5F6A - strange parasitic?

[sunnydaze]

Hello everyone,

I was reading the metroamp forum about startup trouble on newly built JTM 45: http://forum.metroamp.com/viewtopic.php?f=15&t=27609

and read where Structo was helping the guy out and mentioned the screens should read a couple volts less than the plates. I've got a Bassman I built about six months ago. It was my first build, and from a kit. The amp does sounds amazing. Anyway, I checked my the voltage chart I made when I first built the amp and found that I had recorded the same voltage for the screens and the plates. Decided to check it out again. My wall voltages are lower than my PT primary is set for, so my voltages are on the low side. But I get a plate reading of about 402 DCV and one or two volts higher on the screens.

Funny thing though on V4, with the volume near dimed I got a kind of high pitched sqeal (like reversed OT primaries) when I touched the plate with my meter (and other probe grounded to the chassis). Also, the voltage drops about to about 384. This does not happen on V5. First tried swapping the tubes, and the anomaly stayed with the socket. There is no audible problem until I touch the plate with my meter probe - and only then when the volume is near dimed. I'm not sure why this only happens on V4 and only with the probe attached and the volume near dimed - I turned the volume up with the probe attached and no voltage drop till almost dimed. Since (I think) this only happens when measuring the plate voltage on V4 with volume dimed, I'm sure could live with the condition. But it is bugging me.

V4 connects to the OT on the blue lead, brown is going to V5. The fact that this anomaly only occurs only when the meter is hooked to the plate of V4 and volume is cranked makes me think the possible causes would be somewhat limited. Unfortunately, all I can think of is it must be a small parasitic that is like positive feedback to the V4 side of the OT. But what does that mean? What should I be looking for?

Any comments would be appreciated.

thanks
mike

[Bob-I]

First off, I wouldn't be testing voltage with the volume dimed, short the input and test. Your meter is causing the problem. If the PO is there without the meter I'd troubleshoot that. Voltage drop when PO hits is normal. The tubes are conducting high current and causing the power supply to sag.

You shouldn't see voltage higher on the screens. Double check your connections and make sure that that screens are connected to the correct node on the power supply. that said, 2 V is not a big issue, if it sounds good, play.

[sunnydaze]

First off, I wouldn't be testing voltage with the volume dimed, short the input and test. Your meter is causing the problem. If the PO is there without the meter I'd troubleshoot that. Voltage drop when PO hits is normal. The tubes are conducting high current and causing the power supply to sag.

You shouldn't see voltage higher on the screens. Double check your connections and make sure that that screens are connected to the correct node on the power supply. that said, 2 V is not a big issue, if it sounds good, play.

Thanks for the quick reply. There is no PO (parasitic oscillation) without the meter, so I should probably not worry about it - just seemed strange that it only happens on the V4 side.

The screens are connected as they should be. The discussion on the metro amp site was saying the JTM 45 OT's has a high primary resistance (higher than the choke) causing this to happen - they were talking about a JTM KT66 OT though. I'm using 6l6's, but I am using a huge MM Axiom OT, probably pretty high primary resistance, too. I'll probably put a resistor in series with the connection to the screens to lower the screen voltage at idle. Seemed to be the consensus on the metroamp forum.

Could you please explain what you mean by short the input and test?

thanks
mike

[sunnydaze]

to Bob-I: after reading your note over a few times, I guess when you wrote "short the input and test. " - You were describing how I was causing the problem when measuring. My first time through the note, I thought you were recommending a I try different test. Please let me know if you had something else in mind.

I'll post an update after I put a resistor in series with the screens.

thanks
mike

[sunnydaze]

delete double post

[tubeswell]

FWIW it doesn't matter if the screen is at the same voltage as the plate because the screen grid has such a small target area compared to the plate, that most of the electrons hit the plate (and not the screen) anyway. The few that hit the screen merely contribute to the relatively small screen current.

[Ears]

FWIW it doesn't matter if the screen is at the same voltage as the plate because the screen grid has such a small target area compared to the plate, that most of the electrons hit the plate (and not the screen) anyway. The few that hit the screen merely contribute to the relatively small screen current.

Tubeswell your first sentence and reasoning is correct, but keep in mind that increased screen current occurs when plates fall below screen potentials and screens attract the plate's secondary emission and electrons of plate current (when between the screen and anode) that have low enough energy to be attracted back to the screen (so-called virtual cathode effect). I assume having a higher screen to plate potential at idle will allow a virtual cathode to form more easily during the audio cycle and so shorten screen life. A couple of volts at idle won't make much difference but higher differentials sure will if the tubes are driven hard.

[Ears]

Double post, some dang idiot doesn't know the difference between edit and quote buttons

[Bob-I]

to Bob-I: after reading your note over a few times, I guess when you wrote "short the input and test. " - You were describing how I was causing the problem when measuring. My first time through the note, I thought you were recommending a I try different test. Please let me know if you had something else in mind.

I'll post an update after I put a resistor in series with the screens.

thanks
mike

What I meant was unplug the input and make sure the input is grounded. You don't want any signal coming in confusing your idle voltage readings.

[sunnydaze]

to Bob-I: after reading your note over a few times, I guess when you wrote "short the input and test. " - You were describing how I was causing the problem when measuring. My first time through the note, I thought you were recommending a I try different test. Please let me know if you had something else in mind.

I'll post an update after I put a resistor in series with the screens.

thanks
mike

What I meant was unplug the input and make sure the input is grounded. You don't want any signal coming in confusing your idle voltage readings.

OK, got it - was definitely over thinking it.... Haven't had a chance to do anything else yet.
thanks
mike