Kofi Annan in: "Pushed and Pulled Back!"

[Kofi Annan]

Back to your original post...is the amp going to sit on a book shelf?

I'm not sure. Possibly (I haven't seen his room yet). Would that make a difference?

 

[rknize]

Picking up the ST70...an EL34 amp would be kind of heavy. I have an EL84 amp destined for the bookshelf in our office. No feather, but not so heavy I would fear for the structural integrity of the shelves.

Just more contradicting food for thought. 🙂

 

[Kofi Annan]

Picking up the ST70...an EL34 amp would be kind of heavy. I have an EL84 amp destined for the bookshelf in our office. No feather, but not so heavy I would fear for the structural integrity of the shelves.

Just more contradicting food for thought. 🙂

Based on my conversation with friend last night, this should be no problem. Of course, friend and I had just consumed two Scaldis (12%), two Founder's Red's Rye (5.4%) and a 750mL of Malheur Brut Reserve (11%), so many problems were disappearing at that time.

I'm looking more closely at the Bevois Valley (Morgan Jones) design again. I built this some time ago and I loved the results. I think I even have my NFB tuning notes. I can't post this design online as it is proprietary, but everyong likely knows this one cold.

More analysis to come.

Kofi

 

[rknize]

I built one of those about 10 years ago now. It had stability problems and I lacked the skills and equipment to address them. The OPTs and power transformer went back to the amp from which they were stolen and the remains were pilfered for parts a few times and ended up in a box. I recently found the old chassis and it will likely have a second chance at life.

 

[Kofi Annan]

I had some oscillation issues with it and while I was eventually able to mitigate them, it was a bit of a struggle as I had poor equipment and I was / am a dumbass.

Anyone with good / bad experience around the Baby Huey? This looked to be a very popular amp design and was a close cousin to the Bevois Valley.

Kofi

 

[tympani1d]

why not just build this?
http://www.diyaudio.com/forums/tubes-valves/151206-posted-new-p-p-power-amp-design.html

 

[Kofi Annan]

why not just build this?
http://www.diyaudio.com/forums/tubes-valves/151206-posted-new-p-p-power-amp-design.html

I'd actually love to, but its fixed bias and I want a no-fiddle amp for my friend. Wouldn't he need to twiddle dials, etc. if he does some tube rolling?

Kofi

 

[Michael Koster]

I'd actually love to, but its fixed bias and I want a no-fiddle amp for my friend. Wouldn't he need to twiddle dials, etc. if he does some tube rolling?

Kofi

Uh-oh, the requirements list is changing in front of our eyes... ;-)

I think you might consider class A with cathode bias to meet this requirement in a simple way. A pair of EL34s should get you in the >15W ballpark. That narrows it down some.

Michael

 

[tmhajw]

Sorry to hijack your thread Kofi but as a newbie pp builder myself (done a couple of SEs) I liked the look of the PP1c Allen Wright schematic posted by Bas on page 3, using ecc88s and EL34s but (naturally) have some questions....
Unfortunately no voltages are marked, so what is that B+1 and B+2 going to work out as? (sorry only done valve rectifiers so far...) as I thought these might work out rather low.....
Would this design suit a passive preamp?
That -ve bias point c only uses one diode?
Thanks for your time
Thomas

 

[rotaspec]

I have just finished (well, almost finished - no cosmetics done yet) my first EL84 PP driven by 6N1P cathodynes. If you are going to be playing with design of power supplies, then get PSUD. Its a Windows freeware simulator for PS design. It will answer many PS questions. It allows the use of valve or SS rectification.
You can download it here:
PSUD2

I did a quick model of the psu on page 3, and with 100mA load on B+1 and 5k resistor across B+2, the voltages are close to 400v and 200v respectively.
C voltage rail uses a half wave rectifier. It's simple and all that's needed for a low current supply.

 

[tmhajw]

Thanks for your reply.
Unfortunately I've never managed to get psud to work on my admittedly vintage mac....
Kind regards
Thomas

 

[rotaspec]

Yes, if you have a PPC Mac you are probably out of luck. It runs fine on my Intel Mac under Parallels.

 

[klm1]

tube rollin'

Kofi, check the K'wood thread in SS for info about "adjustable bias" in tube amps-sounds a little like what yellowjackets do for tubes, or Ken Fischer did...check it out.

 

[Kofi Annan]

OK-- so I spent the better part of last night reviewing the Allen Wright PPP1c EL34 and I have some questions.

1. The design indicates that the "-" input should be shorted to ground in case of single-ended input. This is confusing to me because (a) I am stupid and (b) that would mean the second pair of cascoded ECC88s ain't doin' nuthin' (see (a) for the reason I think this.

Can you help me understand what happens to the second cascode when a single-ended input is present?

2. The "B+3" on the second cascode-- is this really the B+2 or am I missing something?

3. Someone mentioned that the total circuit is consuming 100mA for a side, but I thought the ECC88s would do 30mA and the PP EL34s would do 140mA (2x70). Of course I'm wrong, but please someone tell me why.

4. The B+2 potential divider on the filament supply-- is this to lift the AC supply for the heaters?

5. Also-- are monoblocks or separate PSUs advisable here? This increases the cost but may be the only reasonable way to implement the supply given the current demand.

That's all for now... more dumb, dumb questions to follow

Kofi

 

[Eli Duttman]

The 2 halves of AW's differential circuit are coupled, at the cathodes. It's the same deal as grounding (via a cap.) the grid of the non-inverting triode in a Mullard circuit's LTP splitter.

 

[Kofi Annan]

The 2 halves of AW's differential circuit are coupled, at the cathodes. It's the same deal as grounding (via a cap.) the grid of the non-inverting triode in a Mullard circuit's LTP splitter.

I think I understand. So, this means ditch the 100R and 100K resistors on the lower cascode and ground its grid directly, right?

 

[Kofi Annan]

No love for my dumb Kofi questions?

 

[Eli Duttman]

No love for my dumb Kofi questions?

Take it easy breezy! You're on the right track. Keeping the 100 Ω grid stopper on the non-inverting side might be a good idea. Just ground that "puppy".

 

[navin]

I'd actually love to, but its fixed bias and I want a no-fiddle amp for my friend. Wouldn't he need to twiddle dials, etc. if he does some tube rolling?
Kofi

If your friend is rolling tubes then he is fiddling anyway and a little bias control wont be such a hassle.

OK-- so I spent the better part of last night reviewing the Allen Wright PPP1c EL34 and I have some questions.
5. Also-- are monoblocks or separate PSUs advisable here?

You know if I were making monoblocks you have 2 amps (boxes) anyway. In this case it might make sense to have the PSUs for both channels in one box and the small signal components and OPTs in the other box. A pair of connector cables could bring power from the PSU to the amp. This way you'll have less hum to worry about.
http://www.hirose.co.jp/cataloge_hp/e23610002.pdf

these (Harting) are available in India, I am sure you can get them in the US as well
http://www.harting-usa.com/imperia/...lectronic_connectors_brochure_73000000143.pdf

 

[planet10]

If you run into a vintage ST-70 amp for low $:

The latest boards for ST-70 are Gregg or Geek on diyaudio from classicvalvedesign.ca.

I can speak to how good Geek's amp is... he borrowed my 2 cratered ST70s and returned a working DynaMutt for the loan... it handily outperforms a BIG $5k commercial amp we compared it too. (i was so thrilled i tipped him a couple Mk III OPTs)

dave