1. ## Drop Resist Calc

I searched this topic, but I think I am more confused now than before.
How DO you calculate the Ohm value needed for a given "dropping" resistor.?
I am using Two 6K6 power tubes in a P-P, fixed bias configuration.
I am using the PT and Pi Filter from the below schem for the Eico Sig Gen. The reservoir cap goes to the OT CT. The first cap after the choke goes to the screens.
Now I want to add another filter cap for a 6SL7 LTP Phase Inverter. How do I calculate the Ohm value of that resistor.? For that matter, what plate voltage (from what I have available) should I run the PI plates at.?
Then I will need Caps and resistors to feed B+ to a 6SL7 tremolo circuit that uses both triodes.
And I will need B+ for both triodes of another 6SL7. The first triode will see the main guitar input, then a Fender BF tone stack, then the second triode before it hits the PI.
I am thinking I will need another filter cap and dropping resistor for those two tubes as well. Perhaps I need an RC Filter for each tube.?
I know this is a lot of info. I am in process of learning the J-Schem program so I can make a schematic. In the mean time, I just have to try and explain it to you the best I can.
Thank You

2. You have to know what you want to accomplish first. If you don;t know what voltage you want at a point, we cannot calculate a resistor to make it.

It is all a matter of Ohm's Law. All the current through the dropping resistors goes somewhere. Almost all of it through the tubes. SO each triode, how much current? My general rule of thumb for crude estimates is 1ma per triode. A dual triode then draws 2ma if its halves share B+ node. SO start with a voltage at some point you want to drop from. Calculate with Ohm's Law what resistance at 2ma drops the desirted amount of volts.

Remember to that all current downstream flows through the up stream too. In other words, all the small tubes plus the power tube screen currents flow through your choke. Now the screens branch off, and all the small tube currents are left. If there is more than one stage of preamp B+, then keep in mind that the lowest voltage tube currents are sharing a path with the higher voltage stages currents.

3. Hey Enzo -
I am really not too familiar with octal phase inverters.
I am looking for a schem that uses a 6SL7, as a LTP, in a circuit similar to mine.
I really do not know what kind of plate voltage I should have for this PI.
Thanks

4. Dear Trem, are you trying to recycle that Eico oscillator into a guitar amp?
You can't.
That PSU certainly is not enough.
Why don't you post the full schematic of the amp you want to build?
Please name all parts (such as R1, R2, C1, etc.) so we all refer to the same.
Don't worry about J-Schem, just draw by hand on a piece of paper and scan it.

5. J M Fahey -

V1-6SL7
V2-6SL7
V3-6SL7
V4 and V5-6K6
V6 6X5

That is exactly what I am trying to do.
I do not have a schematic unfortunately.
What I had in mind was:
Guitar into V1a,
then into Fender BF Treble/Bass tone stack,
the into V1b,
then into V2 LTP phase inverter,
then into V4 and V5 that is P-P fixed bias.
V3 is foot switchable Tremolo from the Ampeg schem below.
So are you saying the PT and Choke from the Eico are not up to the task.?
Thank You

6. The power supply in the Eico is designed to make a voltage appear at the output, but no power to speak of. It has a 6K6 power tube at the output, but it is wired as a triode and as a cathode follower. All it has to drive is the output into worst case probably 600 ohms. A low powered amp is all you could reasonably expect. Nothing wrong with that. But not a lot.

Octal doesn't change how the tube works, it just describes the size of the connector on the bottom. A 6SL7 has about the same gain as a 12AT7 - and 12AT7 is common enough in Fender PIs. I think, if I recall, the 12AT7 has a greater dissipation, but that really isnl;t the4 issue in a PI.

7. Hey Enzo -
Guess I did not give this much thought. Glad to have the experience of you and JMF.
I believe the SL7 has am amp factor of 70.
But back to the issue at hand.....
Am I reaching too far with this do you think.?
Will the PT handle what I have in mind with these Two 6K6 power tubes.?
Would it be more reasonable/sound better to maybe think about a single ended output with just One of the 6K6 tubes.?
I would like to go with a 2 x 6K6, but not if it is a stupid idea with what I have.
Not that it matters, but this is the OT I bought...
1750E Hammond Manufacturing | Mouser
Thank You
btw. i am not expecting power like you would have with a fender princeton. but i would like the amp to sound/respond decently. if i can achieve that with the 2 x 6K6 configuration, I would be happy.

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