New build Complete: 45W, four 6V6, non-HRM

[ToneMerc]

307-0-307 PT
4H 100mA choke

No V1 LNFB
Plate loads: V1: 150K/120K & V2: 100k/100k, 10uf all around
Mid cap: .035,
Slope: 123K
V2 snubbers: 265pf
V2b cap: .0086
R/J cap: .0019
150K OD resistor, with 100K level
100K/110K(my first mind told me 91K/100K) with 15K trimmer

Dropping string: 820>16K>1.6K

JJ83S V1: 196/187V
JJ803S V2: 198/191V
Sovtek balanced LPS V3: 275/283V
JJ 6V6 V4-V7: 406V plates/405V screens

Bias voltage: 21.2, 21.8,23.1,23.4

I'll let it idle burn for 2-3 hours, then I'll pump in a 1Khz signal and AC balance the PI.

TM

[talbany]

Steve
Not beat a dead hoarse here and sorry if I came off a bit crass in my last post.. But I did have chance to talk briefly with Jim today and he mentioned not all NOS 6V's are the same and (like you stated) that the Sylvanias and JJ's held up just fine under those higher voltage applications..(Tung Sol's might be OK but didn't say for sure) However he did not recommend the GE's and RCA's for use in his amps and therefore never used them..He also mentioned his amps were designed more for tone and not extended tube life..Anyway I thought this was worth the post and hope it helps you and others here!!
IMO Sylvania's one of the best sounding 6V's ever made!!

TM Good Luck with the smoke test!

All The Best!!

Tony

[M Fowler]

TM,

Great looking build.

The rest of you thanks for the education on 6V6's

Mark

[renshen1957]

Steve
Not beat a dead hoarse here and sorry if I came off a bit crass in my last post.. But I did have chance to talk briefly with Jim today and he mentioned not all NOS 6V's are the same and (like you stated) that the Sylvanias and JJ's held up just fine under those higher voltage applications..(Tung Sol's might be OK but didn't say for sure) However he did not recommend the GE's and RCA's for use in his amps and therefore never used them..He also mentioned his amps were designed more for tone and not extended tube life..Anyway I thought this was worth the post and hope it helps you and others here!!
IMO Sylvania's one of the best sounding 6V's ever made!!

TM Good Luck with the smoke test!

All The Best!!

Tony

Hi Tony,

Good point on the GE & RCA tubes, and I totally agree as to the Sylvannia being great tubes. I am glad you posted as to Jim Kelley's comments on his design reasons. Always something to learn on Ampgarage.

Back to Tonemerc thread.

Steve

[marshamps]

Any opinions on the best half power approach for 4 6V6's??? 10K Series Resistance on the Cathodes of two tubes, Pentode/Triode switch, other?

[Structo]

I haven't tried it but I know several of the Dumble ODS amps utilized that 10K cathode thing.
Look at Dumble #124 to see how it is wired.

[marshamps]

Xtructo, thanlks! I always wondered what that really does to the impedance of the primary? And if it doesn't create a mismatch????I guess ill have to try and see what it does.

[Structo]

You know, I really don't know.
But now that you mention it, it probably does raise the primary impedance.

I suppose one could try it and then switch the impedance down one.
Like 8 to 4, right?

What is strange about the way #124 is wired, is that instead of switching the two inside tubes, it switches V5 and V7.

Can anybody explain what happens here, when the cathodes are now 10K above ground?

[alvarezh]

Marshamps, I read about the theory on that circuit a couple of years ago and I can't remember it well. But I do remember that it's maim purpose was to keep the plate impedance at the same level as if they (the tubes) were operating normally.

Unless my memory is playing games with me, I think it mentioned something about the L.E.D., it's not there just to give a visual cue, it has an specific function on the operation of the circuit.

Like Structo recommended, I would copy the #124 circuit as it is.

My clone is a 50 watter, but the output section is completely wired for four tubes and I did incorporate this circuit although my build is not a copy of an specific serial number. I did this so I could use four 6V6s (JJs, so they can take the voltage), but I have not tried it with them yet, so unfortunately I can't comment on it's use.

All the best.

[martin manning]

Can anybody explain what happens here, when the cathodes are now 10K above ground?

With 10k ohms between the cathodes and ground, those two tubes are going to be essentially cut off. The effective primary impedance will be the same for all practical purposes, and the remaining tubes will be looking at half the load they saw with all four tubes working. The diode really is only an indicator of the half-power mode.

[alvarezh]

Thank you Martin for clearing that up.

Cheers!

[marshamps]

The same plate impedance! That a great half power method. Thanks all! I think I also saw this approach on a Carvin 100 water.

[bepone]

when you simple disconnect two tubes in PP amp(or put 10K in cathode), primary impedance will be the same, BUT YOU MUST REARRANGE OUTPUT SELECTOR FOR HALF POWER option.

this means if 4 tubes works with 4K impedance ratio and you disconnect two of them, remaining two of them will be connected also with 4K ratio. if you don't raise translated speaker impedance via switch(8 oms speaker must be connected to 4 ohm OT tap, 16 ohm speaker to 8 ohm tap. tap etc..),
remaining two tubes will be in danger of overheating because of to big load for them..

[martin manning]

Just to be clear, placing 10k ohms between the cathode and ground on two tubes (LED or not) is essentially the same as removing those two tubes, and there will be a resulting impedance miss-match for the remaining tubes as bepone says.

[Structo]

Interesting, so you are saying the LED is for indication only?

I was reading Merlin's preamp book and he has a small section on using LED's for cathode biasing preamp tubes.
Page 34.

He said it can fully bypass the tube acting as a Rk as well as a Ck.
It ensures that the cathode voltage remains constant regardless of anode current.
He does mention that a 100n snubbing cap be placed in parallel with the LED to minimize switching noise.

Now I don't know how well that translates to using one on power tubes but thought I would throw that out there.