Transformers for 6AQ5/6005 SE Build

[turbofeedus]

I'm planning a 6AQ5 single-ended build. I bought a small stash of 6005W planning to make something, finally getting around to it.
6AQ5 datasheet
Finding xformers has been more difficult than I expected, here's what I'm thinking;

PT: Hammond 261M6
215VAC secondary (non-center tapped) @ 269mA
- plenty of current if I wanted to run two 6AQ5 in parallel for more power
- I'm thinking of using Merlin's Hybrid Bridge rectifier to bring the hopefully HT down to ~300V with a 6CA4 rectifier

6.3VAC filament @ 4A
- again, plenty of current for 6CA4 and two 12xx7, maybe a pentode

OT: One of the Hammond 125SE series
- not sure how beefy to go here, I do want the option as stated above to run two 6AQ5 in parallel

Any thoughts or tips?

[Stevem]

Any Ot that works for 6v6 will work fine for a 6AQ5

[turbofeedus]

Any Ot that works for 6v6 will work fine for a 6AQ5

Thanks stevem. The replacement xformer for a 5F2 Hammond sells is spec'd at 5W. I suppose something around there will work alright.

[Stevem]

I would go with the 10 watt Hammond 123DSE if you will someday run two in parallel.

[turbofeedus]

I would go with the 10 watt Hammond 123DSE if you will someday run two in parallel.

Yep, what I was thinking as well.

[drlowmu@gmail.com]

Its been awhile since people have posted here. I've worked with P-P and SE 6AQ5s and 6005s for over ten years now, several DIY amp designs and builds, and have some Tips to share. My listening experience is with 6005s mostly, GEs.

Be advised, in my personal opinion, after 4 plus decades of designing and building tube amps, a 6005 tube, when executed well, is virtually impossible to beat VS any other Finals tube I have heard. ( Yes, I have since 1981 done optimized, zero NFB, low DCR amps using tubes like 45s, 46s, 2A3s, 6B4Gs, 300Bs, 50's, KT-88s, etc.)

Usually - it's only two stage amps, and usually directly-coupled . The 6005, tetrode operated, not even triode connected, murders them all, easily in my circuits !! The tube itself is inherently far superior to all the others I have listed.

WARNING, both the 6005 and 6AQ5's elements are tightly spaced, close to each other. Rule ONE, use no more than 235 VDC as the Plate to Cathode Operating point. Never ever near / at / or above 250 VDC P-K , LOL. In any quality build, 28 to 31 mA. is FINE for current draw, make it a Golden Ratio plate power Dissipation, 62% of 12 Watts - is superbly intelligent.

TIP : Use more than one Finals tube !! Paralleling TWO 6005s made a MONSTER of a positive listening difference, on my well - wired 104 dB /Watt/Meter ALTEC VOTTs !!!!! Within five months, I will know much more about THREE 6005s in SE Parallel, and it should be - just what I ultimately want.

For highest quality, employ a LOW DCR power supply. My Power Transformer's secondary is usually below 21 Ohms in DCR, two Chokes are only 6 Ohms each, and NOW I always find that two EH 5U4GBs, dual rectifier tubes, flat out are superior in listening performance, easily, to just one. Circuit topology is always two stages, directly coupled, the simplest topology, and the best approach I have discovered in all the last 40+ years of tube amp building and serious listening.

Below is a photo of the proposed TRIPLE, under development presently. Simple early-stage chassis / parts positioning.

Jeff Medwin

[sluckey]

Here's a smokin' hot little SE 6AQ5 amp...

http://sluckeyamps.com/smoky/smoky.htm

[Miket]

Its been awhile since people have posted here. I've worked with P-P and SE 6AQ5s and 6005s for over ten years now, several DIY amp designs and builds, and have some Tips to share. My listening experience is with 6005s mostly, GEs.

Be advised, in my personal opinion, after 4 plus decades of designing and building tube amps, a 6005 tube, when executed well, is virtually impossible to beat VS any other Finals tube I have heard. ( Yes, I have since 1981 done optimized, zero NFB, low DCR amps using tubes like 45s, 46s, 2A3s, 6B4Gs, 300Bs, 50's, KT-88s, etc.)

Usually - it's only two stage amps, and usually directly-coupled . The 6005, tetrode operated, not even triode connected, murders them all, easily in my circuits !! The tube itself is inherently far superior to all the others I have listed.

WARNING, both the 6005 and 6AQ5's elements are tightly spaced, close to each other. Rule ONE, use no more than 235 VDC as the Plate to Cathode Operating point. Never ever near / at / or above 250 VDC P-K , LOL. In any quality build, 28 to 31 mA. is FINE for current draw, make it a Golden Ratio plate power Dissipation, 62% of 12 Watts - is superbly intelligent.

TIP : Use more than one Finals tube !! Paralleling TWO 6005s made a MONSTER of a positive listening difference, on my well - wired 104 dB /Watt/Meter ALTEC VOTTs !!!!! Within five months, I will know much more about THREE 6005s in SE Parallel, and it should be - just what I ultimately want.

For highest quality, employ a LOW DCR power supply. My Power Transformer's secondary is usually below 21 Ohms in DCR, two Chokes are only 6 Ohms each, and NOW I always find that two EH 5U4GBs, dual rectifier tubes, flat out are superior in listening performance, easily, to just one. Circuit topology is always two stages, directly coupled, the simplest topology, and the best approach I have discovered in all the last 40+ years of tube amp building and serious listening.

Below is a photo of the proposed TRIPLE, under development presently. Simple early-stage chassis / parts positioning.

Jeff Medwin

That's really interesting Jeff. I'm currently working on a single ended amp using a Hammond 125ESE (15w) OT and going through the process of auditioning tubes, sounds like I need to try some 6005s.

Are you willing to share any of your circuit diagrams for your parallel single ended amps?

[drlowmu@gmail.com]

My email address is in my first Post. Please contact me privately, and I can certainly help you. Jeff

[drlowmu@gmail.com]

miket,

I hope you and others realize the schematic posted by sluckey is, ( IMHO, IME ), precisely the way NOT to operate a 6AQ5 or a 6005, although I enjoyed this Toaster Oven chassis, his actual work and his Forum post.

That Finals tube as I originally cautioned, should never be run at 250 VDC if one seeks reliability. He set it up at 280 VDC !!! ( I find 235 VDC P-K is fine as a compromise. )

Also, he is pulling 42 mA. of current !!! At 280 VDC P-K that is 11.91 Watts of plate dissipation - from a 12 Watt MAXIMUM-rated plate.

Design using Golden Ratios, 62% of 12 Watts is 7.44 Watts of tube dissipation at idle. Tube life at his existing 11.91 Watts will be dismal, measured in weeks - not years, and the amplifier will actually sound stressed from turn-on, because the Finals tube is being thermally stressed. Question : How well would we humans operate if existing in, say, 140 degrees of heat ???

Perhaps..... adding a low DCR choke INPUT is an approach slucky can use, to reduce his B+.

Maybe a couple of Hammond 159ZAs at 6 Ohms each, to reduce the B+ across the Finals. ( L1/C1/L2/C2 ) ' would have to PSUD that.

His use of a 460 Ohm choke, added series resistances in the B+ path, is FAR too high. Audio Amplifier chokes generally need to be 20 Ohms or less in DCR if you want excellent amplifier pulse response, as in a drum solos, dynamic music, etc. "I", and others who I greatly respect in audio, demand amps with such low DCRs.

slucky however, should retain the Toaster Oven chassis I think !!! Very nice.

Also, a single 6AQ5 / 6005 is not an optimal speaker-driver, as I discovered - even with my 104 dB ALTEC horn system. It truly takes at least two of such tubes, operating in SE PARALLEL, to really fly.

Jeff

[didit]

I hope you and others realize [...]

He has stated quite transparently it's "smokin' hot" ..

[sluckey]

It's very common to see class A output tubes biased for 100% (or more) dissipation. Lots of popular commercial amps do it.

[drlowmu@gmail.com]

I owe an apology to sluckey and an explanation to Miket.

My above first posts here in THIS thread, was done under the impression that this was an amp building Forum for High Fidelity tube amps, my hobby. I have just now come to realize, this is all dedicated to musical instrument tube amps, that interact with electrified guitars. My error !!!! Very little of what I wrote applies. I apologize. Have fun.

Jeff Medwin, Warrensburg, MO, USA