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6v6 PP choosing between 9k and 12.8k tran

Schmidlapper

Schmidlapper

Super Member
I have a great sounding and looking little rebuilt Magnavox 8802 that really is limited by the output transformers, I also have an old pair of 30w PP transformers laying around, so I think to myself project time. I just don't know which load would be the better option. RCA says 10k for 6V6 pp, but my two options for 8 ohms are a 33.8 turn @ 9.1k or 40 turn @ 12.8k, is one better? Is being closer to 10k but lower better than much higher? Primary is ~80 H and 135 ohms.

Reasoning here is some common thoughts are, the iron is the heart of the amp, also some say go 4x greater than needed. These transformers will fit the space with some new drilling and me moving where I put the speaker connectors originally.

Appreciate some veteran input. Thanks.

Bill
 
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I'll refer you to my thread about a modified 8802 where I changed out the output transformers: http://audiokarma.org/forums/index....-magnavox-8802-in-the-gillespie-style.730098/

I used an 8K primary OT in that case, and made some other changes to the power supply and added @dcgillespie's EFB(tm) (tweaked for 6V6) to the output stage to optimize it for that particular OT. The amp sounds wonderful, and produces more power than the original. Schematics are included in that thread, and I'd be happy to answer any questions about it you might have!
 
I've read your thread while researching my own design choices. Right now with the original iron installed I am pretty good up to ~3.7 watts across 50 to 20khz, and it has a pretty nice 1K square wave and the 2K isn't real bad. The low end peters out first, as expected from the stock iron. I'm hoping these better transformers could get me closer to 6 watts or more until the volts run out.

If photofacts is correct and secondary for 4 ohms is 6k, then 12k reflected for 8 ohms. I can get the 12k, but RCA recommends 10k.

thorpej said:
I'll refer you to my thread about a modified 8802 where I changed out the output transformers: http://audiokarma.org/forums/index....-magnavox-8802-in-the-gillespie-style.730098/

I used an 8K primary OT in that case, and made some other changes to the power supply and added @dcgillespie's EFB(tm) (tweaked for 6V6) to the output stage to optimize it for that particular OT. The amp sounds wonderful, and produces more power than the original. Schematics are included in that thread, and I'd be happy to answer any questions about it you might have!
Click to expand...
 
Leakage inductance is proportional to the square of the turns ratio, so OPTs with lower turns ratio perform better at high frequencies, all else being equal. This could be a decisive factor because OPT changes generally come with a requirement to re-optimize the feedback network, and that task gets more difficult as the turns ratio goes up.
 
So if I go with the 9k, I benefit by the lower 33.8 turns, and closer to 10k.
I also have a 26.8 turn, 5.7k tap that would be close to the 6K. I don't think 6k should be the target though.

These were jukebox amp transformers, hence lots of odd taps.


BinaryMike said:
Leakage inductance is proportional to the square of the turns ratio, so OPTs with lower turns ratio perform better at high frequencies, all else being equal. This could be a decisive factor because OPT changes generally come with a requirement to re-optimize the feedback network, and that task gets more difficult as the turns ratio goes up.
Click to expand...
 
Since your OPT allows you to choose between different loads, why not just pick one and then switch the load later on. You could even put in a switch between the 2 choices and then flip between one and the other. But, it is not advised to switch high voltage loads when they are energized.
 
Ultimately I will pick a tap, just wanted to gather input on pros or cons of the choices. Don't really see myself adding a switch will have enough fab work retrofitting the transformers.
I'll pose the question differently, would using the 9.1k tap for a 6v6 pp amp be a bad choice, over the 12.8k tap?
 
Based on my own experiences,I believe that (from the standpoint of impedance only) the 9.1 K will yield better results overall.
 
It runs 263v B+, 263 screens, 246 drivers and 17v cathode with a CLC filtered tube rectified power supply. Was running 271, 270, 254 with the the RC filtering. Much cleaner B+ With choke. I believe the 9k tap is the best choice also. Was using them in a 7868 pp amp, I like the little Maggie more.
 
Is that cathode to plate, or plate to ground? Voltages in the datasheet are cathode to plate volts.
 
yeah that should be good for ~20 volts, which is about perfect. 6CA4 is a fairly decent little rectifier though, not a terrible lot of voltage drop and it handles a reasonable amount of current.
 
Modeled it in PSUII and then stuck the 1N4007s in and upped the first cap to 44uF, the new voltage measurements are now:
Plate 298v
Screen voltage: 297v
Driver voltage: 278.2v

Scoped it, but only had ~1v from my iPad. Looked very good, will retest with 2v source sometime. I am running it with passive pre. It obviously has a fuller sounding bass, and oddly I had to adjust my JBL L19 tweeter control back to mid level, I had found it a little shrill previously and dropped it back some. Will update with the scope results at full line level.
 
Sounds like the output tubes needed the extra bias current, that you gave them, changing to the higher B+ voltage. That likely reduced crossover distortion (making them sound less bright).

The higher B+ may have also increased the gain on the input tubes a bit- making for more effective feedback. That also helps balance things out.

Regards,
Gordon.
 
Hopefully tonight I will be able to do additional scope testing. It sounds quite good. The power supply models well and well filtered. I appreciate all the input, much of my research has been through reading all of you guys posts on here.
 
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