New 5C1 build design question.

[chikov]

One flaw in these amps is no screen resistor, so adding in even just a 250 ohm 2 watt resistor is a good idea!

Yes, I did that. I tried either way - I could not tell the difference audibly but I left 250Ohm screen resistor in anyway.

[chikov]

One more question. I was playing with voltages and tried to use 660V secondary B+ instead of original 600V. The volume increased by just a little but 6V6 Plate dissipation went up to 16W (on 6V6 it should be below 12W, I think). So I raised the value of a Cathode bias resistor to about 750 Ohms and the PD wattage dropped to about 13 which is still a little too high. is it ok to use 6L6 tube instead? my PT rated 100mA on B+, 4Amps on 6.3V and 3Amps on 5V coil. I can re bias it with 10W resistor.

[Kagliostro]

Have you tried to tame G2 voltage ?

K

[chikov]

Have you tried to tame G2 voltage ?

K

I am not sure what "G2" is...

[tubeswell]

One more question. I was playing with voltages and tried to use 660V secondary B+ instead of original 600V. The volume increased by just a little but 6V6 Plate dissipation went up to 16W (on 6V6 it should be below 12W, I think).

A 660V B+! No wonder the 6V6 tube dissipation is high. in these amps the 500R should be ample to achieve ~12W plate dissipation with B+ voltages of between 340 and 370 or so. Sounds like something is wired wrong if you're running the 6V6 plate at 660V. (unless that was a typo?)

[martin manning]

G2 is screen grid.

660v secondary is 330-0-330, correct?

6L6 would require a 4k primary.

[rp]

Yes it is a very modest amp, not like a 5F1, but it has a nice quality with the grid leak and pentode that when you dig in the clean gets some hair on it, at least with the Edcor I used, I think 5K. No real need for a g2 resistor at 300V, it's more essential on the BFs Champs that can hit 6V6s with >400V. IME Best keep cathode R below 500R, Champs don't sound right when you veer from the schematic, if you have to kluge it, kluge back to spec. I assume you mean 330-0-330 AC? I wouldn't worry too much about the math on the dissipation, as long as it isn't red plating run it to the wall, Fender always did. 350-370V sounds good on 5F1s, I'd try for that on the 5C1 you can also try a 5V4, 5AR4 or SS. BTW the smaller tweeds are all about single coils - funny that. And try a bigger brighter speaker if you are trying an 8", or God forbid a 6".

[chikov]

One more question. I was playing with voltages and tried to use 660V secondary B+ instead of original 600V. The volume increased by just a little but 6V6 Plate dissipation went up to 16W (on 6V6 it should be below 12W, I think).

A 660V B+! No wonder the 6V6 tube dissipation is high. in these amps the 500R should be ample to achieve ~12W plate dissipation with B+ voltages of between 340 and 370 or so. Sounds like something is wired wrong if you're running the 6V6 plate at 660V. (unless that was a typo?)

Yes, what I meant was B+ output on Power Transformer secondary. On my 6L6 plate it is only 389 VDC. Yes, I am trying to use 6L6 and so far i like the sound I am getting. I draw about 42 mAmps of current thru it and therefore PD is about 16.5 Watts. The reason why I experimenting with volume is because someone commented that they were getting a lot of overdrive out of their 5C1 build and I get only clean sound. I mean on the scope i get some distortion on my square wave, but I do not hear it audibly. But with my TS10 overdrive pedal it sounds loud and sweet.

[chikov]

One more observation. After I completed the amp, after 2-3 min into playing it began oscillate with a loud screeching squeal. I switched 6L6 with the original 6V6 tube and it started working well again. Good.
[IMG:640:480]http://i409.photobucket.com/albums/pp171/alexjohnne/4065C6CB-A4BC-4A71-BCA2-22F6A686DBBB_zpsonqenvsl.jpg[/img]

[IMG:640:480]http://i409.photobucket.com/albums/pp171/alexjohnne/33BE118D-5B72-4A7E-81E0-522759D44EC9_zpsjqp4naqb.jpg[/img]

[tubeswell]

One more question. I was playing with voltages and tried to use 660V secondary B+ instead of original 600V. The volume increased by just a little but 6V6 Plate dissipation went up to 16W (on 6V6 it should be below 12W, I think).

A 660V B+! No wonder the 6V6 tube dissipation is high. in these amps the 500R should be ample to achieve ~12W plate dissipation with B+ voltages of between 340 and 370 or so. Sounds like something is wired wrong if you're running the 6V6 plate at 660V. (unless that was a typo?)

Yes, what I meant was B+ output on Power Transformer secondary. On my 6L6 plate it is only 389 VDC. Yes, I am trying to use 6L6 and so far i like the sound I am getting. I draw about 42 mAmps of current thru it and therefore PD is about 16.5 Watts. The reason why I experimenting with volume is because someone commented that they were getting a lot of overdrive out of their 5C1 build and I get only clean sound. I mean on the scope i get some distortion on my square wave, but I do not hear it audibly. But with my TS10 overdrive pedal it sounds loud and sweet.

All good. What (precise resistance) does the cathode resistor measure? And how many volts are you getting across the cathode resistor with the 6L6 plate at 389V? If the cathode bias voltage is high, it won't break up very easily.

[chikov]

Last time I checked the resistor was about 753 Ohms and the voltage drop was about 31V. Is it high?

[chikov]

I am confused... I was listening to Gerald weber on biasing type A single ended amps. He said that you can determine the saturation level of a tube by connection you mAmp meter in series with your primary of the OT - between pin 3 (my 6V6 tube) and the lead to the output transformer. When I do that, instead of a around 120 mAmp current I get something very high ( over 1000 mAmps). My Fluke does not register anything above 999 mAmps. He said that you can connect your milli amp meter across the OT primary and see the current this way, but it is also a very high number on my meter as well. What am I doing wrong? I just want to see the level of saturation and bias the tube somewhere in the middle of that number... Am I correct?

[Phil_S]

I think you are talking about the shunt method of checking tube bias. Here are clear instructions on how this is done: http://www.duncanamps.com/technical/lvbias.html

Now, do yourself a favor. Stop trying to measure current directly. There is a safety advisory on the page I referenced above. Read it twice. It explains all you need to deter you.

You can always measure current by indirect means and use Ohm's Law to solve for current. Where I = V/R, when you have the voltage drop over a known resistance, just solve the equation. For what you want to do:
1) Disconnect the transformer secondary. (To do this pull the tube to disconnect pin 3, and sever the connection at the B+ supply, so you only have to detach one wire.)
2) Measure the ohms across the transformer secondary. Write that number on the chassis or on the transformer so you don't forget or lose the number. Ordinarily, I do this before installing the transformer.
3) Reconnect the wire.
4) Reinstall the tube.
5) Turn the amp on.
6) Measure the voltage drop from the B+ supply to pin 3. You can either a) directly measure the drop: one lead on the B+ and the other at pin 3, or b) measure at each point to ground and compute the drop.
7) Divide the voltage drop by the ohms of the transformer secondary.

IOW, a drop of 3V over 100Ω is 30mA.

Really, though, the cathode method works just fine with the 1Ω sense resistor.

[Colossal]

Really, though, the cathode method works just fine with the 1Ω sense resistor.

Here is a big bottle 5C1 with test jacks. The jacks are very useful for monitoring current directly. You want to use a good quality 1% 1Ω resistor.

[chikov]

Thank you, Phil, for all this help. Yes, I will use your advice. For some reason THE OUTPUT TRANSFORMER SHUNT METHOD is not working for me. May be it has to do something with my Fluke multimeter. From now on, I will be using just Cathode resistor method ( or transformer secondary, altho the 1 ohm resistor in the fixed bias amps sounds the easiest). I love the idea of the test jacks. Would you install one for the plate voltage or it is too dangerous?