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Old 20th September 2010, 01:53 AM   #11
w5jag is offline w5jag  United States
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Here are some top and bottom pics of the completed conversion showing the installation of the screen board. The thin orange wire is the screen wiring; the large orange wiring is for the cathode feedback.

Most are using 2.5K to 4K for the plate load on these tubes. I went in the opposite direction and settled on 10K as there is power to spare and I wanted to minimize distortion. I am using the 2.5K tap to maximize the cathode feedback.

The subtle IMDish distortion I was hearing was yet another bad 6146. A fresh set of 6146B's fixed that.

I have 2E26's in the amp now. RCA did publish data points for 2E26 in class A audio use: 250 volts plate, 160 screen, and -14.5 grid, IIRC. Right now I'm running 400 plate, 220 screen, and -17.5 grid, about 14 watts.

There is a short snap of oscillation on startup when the main DC supply has a cathode type tube rectifier as the plate lags the screen which is on a solid state rectifier. A directly heated rectifier solves that issue and makes a quiet start.

All in all, a fun conversion project.

Win W5JAG
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File Type: jpg screen board.jpg (358.4 KB, 753 views)
File Type: jpg Simple SE 2E26.jpg (247.3 KB, 739 views)
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Old 23rd September 2010, 03:51 PM   #12
w5jag is offline w5jag  United States
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Not much work on this project over the last few days.

I finally got around to changing the grid resistance. 220K is probably too high, but I wasn't sure what the "correct" value is. I settled on 100K and installed a 180K resistor on the solder side of the board in parallel with the 220K on the screen side. AFAIK, the 220K grid resistance caused no adverse results, but I changed it anyway.

As these are 175 MHz RF tubes, stability kinda concerns me in this application, but it got a continuous 13 hour run on Tuesday with 6146A's in it at about 27 watts / tube with no issues so it seems stable, even when deeply heat soaked.

I'm thinking about going to a fixed bias setup, but that is not in the immediate future for this amp as I need to get some smaller parts to get under my 1.5 inch chassis. I can't find anything that even suggests using cathode bias for these tubes.

I've been using both 2E26 and 6146A and B and am developing a preference for 2E26, at least in this amp.

When the subject of 6146's comes up on the forums, a lot of the responses seem to be along the lines of just use a circuit for an 807. I'm not sure what the basis is for that, they are really different devices.

Win W5JAG
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Old 3rd October 2010, 11:09 PM   #13
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Sweet! I've got a sleeve of 2E26's that I had no idea what to do with. Excellent work.
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Old 4th October 2010, 03:58 AM   #14
w5jag is offline w5jag  United States
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You don't have to have a separate screen winding on your power transformer to get a low(er) voltage screen supply in your amp.

If you have room under your chassis, you can put a couple of small filament transformers back to back like this:

:117||6.3=6.3||117:

and then rectify and filter it in the usual manner. You should be able to get 175-200 volts DC easy this way, and that will be about ideal for 2E26 and small sweeps like 6BQ6 and 6DQ6.

One RCA data plate ( the preliminary sheet, I think) states that 6146 can be run at up to 400 volts on plate and screen when triode wired. I have not tried that yet.

Some 2E26's flouresce like crazy, and look really good at work.

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Old 10th October 2010, 04:13 PM   #15
isaacc7 is offline isaacc7  United States
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Glad to hear that someone is using 2e26'sin an amp, and with a kit no less... I've always thought that they had a lot of potential, and Lord are they cheap! Any sonic comparisons to more conventional tubes like 6v6's, el 34's etc?
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Old 10th October 2010, 05:35 PM   #16
w5jag is offline w5jag  United States
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Quote:
Originally Posted by isaacc7 View Post
.... Any sonic comparisons to more conventional tubes like 6v6's, el 34's etc?
Can't really say, I'm not an audiophile - I did this mostly for fun and to learn a bit.

I intend to build a better screen supply and a fixed bias supply to run them closer to optimally. A good NFB network would also help; I've been pretty busy but have tinkered a bit with that, nothing worth posting up so far.

That said, even in the current setup which is crude, I see no reason why 2E26 / 6146B cannot sound as good as anything else run in pentode mode.

I didn't think much of 6BQ6* and 6DQ6* when run as triodes, having now also played with them a bit as pentodes, I have a much more favorable impression of them.

2E26's are cheap, but 6BQ6's are even cheaper.
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Old 10th October 2010, 06:18 PM   #17
isaacc7 is offline isaacc7  United States
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Don't have any 6bq6's but I have a bunch of *dq6's around. I'd be interested in reading more about those too...
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Old 1st December 2010, 10:19 PM   #18
w5jag is offline w5jag  United States
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I am still running 6146B's in the amp, and have a few more observations and have made a few changes since the last post.

All 2E26's and 6BQ6 types I have tried seemed stable in the amp, but at least one 6146B example and one 6DQ6 was not. Of these, they were so close to the edge of instability that just moving a tube from one socket to the other would be enough to start or stop an oscillation.

Working through this, I changed the 100 ohm parasitic supressors in series with the screens (R18 and R28) from 100 ohm to a 1.2K device (nothing special about this value -I had them in stock and saved myself a trip to the parts store).

Regarding feedback, right now I am using substantial plate to grid ( Schade? ) local negative feedback around the PA tubes implemented with a 100K resistor from the plate of the PA tube to the plate of the 12AT7 section on that side. This is in addition to the cathode feedback that is enabled. The pentode shrillness is gone, and it still has that "tube" sound, for lack of a better phrase, albeit in a more subdued manner.

It seems to be much better behaved now; the snap of oscillation on start up that required a directly heated rectifier is now gone. It's very quiet - with 96db ish speakers, in the absence of an input signal, I can't tell the amp is on, even with my ear directly against the speaker.

I'm still using cathode bias; for now I don't have the right parts for a fixed bias setup that can fit under my low profile chassis.

Win W5JAG
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Old 1st December 2010, 11:30 PM   #19
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Quote:
Regarding feedback, right now I am using substantial plate to grid ( Schade? ) local negative feedback around the PA tubes implemented with a 100K resistor from the plate of the PA tube to the plate of the 12AT7 section on that side.
I have tried this too with good results. But the last time I mentioned it in another thread I was heavily criticised for suggesting that Schade would work with a triode input tube. Several of the big names here explained that Schade wouldn't work with a triode and the worst possible circumstance would be a triode fed by a CCS. I decided to just shut up and listen to my amp.

If you have nothing better to do, try connecting the Schade resistor to the gate of the CCS chip. I needed a bigger resistor, 470K maybe, I don't remember. Warning don't try it if you don't have any spare chips. I blasted one in half and I don't know why. Replaced the chip and it worked.

Just plugging a 6DQ6 into the SSE resulted in an exploded cathode bypass cap because the bias voltage went up to about 65 volts.
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Old 2nd December 2010, 02:29 AM   #20
w5jag is offline w5jag  United States
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It seems that a CCS loaded driver is very well suited for this type of feedback. I have enough extra CCS chips that I can explode a few and I'll try running it through the CCS when I have a few hours on the bench with it.

I was fixated on global feedback and fooled around with that with mediocre results. When I tried the Schade, I was taken aback by just how well it worked in this application.

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