Amp eats output valves for breakfast

[solitaire]

Sorry for the possibility of exhibiting ignorance here. Got one Chinese pair of EL34s that's shorting and a russian pair that smells like overheating and considerable bites are taken off of the flash getters. Biased them to specs at 26.8 mA = 70% w. a B+ at 650V I believe.

I used a Weber Bias-Rite, set the instrumet at 2000mA - and here's the funny thing: I couldn't get a read out on 10ths of mA but I settled for a flicker between 27 and 26. Plate volts read 42. I thought this was the same thing as B+, which I at a prior occasion had measured to 652V (?). Could it be there's a number missing here, .8 mA in current and a 0 for 420V or something?

Cheers,
Jonas

[RussB]

maybe a bad fuse? lol


What amp is this again?


Get a solid measurment of you plate voltage. Use a good 1000v DMM to read directly off the plate (pin 3)

That's some serious voltage, be careful! It may be exceeding the specs of the tubes you're using.

[solitaire]

maybe a bad fuse? lol


What amp is this again?


Get a solid measurment of you plate voltage. Use a good 1000v DMM to read directly off the plate (pin 3)

That's some serious voltage, be careful! It may be exceeding the specs of the tubes you're using.
Haha... No, most unlikely two bads in a row, the second one being twice the value as suggested.

This is a 2x50W Marshall 9100 poweramp, basically the same as the current EL34 50/50. I believe the trannies used here are the same as for some 50W JCM800s (or was it 900s, now...) so voltage should correspond to what's evident in these JCMs.

At some point when that first fuse popped I was thinking it could have to do with exceding some maximum operating power. I had estimated a 150% output but I learned higher B+ can put out even more power.

Worth adding perhaps is the other channel w. 6L6GCs seems to run very stable at 70% and appear at least as loud.

[twinrider1]

I'm no expert, but it's my understanding that 70% is the max. I'd start by toning it down some. 650v sounds high to me too, but again, I'm no expert. The experts will be along shortly. :-)

[solitaire]

I'm no expert, but it's my understanding that 70% is the max. I'd start by toning it down some. 650v sounds high to me too, but again, I'm no expert. The experts will be along shortly. :-)Yes, it does sound a bit high but Marshall could have done this intentionally for a clearer sound than going with e.g. 500V or so.

Btw: Isn't there a way to set bias by using those 42V as a reference, instead of the B+?

[RussB]

Sorry for the possibility of exhibiting ignorance here. Got one Chinese pair of EL34s that's shorting and a russian pair that smells like overheating and considerable bites are taken off of the flash getters. Biased them to specs at 26.8 mA = 70% w. a B+ at 650V I believe.

I used a Weber Bias-Rite, set the instrumet at 2000mA - and here's the funny thing: I couldn't get a read out on 10ths of mA but I settled for a flicker between 27 and 26. Plate volts read 42. I thought this was the same thing as B+, which I at a prior occasion had measured to 652V (?). Could it be there's a number missing here, .8 mA in current and a 0 for 420V or something?

Cheers,
Jonas


You say in this post that you measured 420pv and 650pv


You need to measure the actual plate voltage, or find someone who can. You're in a bit over your head, and either your methods or tools are not giving you proper reading

[solitaire]

You say in this post that you measured 420pv and 650pv


You need to measure the actual plate voltage, or find someone who can. You're in a bit over your head, and either your methods or tools are not giving you proper readingI did measure the B+ voltage before and the instrument read 652V. Brought this up on different forums and there's where I learned this voltage occur in some amps, as stated above.

The 42V i thought was the "bias voltage" if 26.8 mA was the "bias current". This is what I measured using the bias-rite probe and as I understand it voltages at 40-50V is quite common. I just keep confusing this with the B+, that's all, both involving the plates.

Most likely I just don't quite know how to use the Bias-Rite thing and have forgotten how those 42V relate to bias current.

[solitaire]

I must be thinking about the Grid voltage in the above statement, but that should be a negative voltage, not a positive one. However should, contrary to better knowledge, the B+ be less this would mean the valves are running cooler than intended, not hotter - right?

[WaltC]

bias voltage (negative actually - ) is placed on the grid (pin 5 on 6L6/6V6/EL-34) to control the tube's operating point in grid biased push-pull amps. B+ voltage is put on the plate (pin 3) of these tubes to "attract" the electrons from the cathode (regulated by the grid neg. voltage).

On many "bias probes" the maker installs 1 ohm resistors between pin 8 and ground and then has you measure amperage (mili-amps) across that resistor since, governed by Ohm's Law, w/ a 1 ohm resistor mA will be equal to mV and resistance across a resistor is easier to measure than voltage between the cathode and the plate.

And... 625 Volts DC seems *very* high for any kind of marshall IMHO...

[solitaire]

bias voltage (negative actually - ) is placed on the grid (pin 5 on 6L6/6V6/EL-34) to control the tube's operating point in grid biased push-pull amps. B+ voltage is put on the plate (pin 3) of these tubes to "attract" the electrons from the cathode (regulated by the grid neg. voltage).

On many "bias probes" the maker installs 1 ohm resistors between pin 8 and ground and then has you measure amperage (mili-amps) across that resistor since, governed by Ohm's Law, w/ a 1 ohm resistor mA will be equal to mV and resistance across a resistor is easier to measure than voltage between the cathode and the plate.

And... 625 Volts DC seems *very* high for any kind of marshall IMHO...
Thanx for filling me in! :) Yes, I believe Weber installed a 100 ohm resistor in the Bias-Rite. That's the measured value of it, any way, but from what you're saying that doesn't make sense. That would displace the decimal so I may have set the bias too high or too low without knowing.

Yes, you're right in that we're talking mAs but is measured in mV. Should I have said something differently my head was temporarilly disconnected. ;)

The one I have is the octal BR-AH w. VI option https://taweber.powweb.com/biasrite/br_page.htm It says 2V range and I set it on 2000mV (=2V) but perhaps that was my misstake.

I too find 652V on the high side. Could be we measured the tranny to its centre tap by misstake, or something, and then 420V sounds a little more reasonable. But as said the instrument only read 42V.

[davemccarthy707]

You need to put away the bias rite and haul out a good multi-meter. Solder in some 1-ohm resistors from the cathode to ground and your in business. I always measure each side of the output transformer too to see how closely matched it is wound itself first before starting.

[solitaire]

I felt I sort of had come to a dead end with this issue, so what I did was I measured the Plate voltage of the Mesa Fifty/ Fifty of mine and it measured 47-48, which should be correct, since that schematic specifies B+ as 475V. By that my theory I'm short of a 0 or something is likely to be correct.

Anyone know if 420, 425 or 430 is a common B+ with Marshalls?

[RussB]

May I recommend reading the instruction manual that comes with the bias-rite? If you don't have them, they can be downloaded, or the Weber crew will email them to you. They will also answer your questions in regards to operating their product

Is there a fresh battery in your bias rite?