Liverpool bias resistors and screen resistor ?

[Doxie man]

I have never built a Wreck (yet) but was scratching my head wondering why such huge values (25w) for setting the bias and screen ? I built many EL84 based amps over the years and always used 10w wirewounds.....never once had an issue. Was Ken just being waaaay overly cautious when he designed these circuits ?

[pompeiisneaks]

I'd think it depends on the voltage and current. If I look at an old EL84 datasheet they say two tubes in PP puts out 36mA per tube. If that's at 300V that's 10.8 Watts maximum potential. Now nobody should push their tubes that hard I'd guess, but that would smoke a 10Watt resistor. I think the usual 10 watters are for lower output biasing, not sure in this case, though, I don't know the TW amps very well. If they're running at higher voltages, then it would be even more wattage output through the cathode resistor. As for the grids, they only run at around 4mA so I'd think they'd be fine at 3W (300 * .004 = 1.2 W add some safety)

[Doxie man]

I was going to use (2) 120R/10w resistors on the cathode per PAIR of EL84's ...NOT one for all four

[pompeiisneaks]

That was for one tube. If you're issuing s ten watt for two it's double. But I just was biasing my Gibson eh185 last night and realized it's the cathode voltage drop that matters, which in my case was something like 28V so you're right it's probably more than enough.

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[pompeiisneaks]

To resurrect this one, I'm now building a Liverpool as well, and can't see the need for 25W either, and I couldn't FIND any 25W 130ohms so I just went with the 10W myself as well.

[Blackburn]

Last I looked, Mouser has 25w chassis mount resistors by Arcol. I used a single one at 50R to bias all 4 output tubes in my tube rectified octal LP. I also used one for the 1K in the PS. Also, the proper value when biasing each pair respectively is 120R. I've only ever seen that value used in real 'Pools.

Edit: My mistake on the bias resistor value after checking!

[Doxie man]

I prefer to split my quads into pairs .......each pair having their own bias network

[pompeiisneaks]

Last I looked, Mouser has 25w chassis mount resistors by Arcol. I used a single one at 50R to bias all 4 output tubes in my tube rectified octal LP. I also used one for the 1K in the PS. Also, the proper value when biasing each pair respectively is 120R. I've only ever seen that value used in real 'Pools.

I didn't find any, but then I have issues with their search tool sometimes. I'm going off the schematic here that shows 130. I'd guess that the 120 will push them closer to the edge, and is a tad hot, but I'll check mine.

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[Blackburn]

Last I looked, Mouser has 25w chassis mount resistors by Arcol. I used a single one at 50R to bias all 4 output tubes in my tube rectified octal LP. I also used one for the 1K in the PS. Also, the proper value when biasing each pair respectively is 120R. I've only ever seen that value used in real 'Pools.

I didn't find any, but then I have issues with their search tool sometimes. I'm going off the schematic here that shows 130. I'd guess that the 120 will push them closer to the edge, and is a tad hot, but I'll check mine.

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MY BAD! I had a look at my notes and the schem is correct. It's been a while. Doh! FacePalm! Sorry about that.

[Blackburn]

As for Mouser, they do still supply Arcol/Ohmite, but you're right and not at that value. They do have 120R @ 25w though.

[pompeiisneaks]

As for Mouser, they do still supply Arcol/Ohmite, but you're right and not at that value. They do have 120R @ 25w though.

Since I just bought the 130 10W I'll use those, and if for some reason it seems the current is higher than I'd expect, I'll shut er down and order those. But I'm expecting the 130 would be better suited for the amp, I think my AC30 had a 170 or some such to keep it under 70% diss..

~Phil

[Blackburn]

As for Mouser, they do still supply Arcol/Ohmite, but you're right and not at that value. They do have 120R @ 25w though.

Since I just bought the 130 10W I'll use those, and if for some reason it seems the current is higher than I'd expect, I'll shut er down and order those. But I'm expecting the 130 would be better suited for the amp, I think my AC30 had a 170 or some such to keep it under 70% diss..

~Phil

Most on here would agree that in cathode bias, going with 100% dissipation isn't a biggie, particularly with rugged, old tubes. I have a 50R/25w in both my Rocket and octal 'Pool and I haven't ever had an issue and like I said, it's been a while (years) and I haven't had a single issue, except with modern tubes that were flawed from the get go, but still I've never blown any tubes at all and I'm a cathode bias at 100% dissipation kinda guy and all I've built (I think I'm in the 40s, regarding my amp count, which I don't keep) and haven't had issues, but like I said, I rarely use new production. I'm also not a fan of running stupidly high voltages and that isn't a problem in either amp, particularly the 'Pool, since it's tube rectified. You'll be fine going with 10w.

[pompeiisneaks]

As for Mouser, they do still supply Arcol/Ohmite, but you're right and not at that value. They do have 120R @ 25w though.

Since I just bought the 130 10W I'll use those, and if for some reason it seems the current is higher than I'd expect, I'll shut er down and order those. But I'm expecting the 130 would be better suited for the amp, I think my AC30 had a 170 or some such to keep it under 70% diss..

~Phil

Most on here would agree that in cathode bias, going with 100% dissipation isn't a biggie, particularly with rugged, old tubes. I have a 50R/25w in both my Rocket and octal 'Pool and I haven't ever had an issue and like I said, it's been a while (years) and I haven't had a single issue, except with modern tubes that were flawed from the get go, but still I've never blown any tubes at all and I'm a cathode bias at 100% dissipation kinda guy and all I've built (I think I'm in the 40s, regarding my amp count, which I don't keep) and haven't had issues, but like I said, I rarely use new production. I'm also not a fan of running stupidly high voltages and that isn't a problem in either amp, particularly the 'Pool, since it's tube rectified. You'll be fine going with 10w.

The liverpool is SS rectified Otherwise I think what you're saying makes sense. I don't have a stash of old tubes and can't really afford to buy the NOS ones out there on fleabay, so I stick mostly with new. (I did score about 12 NOS 5R4/5U4's at a hamfest last month though!)

~Phil

[Blackburn]

Since I just bought the 130 10W I'll use those, and if for some reason it seems the current is higher than I'd expect, I'll shut er down and order those. But I'm expecting the 130 would be better suited for the amp, I think my AC30 had a 170 or some such to keep it under 70% diss..

~Phil

Most on here would agree that in cathode bias, going with 100% dissipation isn't a biggie, particularly with rugged, old tubes. I have a 50R/25w in both my Rocket and octal 'Pool and I haven't ever had an issue and like I said, it's been a while (years) and I haven't had a single issue, except with modern tubes that were flawed from the get go, but still I've never blown any tubes at all and I'm a cathode bias at 100% dissipation kinda guy and all I've built (I think I'm in the 40s, regarding my amp count, which I don't keep) and haven't had issues, but like I said, I rarely use new production. I'm also not a fan of running stupidly high voltages and that isn't a problem in either amp, particularly the 'Pool, since it's tube rectified. You'll be fine going with 10w.

The liverpool is SS rectified Otherwise I think what you're saying makes sense. I don't have a stash of old tubes and can't really afford to buy the NOS ones out there on fleabay, so I stick mostly with new. (I did score about 12 NOS 5R4/5U4's at a hamfest last month though!)

~Phil

I actually built it stock first, except for the single bias resistor, but wasn't happy. I added the hole for the 5AR4 after it was already built.... fun!

Nice score on those rect tubes! I love, love, love 5R4s and use them frequently. Using a tube rect drops more voltage than the diodes and helped tremendously with V1 noise. I also tweaked the pre a bit and got to a point where I am quite happy with it. It's a far cry from what the stock circuit is like, regarding tone, but the changes were minimal. The NF resistor, bypass cap, grid leak value, etc.

[pompeiisneaks]

Cool, I may ask you about those tweaks after mine's done if I'm also looking for something different

~Phil