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6SN7 push pull flea amplifier project
6SN7 push pull flea amplifier project
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Old 7th January 2019, 08:54 PM   #301
Lingwendil is offline Lingwendil  United States
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6SN7 push pull flea amplifier project
Those look interesting! Wonder what the frequency response is like. Interesting that they have 20/43% UL taps as well.

Looks like it may be meant for musical instrument use? I would email them and ask for some datasheet or frequency response plots.
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Last edited by Lingwendil; 7th January 2019 at 09:08 PM.
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Old 8th January 2019, 12:34 AM   #302
jhstewart9 is offline jhstewart9  Canada
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For those who may not be familiar with the Firefly here is the original circuit by none other then Doug Hammond himself. Not meant as a regular amp but for guitar effects. Notice the self inverting final stage, makes lots of even order harmonics, 2nd, 4th, 6th & so on.

The 12AU7 Hammond uses in his circuit is not far off the characteristics of the 6SN7.

Beginning in the late 30s there were several of these self inverting PP stages built into various amplifiers for one reason or another. None of them are able to deliver more than one half what the same PP pair could do with a regular drive stage.

I did an exhaustive series of tests a couple of years ago, complete with distortion tests with an HP334A THD Analyzer & Pico Technology Spec A. The results were not good at all. Anyone wants a copy of the paper let me know, all in a PDF.
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File Type: jpg Firefly Rev 3 8W Doug Hammond.jpg (191.7 KB, 394 views)
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Old 8th January 2019, 01:01 AM   #303
Carlp is offline Carlp  United States
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6SN7 push pull flea amplifier project
Also, with the firefly rated at 1.5w I'd wonder about saturation. Maybe not a problem but...

Quote:
Originally Posted by jhstewart9 View Post
Beginning in the late 30s there were several of these self inverting PP stages built into various amplifiers for one reason or another. None of them are able to deliver more than one half what the same PP pair could do with a regular drive stage.

I did an exhaustive series of tests a couple of years ago, complete with distortion tests with an HP334A THD Analyzer & Pico Technology Spec A. The results were not good at all. Anyone wants a copy of the paper let me know, all in a PDF.
With your analysis, are you saying not good for the intended purpose (effects pedal) or for audio purposes?
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Old 8th January 2019, 01:05 AM   #304
Lingwendil is offline Lingwendil  United States
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6SN7 push pull flea amplifier project
Isn't the Firefly meant for a small bedroom guitar practice amp?

I'm primarily concerned with that transformer. That circuit looks gross
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Old 8th January 2019, 02:02 AM   #305
jhstewart9 is offline jhstewart9  Canada
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The cct is very good for effects. But makes a poor audio amp. In spite of that there are examples over many years of it being used for claimed Hifi, even recently. And found one in WW2, a couple more published in the late 50s & early 60s.

All covered in the paper. The example here is out of WW2. Could have made a real amp simply by stuffing in a 12SC7 diff amp, a real phase invertor instead of that 12SH7 pentode.

The Hammond 125A would be AOK in a small PP amp. Looking at the taps selected in the Firefly, could easily go to a higher load impedance, thus improving for fidelity.
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File Type: jpg 117L7GT X2 PP Hifi.JPG (89.9 KB, 389 views)
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Old 10th January 2019, 09:29 PM   #306
cbueche is offline cbueche  Switzerland
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Default NOS 6SN7 tubes for this project only

Hi,

for builders of this project only, I agree to sell a few NOS tubes. Check the "Swap Meet" forum thread.

Charles
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Old 17th January 2019, 08:45 PM   #307
jhstewart9 is offline jhstewart9  Canada
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Default The Betty Crocker Special

Use of a cake tin as a chassis was inspired by an article in Sound Practices magazine, Issue 11.
Al Bryant was named ‘Home brewer of the Month’. One of the article photos shows a cake tin complete with electronics of some kind. Much lower cost than the purpose built thing & works very well.

This amplifier is a PP 6SN7 driving into a Hammond 125E. Taps 1-3 connected to 7.5R result in a plate to plate load of 27K. Trying other close tap ratios confirm this to be the best option. The maximum audio looks like One Watt at which point the cct is clipping. The distortion is 4.3%. At 0.5 Watt the distortion is 2.2%.

The power supply is a regulated lab supply set to 300V. The driver circuit uses a dissimilar triode, the 6EA7. The high mu section is used in the first stage for gain. The other triode is low mu & makes a very good driver for the PP 6SN7 triodes.

When using twin triodes such as 6SL7 or 6SN7 the driver cct is never optimized for one part of the cct or the other. Using a dissimilar triode solves this problem. The 7247 was developed to overcome the problem but never caught on. One section is high mu like half of a 12AX7 while the other side looks like one half of a 12AU7.
Attached Images
File Type: jpg Cake Tin used as a Chassis by Al Bryant.jpg (124.3 KB, 276 views)
File Type: jpg PP 6SN7GTB Amp One Watt.JPG (176.1 KB, 278 views)
File Type: jpg 27K Load C.JPG (110.4 KB, 273 views)
Attached Files
File Type: pdf Betty Crocker Special.pdf (271.1 KB, 19 views)
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Old 17th January 2019, 10:17 PM   #308
Lingwendil is offline Lingwendil  United States
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Neat

I love old school builds like that. I wonder how it will sim with a bit of feedback to the driver cathode, I'm not too familiar with the 6EA7, 6EM7 or 7247.

Ideally, one could use a separate driver tube and splitter tube, but then the layout doesn't look symmetrical anymore
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Old 18th January 2019, 12:15 AM   #309
jhstewart9 is offline jhstewart9  Canada
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Looking at the schematic I see the OPT is not labelled correctly.
Fix it tomorrow.
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Old 18th January 2019, 08:02 PM   #310
jhstewart9 is offline jhstewart9  Canada
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Corrected schematic here this time.
Hammond 125E with some impedance data extended.
Dissimilar twin triodes 7247 & 6EA7 (same as 6EM7).
Attached Images
File Type: jpg 125 PP Series Hook Up Data Extended B 150 dpi.jpg (491.3 KB, 92 views)
File Type: jpg 6SN7 Amplifier E.JPG (66.1 KB, 86 views)
Attached Files
File Type: pdf 7247 RCA.pdf (169.0 KB, 5 views)
File Type: pdf 6EA7.pdf (160.9 KB, 8 views)
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