Use of a PP OT in an SE circuit.....The good, the bad, the ugly!!!

Wharfcreek

Wharfcreek

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In a separate thread I have going I'm looking at building an SE circuit that is designed for a 6N6G output tube. The OT specified in this circuit is a Hammond 125D, which is a 'universal' PP type OT rated for 10 watts. The way the schematic reads the two plate connections are used much like a conventional SE OT would be wired, and the center tap is essentially just taped off. The output selection is shown as pins 2 and 5, which the Hammond chart shows as being an 8.2K PI at 8 ohms. I believe the 6N6G is optimized at about a 7K load at the 280 volts in this circuit. According to the original builder of this amp, it sounds great! He even posts a link to another person who also built the circuit and loved it!

So this brings up the whole question of use of a PP OT in SE application. To the best of my understanding, this is generally consider 'bad form'. I believe at some level, it's probably just a bad idea and could be the kiss of death to an otherwise successful build. However, as this is a relatively low power amp, is it possible that under these lower power conditions the pitfalls of using a PP OT in SE application are essentially nullified?

I know very little about the 'science' of transformer construction. But, I do know that at some point I was given some information that specifically stated that use of PP OT in an SE design is really just NOT a good idea. I think this might have been back when I had an old Fisher amp that had an OT in it that had gone dead on one side of the center tap. The amp still worked, and surprisingly enough the 'negative' audible aspect of being SE on one channel was not as obvious as one might have guessed. Clearly that amp was operating at a higher power level. So, not the same as what this 'flea' power amp would be. Anyway, I'm posting here to get some opinions. Also, I might ask if anyone has any other ideas for possible 'other' OTs in this amp's construction....still SE as well.

Many thanks, Tom D.
 

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The issue with using a PP output transformer in an SE circuit, is core saturation. In a push pull application, the DC current through each tube cancels in the output transformer, so the transformer sees only an AC signal. In the single ended transformer, there is no cancellation, therefore, a single ended transformer uses a gapped core, which is less efficient and therefore heavier and more expensive, but also allows for unbalanced DC in addition to the AC signal.

So what happens when you operate a push pull transformer as single ended? The core saturates, or approaches saturation, resulting in less power and much greater distortion at low frequencies.

Compare the weight of single ended and push pull transformers with comparable specifications to understand the penalty of dealing with unbalanced DC.

Why is that Hammond okay? Some universal output transformers have a gapped core and a center tapped primary, to allow use as single ender or push pull. These are low cost "universal" transformers with poor performance by hifi standards, originally intended as universal replacements so radio repair men could have a limited stock of spare parts and repair any radio with a bad transformer. This is a very non ideal transformer for a hifi amp, not because it is universal but because it is cheap and has non hifi performance.
 
Well, that rather puts a halt to my initial plans. As it works out, I'd originally purchased this 125D OT for use in a 'micro-Deluxe' amp build, which is a guitar amp. There's a gent named Rob Robinette who does a lot of oddball stuff, including having posted some info about making a 'mini' version of the old Fender 'tweed Deluxe' amp, originally made by Fender with a push-pull 6V6 circuit. Rob's circuit uses this 125D as a push-pull but for use with the two triode sections of a 12AU7. I believe the load factor is like 22.5K PI when doing this at 8 ohms on the secondary. I guess you get all of about 1.5 watts out of it. Almost not worth the effort, but 'hey'...... it's a 'project' and something different. Most guys are doing away with 'big' amps these days anyway.

So, back to the 6N6G: Looks like our friends at Edcor produce the XSE10-8-8; a 10 watt SE open frame OT with an 8K PI @ 8 ohms on the secondary. Not overly expensive at under $25 each. So, maybe in the interest of resurrecting this 6N6G project as a 'stereo' amp, and doing it with some level of success in the end, maybe I'll put out the $75 for two of these OTs plus the $25 in shipping and maybe by the time I get them from Edcor I'll be ready to build the thing.

Thanks Max! BTW, if I DO complete this project, and if I've not completed the micro-deluxe amp build, maybe I'll build one channel with the Edcor and the other channel with the Hammond and see how the two compare to each other. Honestly, I gotta believe that the Edcor will be hands-down the winner! AND, if I'm not mistaken, it's even less $$ to purchase the thing in the first place. Shipping may be less on the Hammond, as Edcor is NOT bashful about charging for shipping. But, I've had pretty good luck with Edcor stuff of late. Maybe they're 'upping' their game in the Design and Manufacturing depts!!

Tom D.
 
Depending on the guitarist, the added distortion from using that transformer in a guitar amp may be desirable?
 
While a bit 'off topic', I'll just state for the record that (to me) a guitar amp has many of the same qualities as the guitar itself. As such, it should be considered as much an 'instrument' as the guitar. Like attempting to build a guitar, you can build an amp, and it 'may' or 'may not' be 'playable' to your tastes or desires. Yet, to the next person, it may be 'perfect' (for a specific purpose at a specific time). I've built a LOT of both guitars and amps. Been around the music business for a while too. When considering the production stuff, you can take 10 of the same guitar off the store shelves, and of the 10, there may be 2 or 3 you'll discard immediately for one reason or another. Likewise, there may be one or two that you have an exceptionally difficult time choosing between if you're trying to find....'The One'!! You'll eventually settle, or keep looking, but in reality, amps are the same way. Players just don't usually put as much effort into the choice since they don't hang the amp off their shoulders, nor is it 'body art' when on stage. But, to the discriminating musician, they well almost all agree, out of 10 'similar' production amps, a couple will sound bad right out of the shoot, most will be OK, but a couple will be 'great' for one reason or another. The you get down to the 'choosing' stage, and again, it can be a real source of frustration.

Anyway, as Lavane points out, using a 'poor choice' of transformer can indeed add to the sonic signature of a given amp. As an example, back in the day, Fender built that Tweed Bandmaster amp with three 8 0hm speakers and used the same 2 ohm output transformer on it that was used in the 4 X 10 Bassman and Super amps (and later the Super Reverb). The impedance mis-match in the Bandmaster produced enough of a change in sonic signature that it became quite popular among a LOT of guitarists.

Some times ya just gotta give something a try before you either sing it's laurels or sign it's death certificate!!
 
different goals. Instrument amps are indeed part of the instrument. Hifi amps should be designed to be as invisible as practical, adding distortion or whatever should only happen to the extent it can't be avoided.
 
I can't say I've heard a case, where transformer saturation in the bass, has made a guitar amp sound better, at least to me. It just makes it sound thin and tinny, in many cases.

If you like that hard-core extreme Marshall- turned-up-beyond-11 "rip your face off" sound, then maybe it would be what you want... but for that Fender-esque "bluesy" sound, not so much.

Regards,
Gordon.
 
Wharfcreek said:
In a separate thread I have going I'm looking at building an SE circuit that is designed for a 6N6G output tube. The OT specified in this circuit is a Hammond 125D, which is a 'universal' PP type OT rated for 10 watts. The way the schematic reads the two plate connections are used much like a conventional SE OT would be wired, and the center tap is essentially just taped off. The output selection is shown as pins 2 and 5, which the Hammond chart shows as being an 8.2K PI at 8 ohms. I believe the 6N6G is optimized at about a 7K load at the 280 volts in this circuit. . . . Also, I might ask if anyone has any other ideas for possible 'other' OTs in this amp's construction....still SE as well.

Many thanks, Tom D.
Click to expand...
The data sheet for the "universal" PP 125D clearly states that it's not for SE use and suggests something from their 125SE series instead. The specs of the 125D also list a frequency response that only goes down to 150hz.

For use with the 6N6G, the 125DSE can handle the current and its frequency response goes lower, down to 100hz. But the tube wants to see a 7k load and the choices available are 2.5k, 5k, and 10k. Digi-Key has them for $64 and you can get free shipping.

For guitar use, I'd try one of the standard 10w OTs sold by Musical Power Supplies (MPS). They have dual primaries (5k and 7k) and multiple secondaries. They're available immediately via eBay or their website for about half the cost of the Hammond SE model.

With Edcor you'll likely have to wait at least 3 or 4 months to get one.

For stereo use, MPS offers a 10w Hi-Fi version with a 7k primary. Like Edcor, they are made to order but don't seem to take as long. I ordered a pair for an upcoming project (SE 47 SEP) and the delivery time was considerably shorter than the 15w 8k OTs that I got from Edcor.

You also need to consider the output of the PT. If you look at the data sheet for the 6N6G you'll see that power output is determined by the plate voltage because the tube is cathode biased using a resistor that's inside the tube. At a plate voltage of 250v, output is 2.5w. At 300v, it's 4w and at 325v (which is the max), it's 5.2w. All power ratings are at 5% distortion.

IMO, the schematic you posted is probably using the 12AX7 because it uses tone controls. 12AX7 might also be good if it's a guitar amp that might benefit from a hot signal.

But if you're building a stereo version, I'd look at a lower mu tube for the input because the 6N6G is fairly easy to drive. Having just built an amp using the 26 and 6N6G

https://www.audiokarma.org/forums/index.php?threads/the-nuance-my-inverted-set-diy-project.990205/
I would recommend using a DHT input tube. But if you go with an indirectly heated tube and want to match the ST shape of the 6N6G, you could try a 27 or 56. If matching the ST shape isn't important, I'd look at a single 6SN7, 6CG7 or 12BH7. Or maybe use a pair of 2C22s with their wacky dual top caps.
 
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