New amp build from old pa - transformer question...

[Phil_S]

If you want 8W, IMHO, a SE 6550 into this OT is not the way to go. We don't know what the impedance ratio is for this OT or what B+ you expect to run. Those questions go directly to tube selection. Just guessing here, but I want to suggest building a PP amp with a lower power tube. Maybe 6BM8, EL95, 6AK6, something like that. Give the size of the OT, I'd guess you won't be able to overdrive it with those tubes, so it should give gobs of clean headroom. You might even consider a single 6SN7 in PP configuration (2 triodes, about 7.5W), either self-split (save a tube) or concertina PI, great clean tone.

[MikeR670]

If you want 8W, IMHO, a SE 6550 into this OT is not the way to go. We don't know what the impedance ratio is for this OT or what B+ you expect to run. Those questions go directly to tube selection. Just guessing here, but I want to suggest building a PP amp with a lower power tube. Maybe 6BM8, EL95, 6AK6, something like that. Give the size of the OT, I'd guess you won't be able to overdrive it with those tubes, so it should give gobs of clean headroom. You might even consider a single 6SN7 in PP configuration (2 triodes, about 7.5W), either self-split (save a tube) or concertina PI, great clean tone.

Very interesting, thanks.....using a 6sn7 in pp self split two triode mode - does this mean one secondary to each plate of the single tube and center tap secondary to b+?

I measured the pt, it gave 400-0-400. I noticed on the old schematic that the ot b+ was 590 volts, 515 volts into the 6550C grids, then around 300 volts into the 12ax7 preamp tubes. I thought I saw one line carrying around 400 volts to something else - so then I might be able to get a b+ at say 450 into the ot, to power the single output tube?

[Phil_S]

See if you can get a few old stock USA made 6SN7. (I don't know if they were made by the Euro brands back in the day, but those would be fine, too.) They are priced from cheap to moderate. I look on eBay for multi tube lots. I have no idea if new production is any good.

I built this, attached. You can do it with one dual triode. As I recall, plate voltage was just a hair below 450 and I think you should really look to get within the data sheet spec. I used a pair (4 triodes) because of the OT I had on hand. Using 2 tubes isn't that much louder (I'm going to guess the rig is 10W) and it made a better match.

If you want self split, look at Rob's page here: https://robrobinette.com/Deluxe_Micro.htm

Do you have a choke? If you take the 400-0-400 and build with choke input power supply, you'll have no problem getting it right. I have a build thread here with a choke input power supply. I tried several chokes and there is quite a bit of info if you care to read through. I think this is it: http://ampgarage.com/forum/viewtopic.php?f=6&t=28753

If your power tube is 6SN7, you can probably use just about any choke. What gets expensive with a choke is the high mA rating you'd need, for example, to run a pair of 6L6 or EL34. In this case, your ordinary 5H-15H 40mA-50mA chock ought to be well over what you need.

[MikeR670]

See if you can get a few old stock USA made 6SN7. (I don't know if they were made by the Euro brands back in the day, but those would be fine, too.) They are priced from cheap to moderate. I look on eBay for multi tube lots. I have no idea if new production is any good.

I built this, attached. You can do it with one dual triode. As I recall, plate voltage was just a hair below 450 and I think you should really look to get within the data sheet spec. I used a pair (4 triodes) because of the OT I had on hand. Using 2 tubes isn't that much louder (I'm going to guess the rig is 10W) and it made a better match.

If you want self split, look at Rob's page here: https://robrobinette.com/Deluxe_Micro.htm

Do you have a choke? If you take the 400-0-400 and build with choke input power supply, you'll have no problem getting it right. I have a build thread here with a choke input power supply. I tried several chokes and there is quite a bit of info if you care to read through. I think this is it: http://ampgarage.com/forum/viewtopic.php?f=6&t=28753

If your power tube is 6SN7, you can probably use just about any choke. What gets expensive with a choke is the high mA rating you'd need, for example, to run a pair of 6L6 or EL34. In this case, your ordinary 5H-15H 40mA-50mA chock ought to be well over what you need.

Great information, thanks very much! I do have a choke from the old unit, so I'll read through the choke thread see if I can make a start on it now:) I'll start looking for tubes.....

Really appreciate your input and ideas!!

[MikeR670]

See if you can get a few old stock USA made 6SN7. (I don't know if they were made by the Euro brands back in the day, but those would be fine, too.) They are priced from cheap to moderate. I look on eBay for multi tube lots. I have no idea if new production is any good.

I built this, attached. You can do it with one dual triode. As I recall, plate voltage was just a hair below 450 and I think you should really look to get within the data sheet spec. I used a pair (4 triodes) because of the OT I had on hand. Using 2 tubes isn't that much louder (I'm going to guess the rig is 10W) and it made a better match.

If you want self split, look at Rob's page here: https://robrobinette.com/Deluxe_Micro.htm

Do you have a choke? If you take the 400-0-400 and build with choke input power supply, you'll have no problem getting it right. I have a build thread here with a choke input power supply. I tried several chokes and there is quite a bit of info if you care to read through. I think this is it: http://ampgarage.com/forum/viewtopic.php?f=6&t=28753
S
If your power tube is 6SN7, you can probably use just about any choke. What gets expensive with a choke is the high mA rating you'd need, for example, to run a pair of 6L6 or EL34. In this case, your ordinary 5H-15H 40mA-50mA chock ought to be well over what you need.

I like the look of your build there, would it be doable to substitute the preamp tubes for 12ax7 or 12au7? I have lots of them and then I'd only need to buy one power tube....

[Phil_S]

LOL, I used 6SL7 because I had those. It was a while ago. It looks to me like I treated them as if they were 12AX7's. You may want to fiddle with cathode resistors and bypass caps on V1.

V2 is the concertina PI (a/k/a cathodyne or split load inverter.) If you can find a concertina done with a 12AU7, you will know what values to use for plate and cathode resistors -- I'd guess they are more like 15K vs 56K, but am not sure. That one is for a 12AX7 -- look at the 5E3. edit: I snooped around and found this 12AU7 concertina at MEF.

I'd even suggest 12AU7 for the power tube, but I don't think you are going to get the plate voltage low enough.

[MikeR670]

LOL, I used 6SL7 because I had those. It was a while ago. It looks to me like I treated them as if they were 12AX7's. You may want to fiddle with cathode resistors and bypass caps on V1.

V2 is the concertina PI (a/k/a cathodyne or split load inverter.) If you can find a concertina done with a 12AU7, you will know what values to use for plate and cathode resistors -- I'd guess they are more like 15K vs 56K, but am not sure. That one is for a 12AX7 -- look at the 5E3. edit: I snooped around and found this 12AU7 concertina at MEF.

I'd even suggest 12AU7 for the power tube, but I don't think you are going to get the plate voltage low enough.

Awesome, thanks - I thought about using a pair of my 12au7s for power tubes because I noticed that the b lines available are A 520v, B 515v C 390v D 315v and E 290v. So I could maybe run the C 390v b line into the Ot. Wasn't sure if that would fly, as it's expecting A 520v..... and that line already gets used to feed the 12au7 pi tube.... but. then I could have used my 12ax7 s for preamp and pi duties?

[MikeR670]

Phil, sorry to monopolize ya on this :/

If my existing circuit uses a string of dropping resistors and 100mf can caps to achieve all these different line b voltages, does this mean I can tailor my voltages and line bs to suit my requirements for power tubes, preamp tubes and pi? It seems too easy - and I suppose the biggest issue is that the ot is expecting the largest line voltage of 520.....I think if I can solve this mystery I can move forward with the build.

[Phil_S]

Phil, sorry to monopolize ya on this :/

Really, it's OK. This is what the forum is for.

Remember, you may not like the build from my schematic, so you are taking a chance. The good thing is that you can choose whatever tonestack you favor. With an extra unused triode, you can build a cathode follower and use the extra triode for recovery. You just don't want to hit the triode power tube with too much of a swing or it will sound like poo, IMO.

The rule of thumb with choke input and solid state rectification is that you'll get B+ = 45% of 800, or around 360. If this is an old 110v or 115V primary, and your line voltage is 120 or more (common), you'll see the differential push the B+up. If you use a tube rectifier, you can drop more voltage. Design max plate voltage for a 12AU7A is 330V. You can probably get below 330 with a 5Y3GT (not the fake Sovtek) or a 5U4GB. It would be a good idea to model it in Duncan's PSU.

I don't think you need 100uF caps. 20uF caps should be plenty of filtering. Yes, to a certain extent, the power supply ladder design will control the voltage at each node. It is a function of current draw and other stuff. It can all be modeled.

The OT won't care what voltage it sees.

[MikeR670]

Thanks for all this great info, you've helped a lot!

I'll refer to my Dave hunter and Morgan Jones books to brush up on some of your points and make sure I get them fully- but I'm understanding almost all of it and it's very encouraging

Thanks again!