amps redplating at full crank

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amps redplating at full crank

Postby David B » Sat Jul 04, 2015 4:18 pm

Many years ago I had this problem with a 70 Superlead running EL34's so decreased the 82k in the inverter to 68k, as to not drive v4-5 as hard. Worked OK but I preferred the original circuit & ran 6CA7's instead.

I have heard increasing the tail resistor to 20k can help, also lowering the voltage a bit with a variac can also calm down the situation.

Just wondering if anyone else had experienced this and had perhaps another solution running EL34's in amps prone to this problem?

Thanks,

David
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Re: amps redplating at full crank

Postby dai h. » Sun Jul 05, 2015 6:14 am

some related threads on the topic:

http://vintageamps.com/plexiboard/viewt ... =1&t=98659

some of the suggestions from the above thread:

-use tougher tubes
-lower grid load/bias return Rs

http://www.lespaulforum.com/forum/showt ... ating-help

btw, happy 4th (and a slightly belated Canada Day to denizens of the Great White North)! 8)
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Re: amps redplating at full crank

Postby David B » Sun Jul 05, 2015 10:39 am

Thanks Dai,

Yeah I forgot mentioning lowering the grid leak resistors, I had one SLP set up that way & no redplating with EL34's .
I had sold those 100's years ago but my recollection the one with 100k grid resistors installed sounded pretty good..
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Re: amps redplating at full crank

Postby dai h. » Sun Jul 05, 2015 5:39 pm

I tried 82ks in a guinea pig 50W (the amp has the supposed EdWig Van mod with big 250uF(?) across V2a cathode which is probably too bassy for the Normal ch.) to try out Sylvania 6550s and the Normal channel (with individual 8uF filter IIRC) seemed pretty good (oh, also 22nF coupling C out of PI). I guess the bass rolloff from the 22nF + 82ks counteracted the bassiness (the Lead ch. did seem a bit lacking in bass though--maybe shouldn't have been too suprising).

There's also some threads regarding red plating at high volume (@metro bbs & amp garage) where Randall Aiken assesses that some of the old Marshalls had saggy (poorly regulating under heavy load) power transformers, and that was actually helping those old amps to *not* red plate the output tubes at high output, but since those threads are about both tubes, may not be related.
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Re: amps redplating at full crank

Postby David B » Sun Jul 05, 2015 6:35 pm

I checked the obvious stuff, sockets, solder joints, .022ufs aren't leaky , bias caps were OK but old so those got changed.
The PT sags about 50 volts down to about 398 so I was a bit surprised to see this though it could be the tubes.
I have some old Svets in there now that seem OK so maybe you hit it with the more robust tubes, if not I'll change those grid resistors.. fingers crossed.

Yeah I found Randall's comments, good stuff, what's interesting is this amp has an OT with a primary impedance of around 3K not the typical 1.7k Dagnall C1998.

BTW the bias offset when the amp is cranked is about 12 volts, looks like all Marshall have this to varying degrees..

How did you like the 6550's?
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Re: amps redplating at full crank

Postby dai h. » Mon Jul 06, 2015 6:35 am

(subjectively) seemed to have sort of a smooth sound when distorting (I'd expect them to sound the same as their 6CA7--they look pretty much identical save for the base (I have one pair of SYL 6CA7 but haven't tried them out in a really long time)). I like the SYL 6550s but they seldom show up on the NOS/old tubes market. The tubes I've tried in Marshalls that I've liked seemed to exhibit that "smooth breakup" phenomenon. But I think everything around them needs to be right for what a particular person is going for also (circuit, speakers, etc.).
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Re: amps redplating at full crank

Postby David B » Thu Jul 09, 2015 9:17 pm

Good to hear, always wanted to try the 6550's & I liked the Sylvania 6CA7's
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