a 0.05 uF coupling capacitor feeding the IT primary. The 6C5 is fed plate voltage thru a plate resistor and NOT the IT. Thus there is no DC thru the primary as the coupling capacitor is also blocking the DC.

I saw one other amp with a coupling/blocking capacitor like this but most of the others do feed DC thru the IT primary. That is a big difference. Probably not just the LF response of the IT, but the impedance the 6C5 sees (and the impedance the 6L6's see). Chicken or egg, or both. Change that to BIG.

There's enough mystery around what the 6L6 power tube grids reflect to the 6C5, and running the IT 'dry' (no DC) vs. 'wet' (DC thru primary) means the multiple amps that use the T-8790 with DC or without DC thru the primary, and with different driver tube and power tubes, suggests the T-8790 transformer 'isn't too fussy'. It's used in two or three or maybe more Gibson amps.

I have a '150 with the IT PI, but I bought it modified, so I am not sure I trust what's been changed. It has 6PS orange drops on the top side of the tag/strip board. Last time I looked inside I didn't realize the preamp tube cathode bypass capacitors would be hiding under the tag/strip board. It's bowed (like maybe the capacitors underneath are GONE), and there are 7 electrolytics (all HV) where the Aerovox wax-potted cardboard box originals were. The only original capacitor is recognized was the postage stamp mica (presumably 75 pF, if I can trust the Gibson schematic of the week).I could not see any capacitors under the board, but also didn't know that's where they originally were. Back in this week with a borescope & dental mirror (and magnifying glass & tweezers)

I've been trusting the Style III schematic someone posted because they couldn't find one. But I don't know what the hell is in my '150, or whether it's 'correct'

I bought a Mercury IT PI for a future '150-III inspired build a friend would like. Also because who the hell knows what the T-8790 properties actually are? I will spend some time figuring out turns ratio and inductance on the Mercury, although I don't trust iron core inductance measurements at test instrument arbitrary drive level...at least I can compare (hopefully) the Gibson transformer with the tubes pulled. It SHOULD match the Mercury (actually, vice versa). And figure out whether my amp actually has a coupling/blocking capacitor feeding the IT PI.

My '150 works, sounds good, and has lots of bass (has the normal mode/bass mode switch), so I guess it was modded by the previous owner (parts like 6PS Orange Drops and Hammond PT all have 2007 date codes) carefully, one part at a time, since there didn't seem to be a reliable schematic for the Style III that long ago.

I think I have a love/hate relationship with Gibson.