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Old 18th April 2012, 02:10 AM   #1
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Default Bogen RP40A transformers....what to build with them?

I cannot for the life of me find the schematic for this receiver, but have seen the RP40 which is NOT the same. Anyway, I do however assume that the output stage is the same in both because they are part of the same series and the tubes are the same 7355s. I believe the output trannys are 6.6K, but no UL taps. The receiver was spec'd at 20W, so I am unsure if I really want to push it since they are not super beefy.

The interesting thing is that the B+ must have been around 300-325v. I get 234VAC with 120VAC in with no load.

Any suggestions on what to build with these transformers? I'm leaning towards 6L6 PP in triode with a fixed bias. I'm not sure if I can get 20W out of them though.

Any other thoughts to get 15-20w in triode? Any schematics?

Thanks!

Blair
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Old 19th April 2012, 04:20 AM   #2
rongon is offline rongon  United States
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Hi Blair,

Quote:
The interesting thing is that the B+ must have been around 300-325v. I get 234VAC with 120VAC in with no load.
You didn't say if the power transformer is 234V-0-234V (468VCT) or if it is 234VAC and 0 (not center-tapped, in other words).

With no load, the voltage will be much higher than fully loaded. I wouldn't be surprised if the loaded voltage would be more like 200VAC. I'll bet that transformer was used with a voltage doubler power supply (a lot of 1960s tube receivers did that). It probably got something like +400VDC, but at only 200mA max current.

6.6k ohm primary is good for a number of different tube types. But with only 200mA available from the power transformer for both channels, your choices are pretty limited.

What about 7591's? You wouldn't get a lot of power, but they'd be really easy to drive cleanly. They're good for 20 watts max dissipation, so if the B+ was 400V, that would be 50mA per tube. That would work, I think...

Eli Duttman's El Cheapo or gingertube's Baby Huey would be great for adapting to 7591's. That would be a worthy project.


Quote:
Any other thoughts to get 15-20w in triode? Any schematics?
There's not much in the triode choices that will run optimally into a 6.6k p-p load that won't need more current and a higher plate voltage. EL84 or 6V6 in triode would work better into something like a 10k load, but max plate voltage is about +325V or so. You'd only get about 5 watts per side from those. 7591 should work well in triode and would be ridiculously easy to drive, so could be run from a very simple driver stage (maybe one tube per channel). I would expect only about 8 watts per channel from triode-wired 7591's.

Wow, this is a challenge. Maybe a pair of 2A3 or 6B4G per side. Those want to see something like a B+ of 300V with a plate current of 50mA (15 watts max dissipation per tube). So the power transformer would be just barely adequate, and only if you use the tubes with cathode bias and maybe a choke input power supply filter. But you could get at least 10 watts per side from that.

Add the current for the driver stage, and 2A3's will need a capable one. Typically that would be three 6SN7 (or 12AU7) type tubes, which need about 48mA of plate current for all of them, and 1.8A of heater current. Can you add a second smaller power transformer for the driver stage?


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Old 19th April 2012, 04:24 AM   #3
rongon is offline rongon  United States
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Quote:
I'm leaning towards 6L6 PP in triode with a fixed bias. I'm not sure if I can get 20W out of them though.
With 400V at the plates, and 50mA through each 6L6, I'd expect only about 10 watts per channel in class A. You could add a hefty driver like Tubelab's PowerDrive and run them into class AB2.

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Old 19th April 2012, 01:54 PM   #4
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Hi Rongon,

Most of the Bogen stuff did not have a doubler from what I see. He used lower voltage B+ and ran the tubes very conservatively. The RP60, which is similar, ran 335V B+ on the 7355s for 30W. I suspect that using a FWB, you are spot on at around 290-300VDC. I think for simplicity sake, I will use pentode to keep it simple. That should keep me at my 20W or so. I am looking at a schematic by Tim Williams:

http://webpages.charter.net/dawill/t...reo_6L6_PP.gif


Since I already have a fixed bias set up on the top plate, I will convert it and use a different front end to accomodate formy pre drilled chassis

Here are a few shots of what I'm working with.

I'll post in another post
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Old 19th April 2012, 02:01 PM   #5
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Here are a few images.
Attached Images
File Type: jpg IMG_0545.JPG (590.6 KB, 314 views)
File Type: jpg IMG_0547.JPG (700.7 KB, 297 views)
File Type: jpg IMG_0548.JPG (579.6 KB, 280 views)
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Old 20th April 2012, 05:47 PM   #6
rongon is offline rongon  United States
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Quote:
Originally Posted by deicide67 View Post
I am looking at a schematic by Tim Williams:

http://webpages.charter.net/dawill/t...reo_6L6_PP.gif

Since I already have a fixed bias set up on the top plate, I will convert it and use a different front end to accomodate formy pre drilled chassis

A 2-stage LTP to PP design. I like it. I built something like that using 6N30P LTP to PP 2A3s, but it needs a preamp with about 10x gain.

Keep us posted on how it turns out. I'm very curious how you end up liking it.

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Old 20th April 2012, 08:41 PM   #7
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Click the image to open in full size.Yes,

I think I'm going to try something like this.http://i1263.photobucket.com/albums/...4/c5afda31.jpg

The drawing is pretty crude, but you get it.

Blair
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Old 20th April 2012, 08:44 PM   #8
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Click the image to open in full size.
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Old 21st April 2012, 12:25 AM   #9
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In order to get a satisfactory damping factor in full pentode mode, NFB of some kind is mandatory.

Open loop linearity in full pentode mode is better, if g2 B+ is regulated.
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Old 21st April 2012, 12:34 AM   #10
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Hi Eli,

I know it is much better to have a regulated G2, but I am not sure is I have the space for it. I thought about a choke feeding it from the B+, but again it is a space issue.

NFB is an option, but I'm not sure where/how to tap it into the front end.

Thanks,

Blair
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