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Old 21st March 2012, 01:59 AM   #1
mfaughn is offline mfaughn  United States
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Default Musical Machine (triode PP EL34) question(s)

I am pretty sure that I would like to build a MM with the iron I have. Can someone double check that I'm not way off base? The output iron is has ~4.2K primaries. I know this is a bit lower than optimal. The power transformer should be able to do ~365V if sand rectified...also a bit lower than optimal. It doesn't have to be perfect, just have a reasonable chance of working.

I'm attracted to this amplifier because it seems to be one of the better fits I can find for the iron I have, I have EL34 tubes languishing in a drawer, and its a relatively simple circuit. Also thought about a Cit V. If anyone has any other suggestions I am all ears. I'm not really up to designing something else -- I'm slowly working towards being able to do that but between the day job and kid wrangling I'm best off implementing somebody else's circuit for now.

PS - In case you are wondering, the EL34 version of Poindexter's Musical Machine schematic has been posted in several other threads. No need to re-post it again here.
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Old 21st March 2012, 02:30 AM   #2
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The Cit. 5 is a very nice implementation of Mullard style circuitry. However, 12BY7s are out of production and (IMO) best left for existing units. You can get the same high gm, low CMiller, and large stage gain out of a 6922 in cascode. A LR8 regulator in the cascode's B+ feed takes care of PSRR issues. The 300 mA. of heater current saved can be used to replace the 6CG7 with an ECC99. High gm is a good thing in an amp with a GNFB loop, as it provides resistance to slew limiting from the HF error correction signal.

"Fixed" bias is must, given your low B+ rail voltage, regardless of the topology you settle on.

Send Poinz an EMail and discuss the "Musical Machine" implementation with him.
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Old 21st March 2012, 10:30 AM   #3
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With 4.2K primaries I'd rather use 2a3s or some version of them like the Russian 6B4G equivalents on ebay. 2a3s need humbucking pots to be quiet, but that's no big deal. I also built a similar circuit with PPP 6V6s in triode, like the MM

I've been getting very good results by using the cheap Hammond 124b interstage (half nickel) as a plate choke for a differential pair - just the 90K side which has a centre tap. I've used it with 6SN7 diff pairs so far.

With or without these modifications the MM is a nice design.

andy
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Old 21st March 2012, 12:09 PM   #4
mfaughn is offline mfaughn  United States
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Hey, hey!! The 2A3 / 6BG4 was totally off my radar for some reason. I like that idea. It would be my first foray with a 'real' triode power stage. I'm going to look into that some more. Thanks.
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Old 21st March 2012, 08:46 PM   #5
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mfaughn,

I have a breadboarded 6B4G Musical Machine based amp and it sounds very nice indeed. It uses the 6GK5 LTP with CCS in the tail and 5k a-a outputs. I also used grid chokes on the output tubes. My speakers aren't such high sensitivity that I've needed special hum cancelling circuits. I have a single hum null pot in each channel, but it doesn't make much difference. I think separate filament transformers and hum pots for each tube would be required to get it quieter.

Good luck with your project.
John
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Old 22nd March 2012, 12:30 AM   #6
rhone is offline rhone  Australia
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Quote:
Originally Posted by mfaughn View Post
Hey, hey!! The 2A3 / 6BG4 was totally off my radar for some reason. I like that idea. It would be my first foray with a 'real' triode power stage. I'm going to look into that some more. Thanks.
Hey,
I recently converted my PP triode EL34 amp to 6B4G with great success. This was also my first try at DHTs. Input circuit is Allen Wright's PP2C using 6H30. A great choice if you don't mind a bit of "sand" scattered around.
A big advantage of 6B4G is that US NOS is available at very reasonable prices. On my 90dB efficiency speakers hum is a non-issue.
Pete Milletts site has a nice circuit to give you some ideas.
Rod

Last edited by rhone; 22nd March 2012 at 12:34 AM.
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Old 22nd March 2012, 12:31 AM   #7
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Stuffing a few numbers in Broskie's PP Triode software, I get:

400V/60ma/5K (assuming typical MM setup)

21W (seems a bit high)
12W class A
3rd harmonic 3.05%
5th harmonic .132%

365V/70ma/4.2K

16.6W
13W class A
3rd harmonic 2.53%
5th harmonic .132%

How much current can you get out of your power transformer?
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Old 22nd March 2012, 01:49 AM   #8
mfaughn is offline mfaughn  United States
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I don't suppose you have a schematic for the 6B4G MM? Diagrams help me think and/or understand a bit better sometimes...

Last edited by mfaughn; 22nd March 2012 at 02:06 AM.
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Old 22nd March 2012, 02:01 AM   #9
mfaughn is offline mfaughn  United States
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Quote:
Originally Posted by boywonder View Post
How much current can you get out of your power transformer?
Hi BW. You might remember a start at an amp design I made a while ago for this same iron. I wanted to use EF86 triode strapped as a voltage amplifier and triode strapped EL34s. You chimed in several times in support of that effort. I'm out of school now and back at work. Less time but no longer augering farther into the red every month. I can afford parts now but a scratch built design isn't in the cards (it was mostly an attempting learning exercise anyway).

Anyways, specs I have on the power supply iron (from Sam's I think):
P: 117V@ 1.8A
S: 140V@ 0.85A AC
S: 6.3V@ 4.4A
S: 6.3V@ 3.8A
S: 48V@ 0.13A DC

Voltage doubler to get ~365V. The amp was donor amp used a quad of 7355 which would have pulled ~300ma I think.
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Old 22nd March 2012, 03:05 AM   #10
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Quote:
I don't suppose you have a schematic for the 6B4G MM?
AFAIK, Poinz has never done a DHT MM.

Quote:
Anyways, specs I have on the power supply iron (from Sam's I think):
P: 117V@ 1.8A
S: 140V@ 0.85A AC
S: 6.3V@ 4.4A
S: 6.3V@ 3.8A
S: 48V@ 0.13A DC

Voltage doubler to get ~365V. The amp was donor amp used a quad of 7355 which would have pulled ~300ma I think.
No way 300 mA. of voltage doubled B+ from an 850 mA. RMS rectifier winding. Approx. 225 mA. is the limit. Will 225 mA. of B+ be sufficient to feed 4X 6B4s and the small signal circuitry?
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