Electricians and those proficient in amp biasing, I need some help please

[Soundstorm]

Ok, this is my first go at biasing my own amp. I am an electrician (not certified, but have learned on the job over the years) so I am used to working on higher voltages and have taken whatever precautions I can to make sure I'm doing this safely.

 

Here's my problem. I had taken the chassis out of it's headshell on the Matamp a few days ago so I could experiment with preamp tubes, but while doing this I noticed the EL34 in position V3 (I'm not sure what the actual positions are, but let's say that looking from the back of the amp, V1-V4 goes from left to right), I noticed that the EL34 in V3 was redplating. I figured this was a biasing problem so I decided to take my first crack at it.

 

Here are my readings: At pin 3, I'm reading 506VDC, which according to the Weber bias calculator, a hot bias should be somewhere in the 35mA range. However, I've noticed a lot of Matamp guys like their's biased at 40mA, my guess is to achieve even more of that Matamp power tube grind.

 

Here's the problem: At V1, V2, and V4 I got an even 40mA reading NOTE: each power tube has it's own bias trimpot. At V3 (the one redplating, I was getting something off the charts). So I turned the amp off, let it cool down, removed the tubes and placed the one that was in V3 into V1. I turned the amp back on, let it warm up, then adjusted the bias trim pot for V3 to 40mA (I'm taking my bias reading from pins 1 & 8, which are jumped). But now, I'm not getting any reading at all from any of the other tubes, yet they all appear to be on and glowing.

 

What I was going to attempt today is throw in a pair of Sovtek Mesa EL34 that I happen to own (and are still new), and see if they bias up. What I'm wondering is what could've caused those strange readings.

 

Here's another thing, I blew the HT fuse when I went to take my first reading. I was going to use an alligator clip, and connect it to Pin 3 while the amp was on, which was obviously not smart because the clip is a lot thicker than the normal lead that comes with the meter. It made contact with another nearby pin (not sure which one), caused a small spark, and popped the fuse. I'm really hoping I didn't fry some component when I did this.

 

The amp requires a 1.6A fast acting fuse, but all I had in the house were 1A slo-blo, so I used that to take my final readings noted above. I'm going run to get the right fuses in a little while.

 

I'm confused here because I know one bad tube can throw the others off in normal situations, but because this amp has an adjustable bias for each tube can it still have the same effect?

 

A huge thanks to anyone that can help walk me through this. The nearest Matamp service center is in Canada (G-Spot) music, and there isn't one in my area that I've heard of, besides, I've always wanted to learn how to do this on my own. Thanks again in advance!

 

EDIT: When I was getting the 0 readings on the other 3, I was still getting a 40mA reading on V3. This was after I had replaced the fuse.

[Soundstorm]

Anyone? I know there's a couple of builders in the forum. Probably just aren't awake yet, lol.

[Hardvalve]

 

Anyone? I know there's a couple of builders in the forum. Probably just aren't awake yet, lol.

 

 

I would suggest joining amp garage, there are some really knowledgable guys there that will help take you step by step.

[Soundstorm]

Ok, I'll give it a shot. Thanks

[Jack Tone]

I'm not an amp tech, but I've got a Spider Valve 100 that has individual trim pots for each tube, and yeah, each one affected the others when adjusted. (what a pain ! )

[ibenhad]

May have blown a screen resistor. I would join TGP and post in:

 

Amps/Cabs Tech Corner: Amplifier, Cab & Speakers Tech Discussions

 

Some good amp techs hang over there.

[Soundstorm]

 

May have blown a screen resistor. I would join TGP and post in: Amps/Cabs Tech Corner: Amplifier, Cab & Speakers Tech Discussions Some good amp techs hang over there.

 

 

From what I gather, to check the screen resistor I should check the impedence between pins 1 (or 8, they're jumped) and chassis and it should read 1K?

[Soundstorm]

No responses at any of the forums mentioned. Maybe I should just contact Matamp directly.

[Soundstorm]

Changed the tubes and getting the same thing. No bias reading on three tubes except for the one that was redplating.

[Hardvalve]

Both of those forums are a lot slower on posting, make take a day or so to get a reply, but they will give you some direction.

[Zozobra]

Its hard to comment without poking about inside the amp with a meter/scope but a few things:

40 mA is too high IMO. Granted some of that is screen current but I always feel that biasing that hot is just begging for premature tube failure for not a great deal of tonal benefit.

 

Pull the tubes and measure all the voltages at each pin of the power tubes and note them. repeat with the tubes installed. This will be critical in working out whats going on.

 

A cathode current of 0 mA could occur for a few reasons. Off the top of my head:

1) the applied bias voltage is huge and the tube is in cut off

2) there is no plate/screen voltage

3) heater voltage is way down although I doubt that to be the case here

 

Using a slo blow tube in a fast blow position is just asking for trouble, especially if there might be a short somewhere. Put the right fuse in before doing anything as replacing a matamp power transformer will not be cheap.

 

If one tube in a 4+ tube pushpull amp is bad it can drag its accompanying phased bulb down with it as the bad tube will runaway and pull excessive grid current which will pull down the bias voltage of its partner.

 

also register on music-electronics-forum. the tech help there is the best you will get.

[Soundstorm]

 

Its hard to comment without poking about inside the amp with a meter/scope but a few things: 40 mA is too high IMO. Granted some of that is screen current but I always feel that biasing that hot is just begging for premature tube failure for not a great deal of tonal benefit. Pull the tubes and measure all the voltages at each pin of the power tubes and note them. repeat with the tubes installed. This will be critical in working out whats going on. A cathode current of 0 mA could occur for a few reasons. Off the top of my head: 1) the applied bias voltage is huge and the tube is in cut off 2) there is no plate/screen voltage 3) heater voltage is way down although I doubt that to be the case here Using a slo blow tube in a fast blow position is just asking for trouble, especially if there might be a short somewhere. Put the right fuse in before doing anything as replacing a matamp power transformer will not be cheap. If one tube in a 4+ tube pushpull amp is bad it can drag its accompanying phased bulb down with it as the bad tube will runaway and pull excessive grid current which will pull down the bias voltage of its partner. also register on music-electronics-forum. the tech help there is the best you will get.

 

 

Ok, so I will set everything at 30 or 35mA once I'm able to adjust it. I changed out the tubes for some new Mesa Sovteks I had lying around and I get the same problem. I will try and check the plate voltage with the tubes out like you said. There seems to be a jumper running across all of them on Pin 3, or maybe it's in pairs, I didn't look that well. I'll report what I find. This is seriously driving me crazy.

 

 

EDIT: I also did get the correct fuses immediately after my original post. The amp still comes on, and will still make sound but I don't leave it on for long and I barely turned it up. But I did sound fine (of course it was at TV volume).

[maggotspawn]

 

May have blown a screen resistor. I would join TGP and post in: Amps/Cabs Tech Corner: Amplifier, Cab & Speakers Tech Discussions Some good amp techs hang over there.

 

This would be my guess.

[Soundstorm]

 

This would be my guess.

 

 

How can I check that? Is it directly connected to pins 1 and 8? What value is it?

[Zozobra]

 

Ok, so I will set everything at 30 or 35mA once I'm able to adjust it. I changed out the tubes for some new Mesa Sovteks I had lying around and I get the same problem. I will try and check the plate voltage with the tubes out like you said. There seems to be a jumper running across all of them on Pin 3, or maybe it's in pairs, I didn't look that well. I'll report what I find. This is seriously driving me crazy. EDIT: I also did get the correct fuses immediately after my original post. The amp still comes on, and will still make sound but I don't leave it on for long and I barely turned it up. But I did sound fine (of course it was at TV volume).

 

 

It'll be in pairs. The plates are supplied by the OT.

 

As people have said an open SGR is one possibility. When the SGR blows it effectively has an infinite resistance (well, not infinite but frigging huge) and a typical EL34 SGR is 1k so any attempt by the screen to draw current (which it still does under quiescent conditions even though its only a few mA) results in the screen voltage plummeting and this will effectively bias the tube well into cut off. Makes sense give the symptoms of your amp. A new set of SGRs will probably fix the problem.

[Zozobra]

 

How can I check that? Is it directly connected to pins 1 and 8? What value is it?

 

 

Between pins 6 and 4. 6 is usually not connected to anything so its used as a convenient tie point to mount the SGR. Value will be something like 1-1.5k, 2-5W.

Pins 1-6 are often jumped with a 1ohm resistor to measure the cathode current.

 

You can check if the resistor is open by either popping it out and measuring it or by measuring the voltage drop across it. If the resistor is okay it will only drop a few volts but if its toast it will drop pretty much the entire screen supply.

[ibenhad]

As zozobra said test the thing. I think it has to be out of the circuit to test it so lift one leg.

[Soundstorm]

 

Here's a pic, the tube placement indicator notch is facing downward at 6 o'clock. If you can help me get my bearings I can test these things. I was thinking the big grey resistor tied to pin 1&8 (on either side of the notch) was the SGR. Guessing I'm wrong? I gotta crash. I'll check back in tomorrow. Thanks for the help, guys!

[Zozobra]

That is confusing to look at! I can see how you shorted something when poking about in there. The SGR is the big grey one on the left of the yellow cap.

[guitarcapo]

You've got a bad resistor somewhere at that tube socket. That's why changing tubes offers no help. I'd measure all resistors associated with that tube socket (plate, screen, and cathode) and make sure they all are of the correct value matching what they should be at the schematic. Test voltages at all the pins there in reference to ground and make sure they match the schematic values.

 

Do you have a schematic of this amp you can post online here?

[Zozobra]

Don't think I've ever seen a GT1 schematic. There are a few reverse engineered GT120 schems but nowt for the new stuff

[Elder Things]

Nevermind what I had posted, take Guitarcapo's advice.

 

Take your voltages carefully. Maybe secure your clips prior to powering on. Deviation from the known red plating socket and the others should point you towards failing components.

[Soundstorm]

This is beyond my skill to heal....

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

....we need Elvish medicine.

[syscrusher]

Here's a pic, the tube placement indicator notch is facing downward at 6 o'clock. If you can help me get my bearings I can test these things. I was thinking the big grey resistor tied to pin 1&8 (on either side of the notch) was the SGR. Guessing I'm wrong? I gotta crash. I'll check back in tomorrow. Thanks for the help, guys!

 

 

1 and 8 tied together is your cathode to ground. That resistor should be black/brown/black. The next one, 2, with that straight bare wire is your heater. 3, next one with that red wire is your plate. 4 is your screen. Should be a 1k resistor by the look of it on that one, brown/black/red. The one on the right in that pic looks like it got a little fried. Those 3 watt oxides are like sparklers when they go. The next one, pin 5, is your grid. Looks like they have a 10k, brown/black/orange, grid resistor with a 100k ,brown/black/yellow, bias resistor both meeting there together. Pin 6 looks un-used not connected to anything, usually is not connected internally anyways. Next one is pin 7, your heater. Then back to pin 8.

 

I'd pull all tubes and take readings on all pins for each tube, jot them down and post them. Pay attention to pin 5 with the negative voltage, should be around -32 or so for EL34s. Check those 1ohm resistors on the tied pin 1 and 8 too just to rule them out. With 3 watters, I doubt they burn up on the cathode, doesn't look it, but just in case. For the bias adjustment, those little blue potentiometers, you really should find you a plastic screwdriver. They it's situated in there, you're asking to short out your bias with the heaters if you happen to slip. Could take out your heater winding in the transformer and that would suck major balls.

 

Those ceramic sockets just sort of suck because the little grips that hold the pins of the tubes tend to get weak over time. Make sure they're tight. I don't like them at all but maybe Matamp found a better supplier then I ever found.

 

Might just got a hold of a bad tube. I've had one do that in mine and took it's sibling with it every time it red plate.

 

I can't see the other socket. Other than that, looks like a nice quality amp.

 

Oh, and don't use the alligator clips unless you're tying the ground tip of your meter to ground. Use your red probe to test each pin. To tight of a mess in there for alligators. Don't forget to change the meter to your lowest mA setting and change the probes accordingly when checking the bias. Don't forget to change them back too. I've burnt out my mA reading many times.

[Soundstorm]

1 and 8 tied together is your cathode to ground. That resistor should be black/brown/black. The next one, 2, with that straight bare wire is your heater. 3, next one with that red wire is your plate. 4 is your screen. Should be a 1k resistor by the look of it on that one, brown/black/red. The one on the right in that pic looks like it got a little fried. Those 3 watt oxides are like sparklers when they go. The next one, pin 5, is your grid. Looks like they have a 10k, brown/black/orange, grid resistor with a 100k ,brown/black/yellow, bias resistor both meeting there together. Pin 6 looks un-used not connected to anything, usually is not connected internally anyways. Next one is pin 7, your heater. Then back to pin 8.J I'd pull all tubes and take readings on all pins for each tube, jot them down and post them. Pay attention to pin 5 with the negative voltage, should be around -32 or so for EL34s. Check those 1ohm resistors on the tied pin 1 and 8 too just to rule them out. With 3 watters, I doubt they burn up on the cathode, doesn't look it, but just in case. For the bias adjustment, those little blue potentiometers, you really should find you a plastic screwdriver. They it's situated in there, you're asking to short out your bias with the heaters if you happen to slip. Could take out your heater winding in the transformer and that would suck major balls. Those ceramic sockets just sort of suck because the little grips that hold the pins of the tubes tend to get weak over time. Make sure they're tight. I don't like them at all but maybe Matamp found a better supplier then I ever found. Might just got a hold of a bad tube. I've had one do that in mine and took it's sibling with it every time it red plate. I can't see the other socket. Other than that, looks like a nice quality amp. Oh, and don't use the alligator clips unless you're tying the ground tip of your meter to ground. Use your red probe to test each pin. To tight of a mess in there for alligators. Don't forget to change the meter to your lowest mA setting and change the probes accordingly when checking the bias. Don't forget to change them back too. I've burnt out my mA reading many times.

 

Thanks! This is a huge help! I only use the clip for ground now. The SGR you said looks a little fried happens to be the problematic tube position. I know this is a newb question, but to test a resistor I just probe both sides of it on the impedance section, correct? Someone mentioned having to disconnect it to get a proper reading. Just FYI I did switch tubes and ran into the same problem in the same position. Hopefully it's the only one that went bad, but all the tubes seem to be affected. I'll remove all the tubes and take readings to post tomorrow. That is after making sure I haven't fried my mA fuse lol