Gibson BR3 help

[Jerry garrcia]

Planning on an all metal PP 6V6 amp. Leaning toward a Gibson BR-3 (only 79 made). Instead I’d 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.
How can I alter the schematic so I can utilise a tube driven PI instead of the 6J5 and interstage transformer PI? Have some 6SC7, 6SL7, 6Sn7, 6J5 and 6N7’s. Wants it to sound as original as possible. The field coil I think I can substitute with a choke and possible a bleeder depending on the PT. A 5z4 rectifier will be used. Any help is appreciated.

[tubeswell]

Planning on an all metal PP 6V6 amp. Leaning toward a Gibson BR-3 (only 79 made). Instead I’d the loctal 7B4 I’m thinking of the octal equivalent 6SF7’s.
My main concerns, due to lack of knowledge, is the PI and  not to use a field coil speaker.
How can I alter the schematic so I can utilise a tube driven PI instead of the 6J5 and interstage transformer PI? Have some 6SC7, 6SL7, 6Sn7, 6J5 and 6N7’s. Wants it to sound as original as possible. The field coil I think I can substitute with a choke and possible a bleeder depending on the PT. A 5z4 rectifier will be used. Any help is appreciated.

So you want to base it on an ancient amp but not use ancient technology and not use an interstage transformer for the PI and not use a field coil speaker?? Why not just base your design on a more modern push-push amp using 6V6s and a couple of octal dual triodes and a 5Y3G rectifier? Oh, hang on a minute...

[Jerry garrcia]

So you want to base it on an ancient amp but not use ancient technology and not use an interstage transformer for the PI and not use a field coil speaker?? Why not just base your design on a more modern push-push amp using 6V6s and a couple of octal dual triodes and a 5Y3G rectifier? Oh, hang on a minute...

Because I do have all the metal octals tubes needed for the whole metal octal amp but no field coil speaker. The 5y3 is a no go since I have a 5z4 and I don’t want any GT’s in it. Also tha voltage drop in the 5z4 is a bit less than from a 5y3 so to skip the field coil and add a bleeder and a low dcr  choke I could get about the same SAG as with a field coil and a 5y3.
I do have an interstage transformer pulled from an old projector but I have never used that PI  configuration in an amp  if it was possible to make it work about the same without the transformer PI it would be much easier… guess I’m just lazy since there is, to my knowledge, no description of a clone build of such particular amp that I can copy/base my design of. I hoped that someone with better understanding of this circuit type could tell me if there is a tube PI that would sound a bit the same, cathodyne, LTP or paraphrase? The field coil I think I can live without since, to my experience with the EH-185 build and comparison, that the tonal difference isn’t much that different between a vintage alnico than from a field coil speaker from my point of view. But what do I know? That’s why I asked.

[66Strat]

The Fender Musicmaster used an inter-stage transformer as a phase inverter. Mojotone has one in stock.
https://www.mojotone.com/Mojo-Interstage-Transformer

Hammond has the 124 series of inter-stage transformers.
https://www.hammfg.com/electronics/transformers/audio/124?referer=968

If those options don't suite your needs, you may want to consider a paraphase phase inverter using 6N7s such as that in the Gibson BR6.

[bmccowan]

you may want to consider a paraphase phase inverter using 6N7s such as that in the Gibson BR6.

That's a 6SN7 in the BR6. Great sounding simple amp BTW.

[Jerry garrcia]

The Fender Musicmaster used an inter-stage transformer as a phase inverter. Mojotone has one in stock.
https://www.mojotone.com/Mojo-Interstage-Transformer

Hammond has the 124 series of inter-stage transformers.
https://www.hammfg.com/electronics/transformers/audio/124?referer=968

If those options don't suite your needs, you may want to consider a paraphase phase inverter using 6N7s such as that in the Gibson BR6.

What about this interstage transformer that I have?

[shooter]

yep, if you're talking about L3 that should work fine.
read about them, they don't take kindly to too much current.

[66Strat]

you may want to consider a paraphase phase inverter using 6N7s such as that in the Gibson BR6.

That's a 6SN7 in the BR6. Great sounding simple amp BTW.

https://www.youtube.com/watch?v=OjYoNL4g5Vg

Yeppers. I missed the S.

[Jerry garrcia]

yep, if you're talking about L3 that should work fine.
read about them, they don't take kindly to too much current.

That’s the one. Thanks! Now it’s time to start to plan the layout. Should be able to use transformer L5 as the OT since it’s PP, 2x6V6’s. Have a new toroidal 40W PP OT but would like this to be a spare parts build. Will have to measure the primaries/secondaries of the OT

[Jerry garrcia]

yep, if you're talking about L3 that should work fine.
read about them, they don't take kindly to too much current.

Pulled transformers from the projector a while ago. My dear wife did some fixing in drawer and now I don’t know which is which. Is any of these an interstage transformer or is there a way to find out?

[66Strat]

Count the terminals for each transformer as indicated on the schematic and compare to the transformers in hand. Compare the wire colors indicated on the schematic for each transformer to the transformers in hand.

[Jerry garrcia]

Count the terminals for each transformer as indicated on the schematic and compare to the transformers in hand. Compare the wire colors indicated on the schematic for each transformer to the transformers in hand.

Done that but some old replaced wires. Any way to check how the DCR would differ between an OT and an interstage transformer?
Or step up/step down voltage handling between the two types?

[Jerry garrcia]

yep, if you're talking about L3 that should work fine.
read about them, they don't take kindly to too much current.

Another question. Have you read about the IT in the BR-3? Do need to find out the winding ratio and specs to compare if the one in the Ampro works or what replacement IT I should get. Hammond have a few different and they do list theirs which for example, the more expensive mercury ones does not.

[Jerry garrcia]

So you want to base it on an ancient amp but not use ancient technology and not use an interstage transformer for the PI and not use a field coil speaker?? Why not just base your design on a more modern push-push amp using 6V6s and a couple of octal dual triodes and a 5Y3G rectifier? Oh, hang on a minute...

I will use the original circuit. Now just need to find a way of getting the info on the specs of the IT in the old ancient amp… with them old Gibsons it’s not that easy.

edited to fix quote... sluckey

[Jerry garrcia]

Found it!
“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.“

Next thing is to incorporate a switch and a 6SC7 tube so I can choose between transformer PI or a Paraphrase PI.

[printer2]

Anybody notice the 56k cathode resistor on the 6V6's? Kind of a power limiter, don't you think?

[sluckey]

Anybody notice the 56k cathode resistor on the 6V6's? Kind of a power limiter, don't you think?

What? You didn't see the decimal point in front of the 5?

[Jerry garrcia]

Anybody notice the 56k cathode resistor on the 6V6's? Kind of a power limiter, don't you think?

I thought it was some kind of typo. The plate dissipation must be really low with that resistor. Or could it have something with the IT? But that shouldn’t affect the bias. I’m on the hunt for the true voltages in the original amp.
What do you think about the possibility to change the 6J5 to a 6SN7 and adjust the circuit so the second half of the 6SN7 could be uses as a cathodyn PI? To be able to switch between the interstage transformer and a tube driven PI? Can’t find info on how much gain the IT provides.

[PRR]

> Can’t find info on how much gain the IT provides.

Didn't you just post this?

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

Figured the same way a cathodyne would be 1:1.9, so a small change.

BUT is this a Stock or a Custom? Waffle waffle waffle? You or the next guy can always put in a cathodyne. Just like a million other small amps. You seem to be in a unique position to fix it like it was built with authentic parts.

[Jerry garrcia]

> Can’t find info on how much gain the IT provides.

Didn't you just post this?

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

Figured the same way a cathodyne would be 1:1.9, so a small change.

BUT is this a Stock or a Custom? Waffle waffle waffle? You or the next guy can always put in a cathodyne. Just like a million other small amps. You seem to be in a unique position to fix it like it was built with authentic parts.

🤯 didn’t realise what I wrote and that the step up ratio would also be the raise in gain factor. I just started with amps 1.5 years ago and can’t stand just follow the beaten path. But I think you’re right regarding the cathodyne PI. I should leave it and make it so close to the original as I can. Will prepare it with the possibility to add a 6X5 for the possible field coil if I ever get my hands on one up here in the cold north.

Didn’t get it about “ BUT is this a Stock or a Custom? Waffle waffle waffle?”

Initial plan vas to just make an amp head but will now do a combo out of the Ampro projector case. I think it will be more authentic looking with just a round hole in the center for the speaker.