Two Single Ended Output Transformers in Series for Small Push-Pull project?

SerpentRuss

Tele-Meister
Joined
Feb 17, 2021
Posts
481
Age
59
Location
North Carolina
I'm noodling with a small, low power, PP bedroom combo made with parts on hand. This includes a Celestion 8 inch driver in 16 Ohm that I'd like to use. I have pairs of 6CL6 and 6AK6 valves possible for the output. I have a couple of center-tapped toroidal transformers that could be used for the OT but they're quite big and beefy and I'd like to save them for something else.

I also have a half-dozen tiny, transmission-line transformers that have a 33:1 turns ratio, which works out to about 8.7K to 8R. Just spit-balling here, but would it be possible to series two of these to join two tubes in PP to one speaker? I've successfully used one of these transformers in an SE 6CL6 project that I gave to a friend. It gets played all the time, sounds great, and is still going strong.

Series Trans.PNG
 

SerpentRuss

Tele-Meister
Joined
Feb 17, 2021
Posts
481
Age
59
Location
North Carolina
Ok,

Either this is a "Duh, how stupid can you be" question or everyone is scratching their head saying. "I don't think that will work, but I'm not exactly certain"

I think it's worth an experiment. It just sucks that something like this takes a while to set up. I really need to build a very simple PP amp to test this; three tubes, as few components as possible.

Will it start a fire? Blow a fuse? End world hunger? Transport the entire galaxy into a new space time continuum? I'll guess we'll find out.
 

andrewRneumann

Tele-Afflicted
Joined
Mar 22, 2020
Posts
1,578
Location
Cincinnati, OH, USA
Are the original transformers designed for single ended use? On a schematic it looks kosher... but I'm wondering since the primaries are separated, you don't have magnetic coupling between them, so will they both saturate with DC current? (Because they won't cancel out each other's core flux.) That is why I ask if they are SE/air-gapped. I'm in the head scratching camp.
 

SerpentRuss

Tele-Meister
Joined
Feb 17, 2021
Posts
481
Age
59
Location
North Carolina
Are the original transformers designed for single ended use? On a schematic it looks kosher... but I'm wondering since the primaries are separated, you don't have magnetic coupling between them, so will they both saturate with DC current? (Because they won't cancel out each other's core flux.) That is why I ask if they are SE/air-gapped. I'm in the head scratching camp.
They're designed for speakers, a 70v or 25V transmission line system, let's assume that since they're only designed for a couple of watts, they don't have an air gap. These are small OTs, about the same size as a Hammond 1750 AX which can only handle 3.5 watts.

I think DC current will be sub 15 mA if I go with a pair of 6AK6 power tubes. They have worked for me on a 6CL6 SE amp already and the current there was likely higher. I'm just wondering if they secondary windings needs to be paralleled for it to work properly. If that's the case, the reflected impedance may be wrong.
 

Bendyha

Friend of Leo's
Silver Supporter
Joined
Mar 26, 2014
Posts
3,122
Location
Northern Germany
Transmission-line transformers are not all the same, some are large, and made to go on the amp end of the line with high voltage and current in the primary, AC and DC to step up the secondary to the 70 or 100V AC line, and then there is the other end of the line made only for the low current 100VAC to be stepped down to the 12 or so volts for the speakers. The bigger first variety can usually work to step down like the second sort, but the little ones you describe, made as step down transformers, might not be so happy having 300V or so of DC running through their primary.
 

SerpentRuss

Tele-Meister
Joined
Feb 17, 2021
Posts
481
Age
59
Location
North Carolina
I copied the wrong thumbs. Here is the entire thread: I didn't find this in my searches because I didn't use the word "Parallel" in my search because I was thinking about series. Google is a fickle mistress sometimes.


Double SE PP Options.PNG
 

andrewRneumann

Tele-Afflicted
Joined
Mar 22, 2020
Posts
1,578
Location
Cincinnati, OH, USA
I copied the wrong thumbs. Here is the entire thread: I didn't find this in my searches because I didn't use the word "Parallel" in my search because I was thinking about series. Google is a fickle mistress sometimes.


View attachment 945401
So if I'm looking at those correctly, the one on the bottom is what you are planning. The impedance ratio looks the same as one transformer. 6:3500 is the same at 12:7000. Is that what that's saying?
 

SerpentRuss

Tele-Meister
Joined
Feb 17, 2021
Posts
481
Age
59
Location
North Carolina
I hear you. The PT that I have laying around for this is 160 volt, so after that is rectified and filtered the output will be below 200 volts. Initially, I was thinking the power section would be a 6J6 as a LTP-PI and then two 6AK6 valves. Pre-amp, two channel, one pentode 6AK5, one triode, possibly another 6J6 run in Parallel. All 7 pin tubes, all run at low volts.

Transmission-line transformers are not all the same, some are made to go on the amp end of the line with high voltage and current in the primary, AC and DC to step up the secondary to the 70 or 100V AC line, and then there is the other end of the line made only for the low current 100VAC to be stepped down to the 12 or so volts for the speakers. The bigger first variety can usually work to step down like the second sort, but the little ones you describe, made as step down transformers, might not be so happy having 300V or so of DC running through their primary
 
Last edited:

SerpentRuss

Tele-Meister
Joined
Feb 17, 2021
Posts
481
Age
59
Location
North Carolina
So if I'm looking at those correctly, the one on the bottom is what you are planning. The impedance ratio looks the same as one transformer. 6:3500 is the same at 12:7000. Is that what that's saying?
Yes, so my little transformers are 8.7K into 8, so the bottom wiring would yield 17.4K Into 16, just about perfect for my design. 20k, would be a bit better. I also found an ANTEK 107 which should yield almost identical numbers. I have tested the small SE for turns ratio, I haven't tested the ANTEK, but it's fairly safe to assume that a 7 volt AC transformer that has an input of 240 volts (Series Primaries) could would have about 18K, which split in half for PP would show each power tube 9K.

I thought I only had a ANTEK 207 on hand, but I did have one more 107, its probably the much safer bet for this project.

Still, it's interesting to think about doing this.
 

Bendyha

Friend of Leo's
Silver Supporter
Joined
Mar 26, 2014
Posts
3,122
Location
Northern Germany
The bottom two PP Parallel are a bad idea, the transformers will saturate with DC and magnetize with no gap, no cancellation....no do.
1643394997124.png
 

SerpentRuss

Tele-Meister
Joined
Feb 17, 2021
Posts
481
Age
59
Location
North Carolina
The bottom two PP Parallel are a bad idea, the transformers will saturate with DC and magnetize with no gap, no cancellation....no do. View attachment 945438
11 mA of quiescent current. I think they'll be OK. I will probably only put the pair in to test, and then replace them with the center tapped toroidal, but testing is too much fun not to do, just to see if it sounds different.
 

Attachments

  • 6AK6_PP.PNG
    6AK6_PP.PNG
    129.4 KB · Views: 9

printer2

Poster Extraordinaire
Joined
May 24, 2010
Posts
8,247
Location
Canada
So you have the possibility of a phase inverter? There is a configuration with the output tubes run as P-P and you flip the secondary of one so the signal is in phase with the other transformer. I can't remember the effects or the ratios of doing different combinations (enough on my plate today) but I looked into two SE transformers way back when (not that I remember them now), a number of possibilities.
 

Bendyha

Friend of Leo's
Silver Supporter
Joined
Mar 26, 2014
Posts
3,122
Location
Northern Germany
11 mA of quiescent current. I think they'll be OK. I will probably only put the pair in to test, and then replace them with the center tapped toroidal, but testing is too much fun not to do, just to see if it sounds different.
It might work, if it doesn't, then you will know. Без муки нет науки
 




Top