6V6 TOS

[stingray_65]

Hey Guys,

Starting up a new version of Tubenits TOS. I contacted Jeff first and he was helpful enough to throw out a lot of suggestions.

So here is my first draft, mostly Tubenit's work, I added some mods he drew up in his "Bells and Whistles" post in the schematic section.

So any how, I need help nailing down the PS a bit, I've got a few ?'s on a few specifics I'll get to soon.

Note that node "E" is not used. I plan on modding this amp further down the road. my board will have a few extra rows and I'll add an extra 9 pin socket to the chassis and wire the heater in it.

We discussed very briefly the possibility of using a 5Y3GT (vintage) to knock the B+ down for the 6V6's and later I can maybe run a 5U4 or 5AR4 to bring the B+ up to 6L6 ranges.

The OT was harvested from an organ that ran 2 7027's in P-P @ 425V so it should be compatible with both 6V6's ran at the lower B+ and the higher stress of the 6L6's when the time comes.

Any comments and suggestions would be appreciated.

Ray

[tubenit]

I'd change the OD Drive to 100k.  

I found the OD tone control to be very useful to me & would suggest considering it. I use a 16mm mini-pot wihtout a knob for the OD tone. It takes up very little room.

Will the PPIMV work in the fixed bias mode?  I don't know why it wouldn't but never thought about it before? What would happen to the bias if the PPIMV is dialed to "zero"?

With respect, Tubenit

[kagliostro]

For fixed Bias the PPIMV must have isolation caps
(also to avoid scratch)



Ciao

Kagliostro

[chocopower]

For fixed Bias the PPIMV must have isolation caps
(also to avoid scratch)

He is using a diferent MV (the LAR-MAR, i think is called) and dont need that extra caps.
The one you post is used in some 80´s Orange OD and yes, it need the isolation caps.

[stingray_65]

Yes, fixed bias works very well.

Tubenit, remember my first TOS? It was fixed bias. I had issues, but it was nothing to with the bias, just my inexperience.

I have the old print I worked from faithfully, I will check it to see if I used the caps, I think that maybe I did use them.

I'm not sure why I left out the OD tone control!

will change OD Drive to 100K and update in my OP

Ray

EDIT
I now see I had 50kL for the OD Drive, should have been 250!

[stingray_65]

The schematic notes I think I used to make the fixed bias on the original TOS

[kagliostro]

The schematic notes I think I used to make the fixed bias on the original TOS

With no scratch noise ?

attached the Bruce Collins version

Kagliostro

[stingray_65]

no scratch at all.

see here was our discussion

http://www.el34world.com/Forum/index.php?topic=10097.0

[kagliostro]

I'm surprised about the fact that the pot don't scratch

but there is always something new to learn

Thanks for the link

Kagliostro

[jojokeo]

Any power scaling?

[tubeswell]

Ray

The 6V6 grid resistors should be 220k each (they are 2M2 in your schematic, which is waaayyy too high)

For the LTP I think you'll get better results with a 12AT7 or 12AX7 with those circuit values. FWIW the LTP in a 5G9 (which also uses 6V6s) has a 10k tail and 470R bias resistor and this drives the 6V6s superbly in that amp, and mightily with a Hotcake in front of the amp (so with the extra pre-amp gain of a TOS circuit, it should be fine with 10k/470R and a 12AX7). If you're going to go down the 12AU7 route, then maybe at least cut the plate resistor values in half to get the benefit of the current to drive the 6V6 grids - but its really overkill I reckon. JM2CW

[tubenit]

The 6V6 grid resistors should be 220k each (they are 2M2 in your schematic, which is waaayyy too high)

Please look again. There are 250k dual gang pots with 2.2M resistors across which makes 220k.  They're correct.

I have the LarMar PPIMV on two current amps.  No scratch issues at all.

With respect, Tubenit

[tubeswell]

Oops wasn't thinking about teh parallel pots - so I am waaayy wrong 

[stingray_65]

No power scaling on this build JoJo, Probably my next one, but that will be a 100W monster.

I corrected the OD Drive pot to 250K as used on my first TOS.

I'm unsure why I listed the PI as an AU7, perhaps from doing schematic work after long shifts? corrected the schemo to an AX7.

Maybe the same reason I left out the OD Tone.

All corrected in the original post

I'm concerned about the choke, will it be too small at 50mA? Should I have ordered the 90mA instead?

[tubeswell]

I'm concerned about the choke, will it be too small at 50mA? Should I have ordered the 90mA instead?

The way you have it in that circuit a 50mA DC choke will be fine. The 6V6 screens don't draw much current even under signal conditions, and the tube current for the other tubes maybe 1-2mA each tops.

[tubenit]

and the higher stress of the 6L6's when the time comes.

I think he is planning to set it up to use 6V6 and/or 6L6 tubes?  So maybe the bigger choke would be an improvement?

with respect, Tubenit

[stingray_65]

I'm concerned about the choke, will it be too small at 50mA? Should I have ordered the 90mA instead?

The way you have it in that circuit a 50mA DC choke will be fine. The 6V6 screens don't draw much current even under signal conditions, and the tube current for the other tubes maybe 1-2mA each tops.

So to start figuring out my dropping resistors, I determine my current draw at each node
@ F, V1 1ma,
@ D, V2 1mA +V3 1.5mA plates +.5 screens = 3mA
@ C, V4 1mA
@ B, V5 + V6 screens .3mA each = 6mA
@ A, V5 + V6 plates 50mA each = 100mA

 Next I figure my supply voltage: 330*1.41-50 = 415

so @ node A I have 415V

@B I have a DCR of 150, and a current draw of B+C+D+F = 11mA, ohms law says I'll have a 1.8V drop so my voltage at B should be about 413.

Am I doing this right?

[stingray_65]

and the higher stress of the 6L6's when the time comes.

I think he is planning to set it up to use 6V6 and/or 6L6 tubes?  So maybe the bigger choke would be an improvement?

with respect, Tubenit

I am, But for the moment , I would like to kind of hammer this PS thing out  

I'll look up the specs of the bigger Choke and update the schem's

IG2 for 6L6GT in AB2 P-P @400V = 8mA

so I may still be ok with the small 40mA

That is to say unless I'm doing my charts and math all wrong, which is a distinct possibility.

Ray

[Willabe]

Hi Ray,

This new build is looking great.

IIRC, the screens on a 6L6 is usually about 10% of the plate? 50mA on the plate, 5mA on the screen? The smaller choke should be fine.


                          Brad     

[tubeswell]

I'm concerned about the choke, will it be too small at 50mA? Should I have ordered the 90mA instead?

The way you have it in that circuit a 50mA DC choke will be fine. The 6V6 screens don't draw much current even under signal conditions, and the tube current for the other tubes maybe 1-2mA each tops.

So to start figuring out my dropping resistors, I determine my current draw at each node
@ F, V1 1ma,
@ D, V2 1mA +V3 1.5mA plates +.5 screens = 3mA
@ C, V4 1mA
@ B, V5 + V6 screens .3mA each = 6mA
@ A, V5 + V6 plates 50mA each = 100mA

 Next I figure my supply voltage: 330*1.41-50 = 415

so @ node A I have 415V

@B I have a DCR of 150, and a current draw of B+C+D+F = 11mA, ohms law says I'll have a 1.8V drop so my voltage at B should be about 413.

Am I doing this right?

Sounds okay to me (but I have shot myself in the foot before)

[stingray_65]

Thanks TW,

So let me go further and figure my dropping resistors.

I'm going to guess at my target voltages to the various nodes.

C, the PI, will be at 310v (looking at the 5G9 Tubeswell noted as being similar)
D, the OD section, will be at 285V
F, Preamp section, will be at 270V

so, if B is at 413, using ohms law to get a 103v drop at 5mA (C+D+F) for node C I'd need a 21K @.515W so 2W would be real safe.
D has a 25V drop at 4mA (D+F) = 6.2K @.1W so a 1/2W is safe here
and F has a 15V drop at 1mA = 15K @.015W so again a 1/2W is overkill.

Did I figure this right?
or am I still in left field.

Ray

[tubeswell]

so, if B is at 413, using ohms law to get a 103v drop at 5mA (C+D+F) for node C I'd need a 21K @.515W so 2W would be real safe.

1W would be real safe:
103V/21,000R = 4.9mA,
and 4.9mA x 103V = 0.5W

D has a 25V drop at 4mA (D+F) = 6.2K @.1W so a 1/2W is safe here
and F has a 15V drop at 1mA = 15K @.015W so again a 1/2W is overkill

Yep pretty much. In reality you just might have to experiment with the dropping string to get the voltages where you want them