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?
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?
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.
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
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.
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.
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
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
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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.
Use Revintages schematic and see if you can work out how his differs from my suggestion. Essentially they are the same but with component values different. You should understand what is going on a bit better if you give it a go.
Essentially he uses a 15K resistor where I specify a 25K.
He uses a 2uf cap where I suggest 100uf (you can shoot anywhere between without much difference), his dropping resistor is 33k mine is 71k.
He also adds in a smoothing cap for the screen of the 807, which is a nice addition, but in my experience makes little sonic difference in this circuit.
Shoog
Essentially he uses a 15K resistor where I specify a 25K.
He uses a 2uf cap where I suggest 100uf (you can shoot anywhere between without much difference), his dropping resistor is 33k mine is 71k.
He also adds in a smoothing cap for the screen of the 807, which is a nice addition, but in my experience makes little sonic difference in this circuit.
Shoog
bigjppop,
The role of the driver in a Schade circuit is to act as a V-I converter, the efficiency of which is proportional to its output impedance, Zo, WHICH SHOULD THEREFORE BE AS HIGH AS POSSIBLE. This is where the pentode has an advantage over the triode due to its (usually) inherently higher Ri.
Zo consists of Rp and Ri+(mu+1*Rk) in PARALLEL. In most RH Schade circuits, the value of the plate resistor (usually in the order of 22k – 33k), must be fairly low to a allow a decent driver current. Because most triodes generally have a fairly low internal impedance, the Ri+(mu+1*Rk) component is therefore usually a significant factor in determining Zo. In the case of pentode drivers, Ri is usually so high that the impact of this factor in determining Zo is usually negligible.
In the RH84 with 12AT7 driver
Rp = 22k
Ri = 15k
Rk = 300
mu = 50 (approx at this OP)
Zo 12AT7 =1/( 1/22k + 1/(15k + (51*300)) = approx 12k7
For Revintage’s 6AU6
Rp = 15k
Ri = 1M
Rk = 180
mu = 150 (approx at this OP)
In this case Ri(mu+1*Rk) factor is so large it barely influences Zo, which is effectively Rp = 15k. This offers a small theoretical improvement in drive capacity over the 12AT7 (Zo=12k7).
Using Shoog’s OP
Rp = 25k
Ri = 1M
Rk = 320
mu = 300 (approx at this OP)
Rp again dominates in determining Zo, which is effectively 25k. This represents an almost twofold increase over the 12AT7 Zo.
Therefore if you are going to use a 6AU6 driver, I would recommend Shoog’s OP. I am unsure how much real world improvement this will confer as there are many very happy builders of the original circuit, some of whom consider it to be the finest single ended EL84 implementation they have heard. I know some forum contributors who’s opinion I greatly respect have put it up against some pretty classy gear, and it has come away giving away very little. Hence my original recommendation.
The role of the driver in a Schade circuit is to act as a V-I converter, the efficiency of which is proportional to its output impedance, Zo, WHICH SHOULD THEREFORE BE AS HIGH AS POSSIBLE. This is where the pentode has an advantage over the triode due to its (usually) inherently higher Ri.
Zo consists of Rp and Ri+(mu+1*Rk) in PARALLEL. In most RH Schade circuits, the value of the plate resistor (usually in the order of 22k – 33k), must be fairly low to a allow a decent driver current. Because most triodes generally have a fairly low internal impedance, the Ri+(mu+1*Rk) component is therefore usually a significant factor in determining Zo. In the case of pentode drivers, Ri is usually so high that the impact of this factor in determining Zo is usually negligible.
In the RH84 with 12AT7 driver
Rp = 22k
Ri = 15k
Rk = 300
mu = 50 (approx at this OP)
Zo 12AT7 =1/( 1/22k + 1/(15k + (51*300)) = approx 12k7
For Revintage’s 6AU6
Rp = 15k
Ri = 1M
Rk = 180
mu = 150 (approx at this OP)
In this case Ri(mu+1*Rk) factor is so large it barely influences Zo, which is effectively Rp = 15k. This offers a small theoretical improvement in drive capacity over the 12AT7 (Zo=12k7).
Using Shoog’s OP
Rp = 25k
Ri = 1M
Rk = 320
mu = 300 (approx at this OP)
Rp again dominates in determining Zo, which is effectively 25k. This represents an almost twofold increase over the 12AT7 Zo.
Therefore if you are going to use a 6AU6 driver, I would recommend Shoog’s OP. I am unsure how much real world improvement this will confer as there are many very happy builders of the original circuit, some of whom consider it to be the finest single ended EL84 implementation they have heard. I know some forum contributors who’s opinion I greatly respect have put it up against some pretty classy gear, and it has come away giving away very little. Hence my original recommendation.
(...)
I am unsure how much real world improvement this will confer as there are many very happy builders of the original circuit, some of whom consider it to be the finest single ended EL84 implementation they have heard. I know some forum contributors who’s opinion I greatly respect have put it up against some pretty classy gear, and it has come away giving away very little. Hence my original recommendation.
+1
Its the drivers ability to push into a low impedance node (the EL84 grid) without distorting that is also important and for that pentodes, been a CCS, are inherently superior. The triode driver works as a variable resistor which is only optimal when driving a fixed high impedance node - the opposite of what we have here.
I also built a RH807 as my first amp. Since then I have built nothing but Schade amps - and all have used Pentode drivers. I feel there are real and not imagined advantages.
Shoog
I also built a RH807 as my first amp. Since then I have built nothing but Schade amps - and all have used Pentode drivers. I feel there are real and not imagined advantages.
Shoog
I've spent the last couple of hours researching rectifiers and PS info and I want to make sure I'm tracking on this one. I've got the 6Au6 tubes, I'll be going with Shoog's operating points listed above.
For my PS, I'm hoping to use the 5v4g (I've got a couple and I think they look cool). I understand it can only do about 175 mA but since this setup should only draw 120 mA max I should be good on that front, correct? Also the 5v4g has a 2A draw so as long as my 5V winding is at least 2A, should be good there as well; correct?
I'm wanting to use a 40-50uf 440V motor run cap in the "first cap" slot. I've read somewhere that the 5v4G doesn't like anything bigger than about 35uf as its "first cap" is this going to be an issue? I'm planning on using the choke listed earlier in this thread (RadioDaze I believe and something like 8H 125mA). I've heard this "first cap" referred to as a reservoir cap, a filter cap, a smoothing cap, and I believe a couple others; does it have an official name? For the last cap in the power supply (220uf 350V) I'll probably use something like like the Panasonic TSHA and then who knows for the bypass.
Am I on the right track for the PS section? Anyone see any issues? Would there be much value in increasing the size of that choke?
Thanks again for helping out the new guy.
For my PS, I'm hoping to use the 5v4g (I've got a couple and I think they look cool). I understand it can only do about 175 mA but since this setup should only draw 120 mA max I should be good on that front, correct? Also the 5v4g has a 2A draw so as long as my 5V winding is at least 2A, should be good there as well; correct?
I'm wanting to use a 40-50uf 440V motor run cap in the "first cap" slot. I've read somewhere that the 5v4G doesn't like anything bigger than about 35uf as its "first cap" is this going to be an issue? I'm planning on using the choke listed earlier in this thread (RadioDaze I believe and something like 8H 125mA). I've heard this "first cap" referred to as a reservoir cap, a filter cap, a smoothing cap, and I believe a couple others; does it have an official name? For the last cap in the power supply (220uf 350V) I'll probably use something like like the Panasonic TSHA and then who knows for the bypass.
Am I on the right track for the PS section? Anyone see any issues? Would there be much value in increasing the size of that choke?
Thanks again for helping out the new guy.
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