Help biasing an amp, PP EL34

[starseeds]

I recently purchased an amplifier at a very low price, but it came without any tubes. I ended up buying a four of Tung-Sol EL34b tubes for it. However, I'm facing difficulties when trying to biasing this thing. My main concern is preventing damage to the new tubes, which cost almost as much as the amplifier itself. The amp in question is the Psvane tc-1 and unfortunately I don't have a schematic for it. Also, I'm puzzled by the fact that in this design, pins 1 and 8 appear to be connected. Here are the various methods I've attempted to figure out the mA..

I acquired bias probes and correctly connected them to my multimeter, but it always reads 0.00 mA.

I measured several voltages:

Voltage between pin 3 and pin 8: 470V
Voltage between pin 3 and ground: 470V
Voltage across the resistor connected to pin 8: 150V
Voltage between pin 5 and ground: 45.9V

In a desperate attempt, I disconnected the white cable going to pin 5 and tried measuring mA by clamping the white cable and pin 5 with alligator clips white connected to my multimeter but it still shows 0.00mA. (shows voltage though)

I’m so lost at this point any advice on what to do next would be greatly appreciated…

[sluckey]

I acquired bias probes and correctly connected them to my multimeter, but it always reads 0.00 mA.

Did the bias probes come with instructions? Some bias probes actually measure  millivolts across a 1Ω resistor and you have to mentally convert that mV to mA. This means you must set your meter to measure mV, not mA.

But your amp has 1Ω resistors already connected to pin 8, so you don't even need a bias probe. Simply measure the mV between pin 8 and ground and mentally convert to mA. That's the tube current. Now measure the voltage on pin 3 and multiply that voltage times the mA found on pin 8. That gives the dissipation in watts.

It would be beneficial to read the instructions on this link....

     http://www.duncanamps.com/technical/lvbias.html

BTW, it's very normal to see pin 1 and 8 connected together in an EL34 amp.

[PRR]

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[starseeds]

Sluckey, PRR, thank you for your advice.

Sluckey, yes the probe did come with instructions and it says to set mA, perhaps it is faulty.
 
Per your advice I took measurements, mV between pin 8 and ground at 44mV. (Mentally converted to 44mA). Voltage on pin 3 and ground was 484v, so 484 x 0.044 = 21.29 watts. This seems to add up, however the max dissipation of EL34b is 25 watts and this is on the bias setting turned all the way low. I set all the tubes to the same 44mA (mV on my meter) and it plays, however it starts to distort with the dial barely at the 9 o’clock position, at pretty low volume. If I set the bias any higher it will go past the max dissipation quickly.

Did I screw up somewhere? Should I try to get it as close to 25 as possible?

[AlNewman]

That seems a bit high.  But it looks like you have a separate bias setting for each tube?  Or at least each side.  When you adjust everything, it should change the voltages across the amp.  Looks like a pretty pimped out amp by the way.  Nice score.

[pdf64]

… In a desperate attempt, I disconnected the white cable going to pin 5 and tried measuring mA by clamping the white cable and pin 5 with alligator clips white connected to my multimeter but it still shows 0.00mA. (shows voltage though)
 …

That was an extremely ill advised action. All I can think is that you didn’t use your meter correctly, as normally that would operate the valve unbiased, wreck the valve and, hopefully, blow a fuse before further damage was done.

[Latole]

… In a desperate attempt, I disconnected the white cable going to pin 5 and tried measuring mA by clamping the white cable and pin 5 with alligator clips white connected to my multimeter but it still shows 0.00mA. (shows voltage though)
 …

That was an extremely ill advised action. All I can think is that you not using your meter correctly, as normally that would operate the valve unbiased, wreck the valve and, hopefully, blow a fuse before damage was done.

You are right,

To many people use their meter before to know it work.
Same with amp repair ; You must learn some basic before, not with you hand on the amp.

[starseeds]

AlNewman, yes thank you! I do have a separate bias adjustment for each tube.  While biasing the amp I ran into another issue, all tubes except tube 3 read around 44 mV measured pin 8 to ground, however tube 3 to ground measures 70V not mV. What could cause this issue? I tried swapping tubes, but it didn't help. Voltage keeps creeping up fast as soon as I turn on this amp and bias adjustment does pretty much nothing to bring it down.

pdf64, latole - understood, at this point I just to get it over with.

[starseeds]

I think I figured it out, the 5 Ohm resistor went out, took it out, meter reads OL, waiting on replacement.

[sluckey]

tube 3 to ground measures 70V not mV. What could cause this issue?

That 1Ω resistor looks like it may be damaged. Measure resistance from V3 pin 8 to ground. If it's anything except 1Ω, replace it.

[sluckey]

I doubt that resistor is 5Ω. Maybe 10Ω. Usually 1Ω. Measure the other resistors to be sure.

[starseeds]

hey sluckey, thanks for your reply, yes the resistor looks kind of charred, took it out of circuit reads OL. Other resistors are very close to 10 Ohm. Colors are brown black black gold (last band is black) you were right - it's 10 Ohm 5% tolerance. (I'm not sure what the last black band indicates though) I'm going to order a replacement, what wattage would you suggest for this circuit? I wish I had a schematic, also now I know why this amp was so cheap.

Another question I have, since these resistors are 10 Ohm and not 1 Ohm, does this have any affect on the biasing method which you taught me in your first post?

[sluckey]

Another question I have, since these resistors are 10 Ohm and not 1 Ohm, does this have any affect on the biasing method which you taught me in your first post?

Yes. Earlier you measured 44mV across what I mistakenly called a 1Ω resistor. That would have equated to 44mA, which is believable for an EL34 tube. ***BUT***, since the resistor is really 10Ω, everything changes by a factor of 10. IOW, 44mV across 10Ω would only be 4.4mA and that's extremely LOW for an EL34.

Bottom line, measure the voltage on pin 8, and divide by 10 to calculate the current.

[starseeds]

sluckey, thank you! That explains a lot! I will order a replacement resistor and update on my progress! Thank you for all your help so far!

[PRR]

It popped a cathode resistor and was sold without tubes because a bad (shorted) tube was in it. Before I put the resistor order in the mailbox, I would also check the G2 Screen resistor on that tube.

[Bieworm]

sluckey, thank you! That explains a lot! I will order a replacement resistor and update on my progress! Thank you for all your help so far!

To simplify things you’d better order 1 ohm 1% resistors. That is the proper and most common approach

[starseeds]

It popped a cathode resistor and was sold without tubes because a bad (shorted) tube was in it. Before I put the resistor order in the mailbox, I would also check the G2 Screen resistor on that tube.

That was a good call, upon very close inspection the screen resistor kind of looks like it's been too hot. I did measure it and it measures exactly 0.978 kOhm so that's within spec. Checked the other three ones and they also measure very close to 1k Ohm. I think it's best not to take any chances and replace.


Bieworm - that's an interesting option. I could order four 1 Ohm resistors with 1% tolerance and swap all four of them, however I'm a little worried, since my knowledge is limited. Wouldn't it affect the whole circuit in a negative way since it was originally designed with 10 Ohm resistors?

[Bieworm]

It popped a cathode resistor and was sold without tubes because a bad (shorted) tube was in it. Before I put the resistor order in the mailbox, I would also check the G2 Screen resistor on that tube.

That was a good call, upon very close inspection the screen resistor kind of looks like it's been too hot. I did measure it and it measures exactly 0.978 kOhm so that's within spec. Checked the other three ones and they also measure very close to 1k Ohm. I think it's best not to take any chances and replace.


Bieworm - that's an interesting option. I could order four 1 Ohm resistors with 1% tolerance and swap all four of them, however I'm a little worried, since my knowledge is limited. Wouldn't it affect the whole circuit in a negative way since it was originally designed with 10 Ohm resistors?

That 1 ohm or 10 ohm would not change anything about the circuit. It’s just a small resistance to ground to enable the measuring of current draw. Don’t worry

[AlNewman]

In order to test the amp before you get your order in, you can actually jump that 10 ohm resistor.  The only reason it's there is to measure.

On that particular tube though, you'd have to find a different way to measure bias.  Your plate, screen, and grid voltages should tell you a lot in comparison with the other tubes.  You could also measure across the transformer if you're comfortable with that.  I don't imagine you have a light bulb limiter?  Even a GFCI outlet would be better than nothing.

Just out of curiosity, was that the tube you were monkeying around with unhooking the grid wire?

[starseeds]

AlNewman, I wasn't aware that I could simply bypass this specific resistor. Thanks for the advice. I'm quite comfortable working with high voltages, having fixed a few antique tube radios long ago in the past. Mostly, I replaced old wax capacitors and even re-stuffed some electrolytics – it was all very straightforward, and I had easy access to schematics online. However, I've never had to deal with setting bias on a class A PP amp before.

Unfortunately, I don't have a light bulb limiter, only a variac and a GFCI outlet. I'm planning to try measuring across the transformer, as you suggested, but I have to read up on it first.

By the way, I disconnected the grid wire from tube #1 as it was the easiest to access. Tube #3 is the one with the open resistor. Now that I think about it, it might be a good idea to replace screen and cathode resistors on tube #1 as well, just in case I caused some future failure points.

[AlNewman]

Watch some Uncle Doug videos on youtube.  He has one called biasing a push pull amp or something.  It would be right up your alley, just remember you're dividing your current over 2 tubes/side instead of 1...  assuming your amp has one output transformer and is push/pull.

I wouldn't think you'd have to replace the screen resistor on tube 1 unless there was a fault of some kind.  I was just curious as to a clue if that was why the resistor went on the cathode. 

[mresistor]

I was going to say that one of the screen ressistors looks like it took a little heat,,  the second from left   good idea to check all the screen resistors..

[starseeds]

AlNewman- I'm going to try to find his video now. My amp has 2 output transformers 4 EL34b tubes, push-pull.

mresistor - Yep it does, I already checked it, it registers fine very close to 1kOhm so within spec but I'm still going to preemptively replace it due to the way it looks.

Edit: might as well replace all screen and cathode resistors, who knows where the amps been, and change cathode 10 Ohm 5% resistors to 1 Ohm 1%.

[PRR]

assuming your amp has one output transformer and is push/pull.

It is stereo Hi-Fi. NOT a guitar amp like most here.

Arguably 10 Ohms "could" give a distortion REduction compared to 1 Ohm. But until the beast works properly, the difference is moot.

Disconnecting G1 CAN burn-up a power tube. It was designed for grit at some specific voltage like zero. If left open, the grid does not know where to go. There is some tendency to go very negative, cut-off; but leakage on socket can easily make it very positive, turned-ON.

[AlNewman]

assuming your amp has one output transformer and is push/pull.

It is stereo Hi-Fi. NOT a guitar amp like most here.

Arguably 10 Ohms "could" give a distortion REduction compared to 1 Ohm. But until the beast works properly, the difference is moot.

That just means you can plug in 2 guitars?

You can still bias the same way, just have to figure out how it relates to your particular set up.  That being said, with a hi-fi amp, I  believe the bias percentage can change drastically, depending on the class of amp you have. 

PRR had some advice about possibly keeping the 10R resistors, and he's probably the guy to talk to about what class your amp is, and where to bias it.

[starseeds]

Guys, thank you for your replies and advice. I just got some 10 Ohm 5% 4w resistors and replaced all four of them. I did the biasing procedure, reading voltages form pin #3 then pin # 8 and adjusting both accordingly. Since it's a 10 Ohm resistor I had to move one decimal point over, also due to this I feel I can get more accurate measurements with my cheap meter. So far I've got:

Tube 1: 443v x 0.038 = 16.83w
Tube 2: 442v x 0.039 = 17.23w
Tube 3: 442v x 0.037 = 16.35w
Tube 4: 442v x 0.038 = 16.79w

I realize I will need some adjustments, but the amp so far sounds absolutely amazing! It's terrifyingly good! It only starts to break up at around 2 o'clock volume dial position. I'm wondering if I should push it any further since EL34b's are rated for up to 25w dissipation, or leave it at 17w. I do want the tubes to last a while.. Any thoughts?

Also huge huge thank you for all your help everyone so far!

[AlNewman]

That looks pretty good to me.  If it sounds terrifyingly amazing, it is probably ok.  I dunno if I would push your tubes any harder, since they cost more than the amp, but you could maybe dial them back a little to make em last.  The amp might still sound amazing at 60 or 50%, you never know unless you try.

[PRR]

In a fixed-bias amp, higher idle current does NOT make higher maximum output.

[starseeds]

AlNewman I took your recommendation to dial it back a little, and aimed for 60% dissipation, to my surprise now, the amp sounds great even past 3 o'clock volume.

Set it as follows,

Tube #1 450v x 0.033mA = 14.85w
Tube #2 449v x 0.033mA = 14.81w
Tube #3 449v x 0.033mA = 14.81w
Tube #4 450v x 0.033mA = 14.85w

I could go even lower but I think this is fine for now as I'm going to re-check everything after I get some hours on these tubes. I'm listening to them right now and the sound is pretty amazing. Thank you for all your help and suggestions, seriously! If you have any more I'd be more than happy to hear them!

[sluckey]

Tube #1 450v x 0.033mA = 14.85w

Probably meant .033A or 33mA.   

[AlNewman]

Hey, congratulations. 
Just playing devil's advocate here, but it's likely you are already at 55% plate dissipation.  But that's not a bad thing.