Hot biased 6v6? Some help plz

Jerry garrcia

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Hi. Just completed a build but getting a bit worried that I will burn the 6v6. Have a couple of Raytheon and JJ’s.
A SS Class A amp with two channels. One 6SC7 and one 6SJ7 and one 6v6.
B+ 362V
B+2 354V
B+3 289V
A 270R cathode resistor for the 6v6.
Did some measurements on the 6v6.
Cathode Resistor 272.3 Ohm
Voltage drop over resistor -18.2V (measured on each side of resistor).
Plate 344.7V
V/R = 18.2/272.2 = 0.0669A = 66.9mA
0.0669 * 344.7 = 23W!!! Shouldn’t it be like 12W max?

Am I totally wrong? If not suggestions plz. Increase the cathode resistor? Or?
 

Jerry garrcia

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Yes, the resistor needs to go up by about two times. Maybe try adding some values in series until you get something in the right area at try.

Also, if you have not seen it Robrob has this nice calculator https://robrobinette.com/Tube_Bias_Calculator.htm
Thanks. Was out doing a 25K trail run and couldn’t let go of that. Found the calculator and will change. Will use a 220K in series to see what happens.
 

VintageSG

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Start with a 470 Ohm and a 22uF 25V bypass cap and take it from there. Anywhere from 95% to 115% dissipation will yield sonic glory :)
 

VintageSG

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Fit it, measure it, see where it sits, listen to it. 6 months of glorious sound from a hot run valve is way better than a year of meh.
Take 12W as a guide, not a hard and fast limit. Older amps often ran crazy values, and sounded spectacular doing it. Up to a certain, nebulous point, the hotter the better in some ( mine included ) ears. You can always cool it down a little with a ( say ) 100 Ohm in series. That nebulous point is just that. If you take a dozen NOS valves, fit and measure them, you'll see a large variance. Some would run happy as a happy thing at values that'd be off the charts, and do so for a long time. Others would have issues at even their designated ratings. In part, it's why Mesa ( and maybe others? ) have graded replacements depending on the dissipation in circuit.

ps. I have a single ended screamer that measures to 112% with no problems.
 

2L man

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I now changed the cathode resistor to a 5w 470R.
Plate 350V
Voltage drop 22 V
Cathode 22.2V
That gives 14.5W. That’s 103% of a 14w JJ 6V6 tube. Should be ok I guess?
Is this Single Ended amp using one power tube or Push Pull A-class amp? If PP then both tubes share the current there flow thru Cathode Resistor and one tube use about half of it.

For A-class 6V6 the 350VDC plate is quite high! What is the OT impedance?
 
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Jerry garrcia

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Is this Single Ended amp using one power tube or Push Pull A-class amp? If PP then both tubes share the current there flow thru Cathode Resistor and one tube use about half of it.

For A-class 6V6 the 350VDC plate is quite high! What is the OT impedance?
I know 😀. Running a Hammond 1760C. 8K primary winding.

The amp is this https://www.tdpri.com/threads/the-t...e-biased-circuit.1100753/page-3#post-11398771

Sounding good but problem with low output from one channel. Any help appreciated.
 

Jerry garrcia

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If this is a singled ended 6V6 amp, why did you use a 270 ohm cathode resistor to begin with?
Made this schematic as my first schematic late one night and it was probably a combination of Jameson, frustration, sleep deprivation and stupidity. It will not happen again I can assure you. The next one will be made earlier since I’m still frustrated, stupid and tend to drink to much.
I learn the most when I makes all the mistakes since I hate to redo things.
In the retrospect I have asked myself the exact same question as you did.
 

2L man

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I know 😀. Running a Hammond 1760C. 8K primary winding.

The amp is this https://www.tdpri.com/threads/the-t...e-biased-circuit.1100753/page-3#post-11398771

Sounding good but problem with low output from one channel. Any help appreciated.
Ok SE understood :)

I would first measure pre amp resistor values and then "operation points" measuring all voltages and then calculate anode and cathode currents which usually should be the same.

Especially for tubes or circuits I don't know yet I trust the Universal Loadline Calculator because after changing few variables I immediately see the loadline.
 

Jerry garrcia

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Ok SE understood :)

I would first measure pre amp resistor values and then "operation points" measuring all voltages and then calculate anode and cathode currents which usually should be the same.

Especially for tubes or circuits I don't know yet I trust the Universal Loadline Calculator because after changing few variables I immediately see the loadline.
I did almost all of that I the build thread.

Did some new measurements. No cord in input jacks.
B+ 362V
B+2 354V
B+3 289V

Resistance/voltage to ground.
Resistance/Volt values 6SJ7:
Bias switch towards the speaker
Plate: 10M / 211 V
Grid: O.L / 5 mV
Screen: 11.8M / 52.5 V
Cathode: 2.2K / 2.4 V

Bias switch towards back
Plate: 10M / 196 V
Grid: O.L / -1.1 V
Screen: 11.8M / 31 V
Cathode: 0R / 0 V

Resistance/Volt values 6SC7:
Plate 1: 10.5M / 175 V
Grid 1: 65K / 0V
Plate 2: 10M / 151 V
Grid 2: 67K / 0.4 V
Cathode: 2.2K / 2.3V

Resistance/Volt values 6V6:
Plate: 10M / 250V
Grid: 127K / 0V
Screen: 10M / 346 V
Cathode 472R / 22.2V
Voltage drop 22 V over cathode resistor

Almost all is in the thread, long one, https://www.tdpri.com/threads/the-t...id-leak-cathode-biased-circuit.1100753/page-3

Haven’t measured all preamp
Resistors yet. Will do that tomorrow or Tuesday.
 

2L man

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meant actual resistor values because they have a tolerance!

But using those voltages and resistor values on other link it 6SC7 look fine. Anode currents are 0,51mA and 0,42mA and combined Cathode current comes 1,04mA which is quite a lot much more? Some error here comes when the Vc=2,3V has only one decimal. Or resistor values are more or less off the numeric value.

Obviously there can come some internal leaks in triodes but if I have ever learn anything of it I have completely forgot it ;)

I have few 6SC7 but have not yet used them and that is not in Universal Loadline Calculator tube list :(

Interpreting datasheet one stage gain comes about 37 but I did not notice anode resistor on numeric table? Just B+ voltage and cathode resistor and there was 700 ohm and next was 1,3k and I placed 1,1k which is half of combined 2,2k in between there. Perhaps datasheet assume that Rc is combined because both triodes have same cathode?

Anyway I think voltage divider after first stage is what I would test tune first begause now it pass only 20% of signal to second stage.
 

Jerry garrcia

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meant actual resistor values because they have a tolerance!

But using those voltages and resistor values on other link it 6SC7 look fine. Anode currents are 0,51mA and 0,42mA and combined Cathode current comes 1,04mA which is quite a lot much more? Some error here comes when the Vc=2,3V has only one decimal. Or resistor values are more or less off the numeric value.

Obviously there can come some internal leaks in triodes but if I have ever learn anything of it I have completely forgot it ;)

I have few 6SC7 but have not yet used them and that is not in Universal Loadline Calculator tube list :(

Interpreting datasheet one stage gain comes about 37 but I did not notice anode resistor on numeric table? Just B+ voltage and cathode resistor and there was 700 ohm and next was 1,3k and I placed 1,1k which is half of combined 2,2k in between there. Perhaps datasheet assume that Rc is combined because both triodes have same cathode?

Anyway I think voltage divider after first stage is what I would test tune first begause now it pass only 20% of signal to second stage.
Tack för informationen.
I'll look in to that. My main problem is that the Pentode is not putting out enough volume. It's like its lacking its full gain capacity. driving me crazy and I cant work :) Just thinking of the amp. The son is frustrated with his stupid dad and wants me to change his amp to the last build instead. I don't want that since this should work. How can a build that only differs in the choice of triode in the other channel (6SC7 vs 12ax7), a different tone circuit and a reverb? Strange.
 

2L man

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I thought it was the triode side which is quieter :(

I don't have much experience but two triode stages can have very much higher gain than a single pentode although one pentode has more gain potential than one triode. Its because second triode amplify what first triode already amplify. I am sure you already knew this!

I use those voltages on Loadline Calculator what the 6SJ7 loadline looks. Or has Printer already done it?
 

2L man

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Jag tror jag har någonting :)

The loadline is very gold!

When I did input anode bias current 0.78mA it did calculate the plate voltage of 211V which is exactly what you have measured. However the g1 voltage comes different but I think its because in calculator there is no Screen current which in yours is 0.23mA. Anode and Screen together are 1.02 and thru 2,2k make 2,24V and your voltage reading is 2,4V.
 

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Jerry garrcia

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Thank you. Sorry. Missed this. Forgot to use the email notification thing.
So regarding the loadline calculator the Bias is perfect?
Then the problem is further down the circuit. Changed a bad pot and made it a bit better but not enough. Much quieter than other builds with a 6SJ7 in that position. Today I changed all components in the pentode circuit and also the coupling cap from a 0.1 to a 0.022 uF.
Will disconnect the tone circuit and bypass the mixing resistors.

Now I’m planning a ES-185 build
 




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