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Old 23rd April 2011, 07:01 PM   #21
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I think I will take the advice of others here and build the amp per the schematic at first, and then try the pentode driver swap.

Just to push Ty's question up again; anyone have any experience with specific pentodes in this setup? Recommendations/values to try? I looked up the 6AU6 and it does look interesting, but I'm not terribly excited that it's a seven pin tube. I was hoping to be able to at least keep the socket the same when it came time to try a pentode.

Any other suggestions?
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Old 23rd April 2011, 09:56 PM   #22
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Quote:
Originally Posted by bigjppop View Post
I looked up the 6AU6 and it does look interesting, but I'm not terribly excited that it's a seven pin tube. I was hoping to be able to at least keep the socket the same when it came time to try a pentode.
No can do. You can't fit two complete pentodes in a noval bottle. Sure, there are a couple odd beasts with common cathodes or something, but they wouldn't be suitable for two channel use. If you want to use a pentode for the driver on the RH84, you will need a separate socket for each driver tube.
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Old 23rd April 2011, 10:15 PM   #23
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How about using two 6AB4 for your initial build? It's equivalent to half an 12AT7, with 7-pin base.
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Old 23rd April 2011, 11:30 PM   #24
Sheldon is offline Sheldon  United States
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Quote:
Originally Posted by bigjppop View Post
"You don't know what you're talking about; figure out what you really need and get back to me. Oh, and this design sucks."
LOL. I've gotten a similar treatment from a well known curmudgeon. Let's call him J. He is opinionated and doesn't suffer beginners well. But, if you spec. it - watts, impedance ratio, standing current. You will get a great product. And his power transformers run cool.

Sheldon
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Old 24th April 2011, 07:20 AM   #25
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Quote:
Originally Posted by Ty_Bower View Post
No can do. You can't fit two complete pentodes in a noval bottle. Sure, there are a couple odd beasts with common cathodes or something, but they wouldn't be suitable for two channel use. If you want to use a pentode for the driver on the RH84, you will need a separate socket for each driver tube.
Rats! I guess you can't have it all. I have no problem with two separate tubes, but the piece of the amp building pie that I HATE the most is cutting the top plate; I hate it so much that I've decided to farm that out to the nice folks at Front Panel Express. I'd hate to have to make two of them if I decide to try a pentode driver.

Good thought on using the 6BA4, something to consider, but I think if I'm going to start with a modded design, I might as well start with the pentode.

Decisions, decisions.
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Old 24th April 2011, 07:23 AM   #26
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Quote:
Originally Posted by Sheldon View Post
LOL. I've gotten a similar treatment from a well known curmudgeon. Let's call him J. He is opinionated and doesn't suffer beginners well. But, if you spec. it - watts, impedance ratio, standing current. You will get a great product. And his power transformers run cool.

Sheldon
So I'm not the only one...

I've heard nothing but great things about his products, but I'm not so sure I'm quite ready to eat humble pie and go back hat in hand... "Please sell me your transformers, pretty please."

If he was the only game in town... maybe; but since there are other good options out there.

Can't win them all.
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Old 24th April 2011, 09:02 AM   #27
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I built this amp as one of my first projects. Its not great as it stands, and Kitic was less than helpful with information - another comudgeon.
Running the front end as a pentode is "essential" for getting this to work properly, and indeed the 6AU6 is the boy to do it. The problem you have is that you do not have the plate voltage on the driver - so choosing an alternative operating point is tricky.
Assuming a B+ of 300V.
I would look to pass 5mA through the pentode anode. The 6AU6 needs to have at least 200v on the plate. This suggests a g2 voltage of 150v (which a simple resistor divider with cap smoothing should supply - screen current 2.1mA so dropping resistor of 71k - smoothing cap of 100uf). With a plate voltage of 200v and a feedback resistor of 100k, 1mA of plate current will be supplied through the feedback resistor, so the plate resistor will need to supply the remaining 4mA (100v/0.004A=25K).
The cathode needs to sit at 2.25V passing current of 5mA anode + 2.1mA screen, for 7.1mA (2.25v/0.0071A=320R).

So
Rk=320R
Ra=25k
rg2=71k with 100uf from screen to ground.

That should get you going - and can be fine adjusted if necessary. This will sound infinitely better than the original.

Things work out better if you have a B+ of 350V.

Shoog

Last edited by Shoog; 24th April 2011 at 09:09 AM.
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Old 24th April 2011, 09:26 AM   #28
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Hey Shoog,
No secret the RH is wrongly designed. I know you have worked a lot with 6AU6 so probably you know best about its sweetspot. Anyway this is what I would do. Only componentvalues differ.
Attached Files
File Type: pdf 6au6el84Schade.pdf (11.0 KB, 389 views)
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Old 24th April 2011, 09:32 AM   #29
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Your running it nearly twice as hard, which shouldn't be an issue, but my experience with the Tabor clone is that it works just great at 5mA in a schaded circuit.
I was getting bored of watching people not answer the question

Shoog
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Old 24th April 2011, 09:43 AM   #30
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Quote:
Originally Posted by Shoog View Post
I built this amp as one of my first projects. Its not great as it stands, and Kitic was less than helpful with information - another comudgeon.
Running the front end as a pentode is "essential" for getting this to work properly, and indeed the 6AU6 is the boy to do it. The problem you have is that you do not have the plate voltage on the driver - so choosing an alternative operating point is tricky.
Assuming a B+ of 300V.
I would look to pass 5mA through the pentode anode. The 6AU6 needs to have at least 200v on the plate. This suggests a g2 voltage of 150v (which a simple resistor divider with cap smoothing should supply - screen current 2.1mA so dropping resistor of 71k - smoothing cap of 100uf). With a plate voltage of 200v and a feedback resistor of 100k, 1mA of plate current will be supplied through the feedback resistor, so the plate resistor will need to supply the remaining 4mA (100v/0.004A=25K).
The cathode needs to sit at 2.25V passing current of 5mA anode + 2.1mA screen, for 7.1mA (2.25v/0.0071A=320R).

So
Rk=320R
Ra=25k
rg2=71k with 100uf from screen to ground.

That should get you going - and can be fine adjusted if necessary. This will sound infinitely better than the original.

Things work out better if you have a B+ of 350V.

Shoog
I really appreciate the detailed outline. I went ahead and bought 10 of the 6AU6 on eBay this morning (10 for $2.00, couldn't resist) so at least I'll have the tube on hand.

Not sure if you're setup to do this easily or not, but would it be possible to show what you described in a schematic? I'm still working on my lingo so things like smoothing cap and plate resistor are difficult for me to translate into a visual image.

I know there's people out there that can crank out schematics in their sleep and I know, at least for this newbie, a picture of this circuit with the pentode driver you suggested would be a HUGE help.
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