Baby SET w/6FD7 and ilk, for Bas and others....

[Geek]

I figure I might as well stop hijacking the "Cheap SE Transformers" topic

Did some more tweaking over the last couple of days.

Main circuit - only changed NFB resistor and dB value.

New F3: 19Hz to 23KHz @ 1W into 8 ohms.

Max clean power into 8 ohms, 2.85W. 3W @ roughly 8-10% (gotta build me a distortion meter). Of course, less NFB distortion correction = less output power.

Square response, ruler straight with only slight corner rounding on the bottom of the trailing edge, due to xformer above 5KHz. Ringing, zero. Spikes, zero. Nasty top-leading edge spike at extreme overload. Spike, not ring.

PS circuit in the next post...

[Geek]

Power supply went through a bit of a makeover. C-L-C-R-C seems optimum.

Feeding the first stage straight to the second C. SS CCS makes the isolation resistor moot.

Because of the added C, the voltage went up and I had to up the "B" resistor value and watt rating.

Floated the filaments at around +60V for hum elimination (as opposed to reduction).

Choke could still be 5-10H though. 120Hz hum is about -80dB down. I'd like it better than -100dB.

Ordered the final parts from Mouser today. Hope to have pics in a week or so.

[Kyle K]

Nice design! Have you thought about trying cathode feedback for the output stage through the OPT secondary instead of global feedback? I tried this on a 6EM7 amp using Hammond 125ESE's and was stunned by the improvement. I don't know how it would compare with other feedback circuits, but it might be worth a try.

Kyle

[moray james]

Have you tried the 6BM8 this is one of the special audio tubes. In Japan the 6BM8 is thought of as is the 300b in America. By Jack.

[leadbelly]

Originally posted by moray james
In Japan the 6BM8 is thought of as is the 300b in America. By Jack.

Really? Do they use it in some special way: ignoring the triode section, triode strapping the pentode section, etc.?

[Geek]

Originally posted by Kyle K
Nice design! Have you thought about trying cathode feedback for the output stage through the OPT secondary instead of global feedback? I tried this on a 6EM7 amp using Hammond 125ESE's and was stunned by the improvement. I don't know how it would compare with other feedback circuits, but it might be worth a try.

Kyle

Thanks!

No I haven't tried some CFB, but shall do so ASAP

Originally posted by moray james
Have you tried the 6BM8 this is one of the special audio tubes. In Japan the 6BM8 is thought of as is the 300b in America. By Jack.

Yes.

they are apples and oranges. Even trioded, you still need ~8K on the 6BM8 for 600mW out. Great for a headphone-only amp. I have some 8K SE OPT's around here somewheres...

If you wanna hear a "1 watt 300B" that's a true-triode, use a 6AH4GT at 250V and 5K plate

[Geek]

All put together for tomorrow's Northwest Audiofest

Schematic with final specs.

Excuse my crummy digi-cam

The switch on the left is "speakers on/off" with the phones jack right below it.


Cool thing is the finish matches my speakers!




That's the power switch on the back. I had put it there to keep the AC wiring as short and to the point as possible to reduce hum. However when I did a bench test, I purposely swung AC line down the side it "would have been" for a front mounted switch and hum was not an issue. So the production unit will have the AC switch front mounted, opposite side of the chassis from the speaker switch.

Also the transformer is covered (I used a surplus Z-mount that was old and ugly) here. The future one will have an exposed Edcor XPWR-106.

[planet10]

I had the pleasure of meeting Gregg yesterday, We got to swap stories 7 interesting tidbuts.

It is a lovely little amp... we discussed a few improvents, but all-in-all a lovely addition to available projects (& Gregg says he is going to get kits together )

Here is my pic with no fancy drapery.

dave

PS: Gregg, did you see where my little red screwdriver went... i think i must have left it on Al's ping pong table.

[Looneytunes]

Hi Geek,

Who's 6AH4GT tubes do you like? I just looked at the GE plate curves, and they look really good. Would you recommend this tube for a headphone amp?

Thanks,
Looney

[Geek]

Hi Dave,

Look foward to swapping a ton more stories in the future!

Originally posted by planet10
It is a lovely little amp... we discussed a few improvents, but all-in-all a lovely addition to available projects (& Gregg says he is going to get kits together )

Hopefully I'll have the info soon

That was embarrassing overlook on the terminals, LOL! I put the thing together and realized, oh... I guess I should have seperated the terminals by the standard 3/4". Nah, who will notice? Dave, the look on your face was priceless when you made the discovery!

Originally posted by planet10
PS: Gregg, did you see where my little red screwdriver went... i think i must have left it on Al's ping pong table.

Yeah, that's where I last saw it.

I almost grabbed it, then realizing it was yours, I put it on the table near the baffles. As one of the last ones to leave, that's where I still saw it as I grabbed my CD changer.

Originally posted by Looneytunes
Hi Geek,

Who's 6AH4GT tubes do you like? I just looked at the GE plate curves, and they look really good. Would you recommend this tube for a headphone amp?

Thanks,
Looney

6AH4GT is harder to get than a NOS 300B. Grab what you can, where you can and don't be surprised to start paying upwards $30/ea. for them

I'd recommend this tube for anything requiring 1 watt.

I actually wanted to go with it first, but availability made me scuttle that plan. I do plan on a limited-release 6AH4GT version of this amp if I can score enough.

Soundwise, it's an absolute 1-watt wonder!