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 NEW  Matt presents bias and operation data for the 6V6 tube in SE operation - 6V6 Single-Ended (SE) Ultra Linear (UL) Bias Optimization.

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PostPosted: 28 May 2011, 09:41 
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Hi Everyone, Yes a 12AX7 can drop in the circuit. Values are quite close for either type. The 12AX7 does allow for greater choice of tubes, but I have been unable to find one I really like better than either the JAN 5751 or in most cases the new Sovtek ones. Yes also you can re-do the circuit for a 12AU7. I did this in an early project posted on this site (about 2 years ago, please ignore the inaccuracies in power measurements, my test equipment has gotten way better since then). You should use the driver portion and feed it to the output stage of the new circuit. Works fine. BTW the ECC802S is a spiral filament version of the 12AU7 (I use the JJ ECC802S also in the Forewatt preamps - lovely tube). If you really want to go whole hog, you can even use the output transformers from the early circuit on the new one. No special advantage as the ones in the newer circuit are excellent and a bunch cheaper. As for the 6N1Ps. Yes I like them too and they can be used with some circuit changes. If you give me a day or so I compute the values and post them here. The two main changes are they work at lower B+ values (I use about 175 across the SRPP) and higher current (about 8ma vs 1.2 in this case). The various resistor values need to be changed and the heater / cathode bias adjusted. In that area, you can eliminate it altogether, but the noise level will be higher.

Good listening
Bruce

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PostPosted: 28 May 2011, 22:11 
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Hi Everyone, As promised here is a modded schematic for using a 6N1P in the "Poddwatt" The gain will be slightly lower than with a 5751, but not a problem. The 12AU7 version with even less gain was fine with about one volt of drive.
Attachment:
Poddwatt 6N1P mod.PNG

Good listening
Bruce


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PostPosted: 29 May 2011, 01:42 
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Bruce: Cost no option, best EL84s? Who's have you tried and liked?

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PostPosted: 29 May 2011, 10:28 
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Hi Mark, The best ones were... believe it or not Sovteks, next EH then new production Mullards. All three were pretty close. Slightly different flavors, all good. The differences in distortion and frequency response were really quite small (under 0.1% and less than 1db). There may well be others out there that are better...but when the one watt distortion levels are in the 0.25% range and flatness is under 1 db from below 20 out to 20K... hard to beat. BTW the kits use EH as we can get matched ones easier than the Sovteks. The drivers are either NOS Phillips or new production Sovteks .... very close in performance.. just small flavor differences.

Good listening
Bruce

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PostPosted: 29 May 2011, 16:57 
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Thanks Bruce.

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PostPosted: 19 Jun 2011, 01:31 
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gofar99 wrote:
As for the 6N1Ps. Yes I like them too and they can be used with some circuit changes. If you give me a day or so I compute the values and post them here. The two main changes are they work at lower B+ values (I use about 175 across the SRPP) and higher current (about 8ma vs 1.2 in this case). The various resistor values need to be changed and the heater / cathode bias adjusted.

Bruce-
Thanks for making all this info available.
Getting set to put together a 6N1P Poddwatt here....what's the correct heater bias for the 6N1P?

Also, will using an 8k primary OT (Heathkit 51-29) be a problem vs the 10k you spec?

Thanks
John


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PostPosted: 20 Jun 2011, 10:59 
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Hi, Glad to assist. I attach the bias to which ever side of the heater circuit is the negative if you use a DC heater supply. If you use AC it doesn't seem to matter. 8K trannies should be fine. The calculated value is in between 8K and 10 K. In such cases I go for the higher one as it seems to result in slightly better distortion readings (in the 0.1 to 0.2% range) in the bass region below 100HZ. It is not generally audible. The trade off is about 5% less output which certainly isn't audible.

Good listening
Bruce

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PostPosted: 20 Jun 2011, 12:14 
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Thanks, Bruce.
About the bias- the 100k/220k voltage divider will still work for biasing the heaters of the 6N1P tubes? (I'm assuming it will....)
Since the PT I'm using is right at its limit, I'm planning on feeding the EL84s with a separate filament transformer, so they will be on AC. The 6N1Ps will have DC heaters.

John


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PostPosted: 20 Jun 2011, 17:15 
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I've read and heard good things said about the EL84 and one tube I have worked with a fair bit is the 6N1P. A Poddy with the two could be a great combo.

I have been asked to build a tube front-end (driver stage) for a newly designed SS back-end (power stage). The request has come from a very respected local SS amp maker who sells all over the world with big acceptance into the States.

My orginal thought is using the 6N1P with plates ganged to double current drive and lower output Z. The load they willl have to drive is 30K. We need about 80Vpp to get 120W out of this SS module. I intend to pre-drive the 6N1Ps with the simple preamp I just completed (12AU7). Suncale is doing the slide rule work and thinks we can do it

Why I bring this up here, which appears OT, is because suncalc pointed out that there is a 6N1P-EB. The EB is a different beast and has the ability to drive harder which is just what we need. By accident, when I was purchasing 8 X 6N1Ps off eBay for $24 (new and in the box) I got EBs. So a win fall here. In the past I have paid $24 each for my 6N1Ps.

So take a look at the EB specs and see if it is not a better fit with the EL84 and what you are trying to acheive.Bruce is the man to decide this. My intention, way down the road of burning amps, is to make a 2W SE EL84. Now it may have a 6N1P driver.

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PostPosted: 27 Jun 2011, 11:56 
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Hi Mark, Sorry for the delay in answering, we were having local fires (30,000 acres and 2400 acres) that came within 1/2 mile of the daughter's house so they evacuated to here, and then within 1 mile of here (nearly had to bail out too). The standard 6N1P is easily able to drive 30K. Depending on the configuration the output Z can be in the 200 ohm range. Output can be in the range of voltages you need as well. Paralleling them will not cause any harm, but probably not be of any particular benefit either. I use one as a cathode follower in my developing phono preamp. Z out is 200 and Vout is +/- 40 with a 200v B+. If you are already getting gain from the 12AU7, I would seriously consider the cathode follower output. With careful adjustment of the components, you can direct couple the stages (I do that in the phono preamp). PM me on my hotmail account for details if you are interested.

Good listening,
Bruce

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