Mini TOS SE with FX Loop Build

[PharmRock]

About to tackle Tubenit's Mini TOS (schematic below).  However, instead of using the 6BM8 as the FX return (triode) and power tube (pentode), I will be using 1/2 12AX7 for the FX return and a 6L6 for the power tube. 

The PT I am using is a Weber W022722, which has the option of 600V CT secondary or a 660V CT (100mA).  The 6.3V heater winding is rated at 4A, and I will be using a SS rectifier (there is a 5V rectifier winding at 3A, so I have options here). I am using the Hammond 125ESE OT.  I have a few questions about these changes and hope you guys can help me out here.

1. I'm guesstimating an unloaded B+ of ~400 to 420VDC using the 300-0-300 winding on the PT with a SS rectifier (1.4 x 300 ballpark). Figuring ~ 90% plate dissipation on the 6L6GC, I would be shooting for around 27W, and with a plate voltage of 400V would give me about 70mA.  Since my PT is rated for 100mA, I think I should be ok with 3 triodes and the 5879 added on to the 6L6 @70mA.  Does this sound right?

2. On cathode biasing the 6L6, I am having trouble determining the value of the cathode bias resistor.  I know that I should be looking at a target grid voltage, and dividing that absolute value by the cathode current (70mA).  What would be the target grid voltage for this situation?  I tried to figure it out using the data sheet (attached) but I can't seem to find anything where the plate and screen voltages are in the neighborhood of what I will have in my amp (~400). The best I could estimate is a grid voltage of -20V, which would give me an Rk of 250-300R.  This matches what I've seen in other 6L6 SE designs.  Any help here is much appreciated!

3. With my PT, the voltages will likely be a little higher than what is seen in the mini TOS schematic with the Hammond 269EX.  Since this is my first time using a 5879, I didn't want to go too high on the plate and screen for this tube.  Are there any recommended voltage ranges on the plate and screen for the 5879?

4. Since I am using a 6L6 instead of the 6BM8, I am losing the "free" triode from the 6MB8 that serves as the FX recovery.  Therefore, I will be using a 12AX7, but only half of it.  The idea of an unused triode drives me nuts, so I figured I could either A) use the other half as the FX send driver (instead of the MOSFET), or 2) parallel the triodes of the FX recovery.  Any advantages to either of these approaches, or just accept that I'll have an unused triode?

Thanks for reading...appreciate any suggestions/guidance you can give.

[PharmRock]

I've taken a stab at modifying Tubenit's schematic to incorporate the 6L6 and 1/2 12AX7(to replace 6BM8), and the Weber PT.

Please let me know if there are any glaring errors.  Will begin working on the layout soon.

Thanks!

[tubenit]

The only thing that I would suggest considering is having the 5879 and the 12AX7 triode FX recovery have it's own filter cap on the B+ rail meaning they share one apart from the V1 triodes.   I think that could be useful and perhaps offer you a quieter amp?


Look forward to following your build.


With respect, Tubenit

[PharmRock]

Just updated the schematic, added the extra filter node.
A: 6L6 plate (V4)
B: 6L6 screen (V4)
C: 5879 (V2) and FX recovery (V3a)
D: 5751 (V1a and V1b)

I have DIYLC fired up and doing the board layout tonight.  Hope to have that posted before too long, but I am slow as molasses when using that program.
Thanks!

[PharmRock]

Tubenit,
Is the OD trim pot typically internal on the board?  I think a lot of Dumble style amps have it that way as a fine tuning of the OD section, then "set and forget". 
My front panel real estate is limited so I'm trying to figure out the best approach.
Also, instead of a mid tone pot on the OD section, I'll be using a switch...I was thinking a typical Fender value of 6.8K as well as a 25K option.  Do those sound good?  I could go much higher than 25K for a hefty mid boost or a pseudo tone stack bypass.
Thanks!

[tubenit]

IIRC,  I think I've done a trim pot internally, on the back chassis panel and on the front chassis panel. I do find it to be more of a "set and forget" feature overall especially when the OD section has 2 gain stages. 

Having said that, the Dumble OD sections have a trim, drive and level around two triode gain stages.  Since you have only one gain stage available, you might consider wanting the trim on the front amp chassis panel? 

I certainly think the 6.8k and 25k would work fine.  If you used a mini-toggle that is on-off-on, you could get 3 choices.
I have used smaller potentiometers for the mid tone pot on several amps & simply left a knob off to save space on the front of an amp chassis.  So, that might be another consideration?

with respect, Tubenit

[PharmRock]

alright....took me a while but I think I have the layout finished. 

Tubenit...thanks for all the info you have shared.  I am putting the OD trim knob on the front panel for now and may relocate it if needed.

I've attached PDF, SCH, and DIY files below.  I'd appreciate all the experts out there if you could look these over and let me know of any errors or improvements.  I wasn't sure about putting filter caps on the board along the edge like that but it seemed to work best for my board. 

Thanks again! I have a few components to order, and then I will get to drilling the board and chassis once everything is squared away.

[PharmRock]

I found a few errors and corrected them on the layout:
1. Tied pin 9 to pin 3 on 5879 tube socket.
2. Made connection between Node C filter cap and power rail on the board.
3. Updated rectifier, standby, and power switch wiring.

I'll be wiring up the OT for a 5K plate load.

Just posting the updated PDFs below.
Thanks!