6AQ5, 6K6 build ideas

[gktamps]

Hi All,

Been checking in regularly despite malware warnings, but haven't built anything recently, and want to build some amps with some transformers I've had laying about.

Would really like to build a 6AQ5 powered amp, and a 6K6 GT amp. For OTs, I've got a Hammond 125DSE output transformer rated at 10K (less if mis-matching), and 10watts. Would this be suitable for a 6K6 GT SE build with one tube? Or better for a 2-tube parallel SE 6AQ5 build? I also have a Weber WSE15 OT (5K), and a Heyboer 5K-SE-M27.

For PTs, I've got a Weber WRVBPT (275vac) and a Heyboer PT-275. I can get a different PT if there's a better match for what I'd like to build.

Any ideas would be greatly appreciated.

Thanks!

Greg

[martin manning]

One of either tube would work in SE into a 10k load at around 300V. A Champ-like circuit would be fine.

[davent]

I built up an Octal One with the 6AQ5 for the output tube quite happy with that one. The Octal One is a derivative of the 5C1 so uses 6sj7 for the preamp.

dave

[gktamps]

Thank you, Martin. I see that I reversed the tubes in my question; meant to ask if a parallel 6AQ5 build was OK with a 10k OT, not 6K6, which I have understood would need 9-10k plate load.

Looking at 6AQ5 specs showing approximately 5.2k plate loading, would a parallel SE configuration be OK with this OT, or should I just use one of my 5k OTs for the 6AQ5 build, and the 10k for the 6K6?

Dave - thank you for the lead. I'll check out the Octal One - I see it's pretty well detailed on SEWATT.com Did you have a thread here on your project?

Cheers,

Greg

[Phil_S]

Look at the 6V6 spec sheet for the 6AQ5. They are electronically equivalent. I think that 5K is too low. The spec sheet (Duncan's TDSL) says 8.5K at 315V. I don't think that's too far from 10K. You may want to do something to knock down screen voltage.

[martin manning]

I see that I reversed the tubes in my question; meant to ask if a parallel 6AQ5 build was OK with a 10k OT, not 6K6, which I have understood would need 9-10k plate load.

Looking at 6AQ5 specs showing approximately 5.2k plate loading, would a parallel SE configuration be OK with this OT, or should I just use one of my 5k OTs for the 6AQ5 build, and the 10k for the 6K6?

Since Za can be estimated (Va-k^2)/Pa max, you can rearrange to get Va-k for your 10k OT and an assumed plate voltage: Va-k = Sqrt(Za Pa max). That will give the voltage for a center-biased, 100% dissipation idle point.

For 1 6K6 and 10k Za, Pa max = 8.5W, so Va-k = 292
For 1 6AQ5 and 10k Za, Pa max = 12W, so Va-k = 346 (note spec Va-k max is 275, which would require a 6k3 Za to center bias)

The screen should be ~50-75V lower than the plate.

If you double the Pa max by paralleling two tubes and reduce the load by half to 5k, the Va-k will be the same.

[davent]

Dave - thank you for the lead. I'll check out the Octal One - I see it's pretty well detailed on SEWATT.com Did you have a thread here on your project?

Cheers,

Greg

Hi Greg, I did and i even found it. Was back in 2010.

Also made their Savage Croaker which is 6U8/6AQ5.

Octal One build thread http://www.sewatt.com/node/20651

... have to be registered there to see anything, blah...

Presently, almost finished and have been for months, a 6AQ5 HoSo56. Will have to see when i feel moved again to dive back in.

[IMG:800:800]http://i216.photobucket.com/albums/cc30 ... iywlcd.jpg[/img]

[IMG:800:267]http://i216.photobucket.com/albums/cc30 ... ueklv5.jpg[/img]

dave

[gktamps]

Hi Martin,

Thanks for taking the time to post this equation. It's time I got familiar with making these calculations so I can move up to the next step of amp building.

Thanks again!

Greg


[/quote]
Since Za can be estimated (Va-k^2)/Pa max, you can rearrange to get Va-k for your 10k OT and an assumed plate voltage: Va-k = Sqrt(Za Pa max). That will give the voltage for a center-biased, 100% dissipation idle point.

For 1 6K6 and 10k Za, Pa max = 8.5W, so Va-k = 292
For 1 6AQ5 and 10k Za, Pa max = 12W, so Va-k = 346 (note spec Va-k max is 275, which would require a 6k3 Za to center bias)

The screen should be ~50-75V lower than the plate.

If you double the Pa max by paralleling two tubes and reduce the load by half to 5k, the Va-k will be the same.[/quote]

[gktamps]

Hi Dave,

Just checked out your thread on SEWATT. Wow! Nice build. I like your use of small sub-boards. You made a gem!

Your current build has lots going on for a low-wattage amp. Looks great.

Thanks for the leads, Dave,

Greg

[gingertube]

If you want to go serious vintage then look at:
http://www.diyaudio.com/forums/instrume ... ifier.html
The tone control around the output tube works really well. The 6J7 triode strapped into the 6J7 pentode sounds lovely but 6J7 are a "bitch" for parasitic oscillation. Substitute 6SJ7 or even EF86 or 6BR7 for the 6J7 for easier build. (the 6J7 has a grid1 top cap, you need to run screened wire up to it and sometimes even solder a gridstop to the topcap.
This amp, as described there with tube rectifier and field coil speaker was just stunning with my Fender Telecaster. Ideal amp to cart to the Sunday arvo, Jam, Nosh, Pissup etc. with the mates.
Cheers,
Ian

[10thTx]

almost finished and have been for months, a 6AQ5 HoSo56

Yrs ago, I built a HoSo56 with 6AQ5's also. Great amp! A friend kept asking me to sell it to him so I don't have it any more. I really enjoyed playing it. Very touch sensitive response with the 5879 V1.

With respect, 10thtx

[JazzGuitarGimp]

Hi 10thTx, the plate load resistor at V1 is 66K?

[10thTx]

Correct V1 is 5879 and had a 66k plate resistor on this amp.

DaGeezer's original HoSo56 had node E with 219v with a 56k plate resistor for 141v on 5879 plate. He used EL84's in his running 350v on plates

With respect, 10thtx

[Kagliostro]

For the 6ak5 now you have a good plan

Here a 6k6 Herzog inspired toy, so you can use also that tube

http://el34world.com/Forum/index.php?to ... #msg172277

[img:947:583]http://i.imgur.com/c6L23wD.gif[/img]

K

[gktamps]

Thanks, Kagliostro,
Greg