JJ 6V6's and output transformer.

[wozt]

Hi, the JJ 6V6's data sheet show ra-a as 10k. Was hoping to make a FDR type amp but the Classic Tone output transformers are 6.6k ra-a.
Is there a suitable alternative transformer out there or a way to compensate for this difference?

[potatofarmer]

FDR amp? And you're looking for a New Deal on an OT?

You'll notice they specify 10k at 250V on the plates. The "ideal" plate-to-plate impedance changes with plate voltage, and also if you want maximum power or minimum distortion. Tubes also work with a range of "non-ideal" plate-to-plate loads.

If you're looking to clone a particular amp, go ahead and use the iron designed for that amp. If you want to pick a new ideal OT, that's going to involve some work with the data sheet. I'd recommend an older one, as JJ's one-page sheet isn't particularly detailed. Also good to look over this: The Valve Wizard -Push-Pull

[jpfamps]

The FDR uses a much lower primary impedance than recommended for 6V6s.

The "ideal" impedance is probably nearer 12k.

Indeed some people prefer to use a 16 ohm speaker in the FDR, which would give an impedance nearer the "ideal".

Not sure why Fender chose the lower impedance; perhaps it was to keep the screen dissipation down.

Regardless, if you are making a FDR style amp, then you can either use the Classic Tone transformer and experiment with a 16 ohm load.

[g1]

Not sure why Fender chose the lower impedance; perhaps it was to keep the screen dissipation down.

I thought the opposite were true, that running into a higher impedance was a way to deal with over-dissipation?

[Chuck H]

I thought Fender chose the lower impedance for more output for a compromise in THD.?.

If you want an FDR clone then it should probably be made just as "deficient" as an actual FDR because if you "correct" the design it won't be a FDR any more.

All sarcasm aside, it's like the others have said. Idealizing for lowest distortion, highest output, whatever, isn't really the goal with most guitar amps and the difference won't hurt anything electronically. Operating the tubes as per the data sheet is probably what you want for a hi fi design, not necessarily a guitar amp. Thoughts on why guitar amps use generally lower primary impedance:

Guitar amps are hard use products that are subject to less than ideal usage. Lower primary may reduce the severity of voltage spikes (and therefor failures) due to "misuse" like clipping

Perhaps the trend can be tracked with the "power wars" when all the manufacturers were trying to offer the most wattage for any given tube compliment. As amps and speakers evolved the lower impedance, and whatever tonal contribution it made, just became part of the sound even after the highest power wasn't the goal anymore. Maybe.

Perhaps a lower primary OT is a little cheaper to make/buy?

Perhaps the designers figured out that since the data sheets most likely represent a resistive load, and speakers have a dynamic load with a much higher average load impedance than the speakers ratings, that using a lower OT primary could be a way to compensate.

At any rate, using the lower than spec sheet OT primary won't hurt anything and the amp will be more the FDR design you want than if you used a different primary.

[Tony Bones]

Hi, the JJ 6V6's data sheet show ra-a as 10k. Was hoping to make a FDR type amp but the Classic Tone output transformers are 6.6k ra-a.
Is there a suitable alternative transformer out there or a way to compensate for this difference?

If you want an FDR type amp then use FDR load impedance. Simple as that.

[Justin Thomas]

There's a "universal" Hammond OT, 15W, has (IIRC) 1.5k-22k:2-16 ratios. Hmmm. For $40 you can wire up just about anything. Okay, not your Major clone, but for screwing around (read, experimenting) I'm sure it won't burn up. If I hadn't got it for a customer's build I'd do it myself. It's probably conservatively rated anyway, so the smoke shouldn't come out.

Justin

[Tooboob]

Everybody's speaking truth.

Understand that the whole impedance thing is imprecise, because speakers aren't really resistors. Look how resistance changes with frequency: http://www.eminence.com/pdf/Legend_1258.pdf.

7.5 to 55 Ohms is a very large range. You want to optimize the sound without over-stressing the tubes too much. It's an ear thing.

[Chuck H]

I wasn't going to go here, but since another OT has already been mentioned...

Hammond also makes an off the shelf unit good to 10W (hi fi) with a 10k primary. Model 1609. I use the 1608 (8k primary) for el84 amps and it sounds fabulous. The lower wattage rating isn't a problem because the unit will never be required to deal with a 20Hz-20Khz program. Half rating the shelf models for use in guitar amps was actually suggested to me by a tech at Hammond! But, if the half rating bothers you they also make a 15W unit with a 5k primary. Just run it into a load of twice the secondary rating and the primary will reflect 10k on the tubes.

[jpfamps]

I thought the opposite were true, that running into a higher impedance was a way to deal with over-dissipation?

As you increase impedance screen grid dissipation increases, plate dissipation will decrease though.

[g1]

Thanks. I was thinking about Aiken's remarks that in cases where red-plating could not be resolved, increasing impedance would help. As you say, this would be plate dissipation, not screen.
Which reminds me of a post some time back, where someone was unable to stop the screens from lighting up. I don't think there was mention of red-plating, so perhaps decreasing impedance would have helped. I don't recall if it was resolved.

[Alan0354]

I ran my 6V6 with the Classic Tone Bandmaster/Vibrolus/Pro Reverb OT with 4K primary. I ran the 8 ohm speaker on the 4ohm tap and bump the primary up to 8K. I got 22W from all the other 6V6 with 400V +B except JJ. I have no explanation with why JJ only got 18W or so. Maybe it's defective tube or something. Never look into that. But the OT works for me.

[Gingertube]

Posted this suggestion before ...
I've beern using these with push pull 6V6 with stunning results.
It is a medium quality HIFI tranny which makes it ideal for a guitar amp. I've also used these with 6GW8 (ECL86) with equally good results.
86PP output transformer.
Even used them with switchable Ultralinear in a 6V6 Guitar Amp.
Cheers,
Ian

[Rhodesplyr]

I got 22W from all the other 6V6 with 400V +B except JJ. I have no explanation with why JJ only got 18W or so. Maybe it's defective tube or something.

I have seen this consistently with many different pairs of JJ 6V6s in several amps, so it's not a defective tube. I have two customers with Fender DRRIs who like to use JJ 6V6s, and I only get about 17.5 Watts out of them before clipping, no matter how the bias is adjusted. Swapping in old-stock Sylvanias brings the output up to 22W. I think it's because the JJ 6V6 is sort of a hybrid 6V6/6L6 rather than a "real" 6V6. I like the JJs in some applications, but they don't behave exactly like an old-stock American 6V6GT.

[Leo_Gnardo]

I have seen this consistently with many different pairs of JJ 6V6s in several amps, so it's not a defective tube. I have two customers with Fender DRRIs who like to use JJ 6V6s, and I only get about 17.5 Watts out of them before clipping, no matter how the bias is adjusted. Swapping in old-stock Sylvanias brings the output up to 22W. I think it's because the JJ 6V6 is sort of a hybrid 6V6/6L6 rather than a "real" 6V6. I like the JJs in some applications, but they don't behave exactly like an old-stock American 6V6GT.

I'll take the small loss of power with JJ 6V6's over the short and sparky life of EH or (fake) TungSol russki's or chinese firecrackers. The difference between 17.5 and 22 watts, same as 1.75 and 2.2 watts. Not much to worry about. For those who judge tone "by the meters", well stick with your high readings and keep plenty of fuses plus spare russian or chinese tubes ready at hand. Some may not like JJ for various reasons (and in some cases I agree), but I'm stickin' with their 6V6 like the old song says "until something better comes along."

My best guess at the power discrepancy is the JJ's actual Rp isn't an ideal match to DR's output transformer. A couple of posts back it's been mentioned that the DR OT primary is a low figure compared to ideal, and one can sub a 16 ohm speaker for better power transfer (plus better tone I hope). I'll have to give that one a try.