superlead with master volume redplates

[martin manning]

Thanks for the effort here, Randall. Very informative in a general way, and much appreciated!

[Roe]

In this amp I suspect oscillation (or too much AC in the output section). many different tubes redplates after playing on 10 for some time, this is not normal even for a superlead. my other 100w runs at 490v without any redplating at all (tubes are RTFs).

edit: measured AC in the output section and this amp actually has a little less ac under full load than my other 100w

[Roe]

this amp - and redplating more generally - has been discussed at the metro board as well. Randall and others contributed a lot to the following tread:

http://forum.metroamp.com/viewtopic.php ... &start=600

[martin manning]

this amp - and redplating more generally - has been discussed at the metro board as well. Randall and other contributed a lot to the following tread:

http://forum.metroamp.com/viewtopic.php ... &start=600

I think Randall has it right- somebody should call PETA*!

*People for the Ethical Treatment of Anodes

[raiken]

It just so happens I have a JVM 100W amp in here that a friend asked me to repair. It has 500V on standby, 480V on the plates at idle, and sags to...get this...408V at full clip! The screens sag from 466V at idle to 340V at full clip!

Compare this to your 30V sag at full clip...

I am measuring right at 100V p-p into 16 ohms, for an output power of around 80W when the clean channel hits clipping.

That's one way around the redplating issue...


Randall Aiken

[Cliff Schecht]

That's some TERRIBLE supply regulation.. I would almost consider that almost unacceptable, although I guess it's all about the sound in the end.

[raiken]

BBTW, in troubleshooting the JVM, I found a blown HT fuse (internal, 1A 5mm pain-in-the-butt), an open 1 ohm cathode resistor with no sign of burning - the guy was running on one side of the push-pull pair only! After I fixed that, I also found a pin on the PI tube that had no solder on it at all - it was making an intermittent connection.

It was odd, because it would work with a JJ tube or the original Marshall tube, but not with an EH, or Ruby, and it would occasionally cut in and out on one side. With the EH or Ruby, the output was cut off on the top, like a half-clipped waveform, because only one side of the PI was working when it wasn't making contact.

The guy was complaining that the entire amp would cut out from time to time. I think the intermittent PI was cutting in and out on the one side he was running!

RA

[lastcaress83]

Hi,

hate to resurrect this old thread on my first post but it's not really worth starting a whole new thread for the same issue.

I've been dealing with a problem similar to this with my Orange OR120.

It's got a 1.9k primary impedance on the OT@16 ohms.
The plate voltage (using a variac @115vac) is 480, screens are ~473 since I'm using 2.2k screen grid resistors.
When it's cranked into the conditions that cause red plating, the plates sag to ~430 and the screens to 370.

It only red plates on one side in the most extreme conditions I can dial in. It's only when I turn the FAC to it's bassiest setting and turn the bass knob past 50% that it starts to red plate. Unfortunately, I really dig the square wave distorted tone that this kind of setting gives.

When the amp is cranked with a lot of bass, one side of the amp begins to work harder than the other. When I really push it, it gets to the point that the one side is doing all of the work while the other sides grids are completely dark until I stop holding a note or the note decays.

If it's any help, I made vids of the red plating and the PI outputs when I started working on it over a year ago.

http://www.youtube.com/watch?v=ZNXxcErkRSE

http://www.youtube.com/watch?v=ZqlE2FIz ... er&list=UL

some additional info:
It's currently running 4 EL34B's but I've used GTEL34M's and winged C's in the past.
Biasing cold makes a little difference but not much.
I've installed new components throughout:
[IMG:640:480]http://i270.photobucket.com/albums/jj10 ... 0_0962.jpg[/img]
[IMG:640:480]http://i270.photobucket.com/albums/jj10 ... 0_0968.jpg[/img]
Hey Randall, do you do tech work? I'm not too far from you and I'm about at the end of my rope on this one.

[backpack]

Calling up Aiken amps would probably be quicker way to find that out.

[lastcaress83]

thanks man, do any of you guys know anything about the cathodyne PI? My understanding is that it's supposed to be more balanced than the LTP but for some reason it's not when I crank mine.

[gldtp99]

Still having trouble with that OR120 ? Cathodyne PI info here:http://www.freewebs.com/valvewizard/cathodyne.html

[lastcaress83]

Hey gltdp99, fancy seeing you here.

I've found that some tubes are better than others but I have yet to find a cure for the imbalance.

I've read the article you linked before but it fails to give any explanation for the imbalanced signals other than the output section is overdriving, which doesn't tell me much.

After reading all of the fixes for the LTP on this and the related metro thread, I was hoping someone may know something I don't.

[Ian444]

When the signal voltage at the grid of the output tube reaches the same voltage as the cathode, the grid starts pulling a lot of current from the PI if the signal tries to swing any higher. The PI cannot deliver any more current to the grid and the waveform becomes clipped. It seems that the output tubes don't distort in themselves, but moreso that they get fed a distorted waveform from the PI, and the output tubes faithfully reproduce this distorted waveform. Its a bit like the chicken and egg thing, the output tubes pull grid current causing the PI to distort, and the distorted signal is amplified by the output tubes...

When the output of a LTP starts to clip, the squared-off output signal of the first section becomes a mirror image in the second section, with the bottom of the signal clipped off instead of the top of the signal like the first section. But the top of the signal (in the 2nd section) will also be clipped trying to drive the other output tube (or pair of tubes), and the imbalance starts there, leading to the change in duty cycle Randall Aiken referred to earlier in this thread. The cathodyne doesn't do quite as well with heavy drive. The best I saw was a Williamson design, that is, a cathodyne driving two common cathode gain/buffer stages, sharing a common cathode resistor (diff amp), as this buffers the PI from the effects of grid current, and the result is equal drive to both output tubes. The same result could be achieved with mosfet source followers after a LTP. The drive to each output tube will still be clipped, but the two signals will be very similar and look like a nice sine wave clipped on the top half only, unlike the roadkill waveforms that you can get without the buffer. As Randall Aiken pointed out though, part of the Marshall sound is from the uneven duty cycle of the LTP outputs under heavy drive conditions. A signal that looks nice, may not sound as good.

No matter which way you look at it, you can't get any more power from driving the outputs with more signal once the grid to cathode voltage approaches zero.

I can't get any more than 35W clean from a pair of Chinese EL34B's or EH EL34 no matter what primary load I give them, that's with 440V B+ under load and screen grids around 400V, and the screens are already dissipating more than max specs at this voltage. Maybe the NOS tubes do better? If I go higher in B+ I would expect red-plating.

Dunno if this helps or not, hope so.

[gldtp99]

Thanks for the post--- i hope it helps lc83 with the OR120 but it helped me better understand the reasons behind some amps i have--- the cathodyne PI>twin triode buffer (w/common cathodes)>output tubes construction in the Gibson Atlas and Titan amps i have---- also similar to Silvertone 1484 and 1485 amps, i believe.
I had been thinking of the buffer twin triode tube in these amps as a booster (that didn't seem to be needed to boost signal strength)---your explaination of the buffer function makes perfect sense.........gldtp99

[hitchcaster]

i was gonna say leaky coupling cap or bad tube but it looks like you've replaced those right?