redplating 1959 w larmar

[planetjimi]

Question: I agree 100% with Randall's load line assessments. But that begs the question: why am I able to build amps with these same parts that never have red plating issues? I'm asking this genuinely.

george

Have you tried cranking an amp with el34's in the dark for a couple mins while slamming on an E chord and looked at the tubes?

[Midnite Zone]

George,

I'd love to know the same the same thing. Like, with my Laney, what did the previous owner do for the first 23 years of the amp's existence?? I assume it must have worked just fine...

And yet, there is no question that my tubes (new Winged C's) were redplating at EVERY bias setting, and that the problem is 100% fixed by raising the plate load impedance...

Perhaps a difference in tube sets is affecting what happens? Maybe a difference in line voltage? Maybe just component tolerances? Maybe variations in the actual impedance of our speakers? Or, maybe most people ARE getting minor redplating & just aren't noticing...

[VelvetGeorge]

All of my amps burn in for 24 hours under full load. If there was any red-plating whatsoever I would know about it.


george

[jbzoso2002]

I got a 1988 JCM800 Super Lead (3 filter caps) 100w
a while back.

When I cranked it in total stock form I got redplateing.

I put in a MetroAmp 68 board and 68 6 filter cap setup
and it redplates when cranked.

So the stock amps do it to. Also my 83 50w with stock
pt redplates.

Jimmy

[Roe]

I've been running flat out for 45 minutes without any signs of redplating whatsoever This is great, absolutely great!

I only disconnected one master volume (connecting the one left to all 4 tubes) and disconnected the second bias circuit. I'm running 4 RTFs at 60% diss (480v). the tubes seems to handle it without any problems. but the mass and the resistors connected get hot

this is with a 27k tail resistor and 82k/56k plate resistors instead of 100k/82k. using stock values (10k tail resistor and 100k/82k plate resistors) I get a little redplating on v4-5 again I'm gonna try to reduce the 82k to 68k tomorrow, and possibly increase the tail resistor again.



BTW: A 1971 superbass I had on the bench redplated a little even with cold bias. amp was stock with the exception of a replaced PT (metro 1203-80).

[AJW]

I have been struggling with redplating on my 100w plexi kit as well. It took over a year for this to crop up. At least I didn't notice it before. My first set of tubes (JJ's) lasted a year of daily use. Since then I would get 3 months out of a set, and that includes a real nice set from Terry. I noticed the red plate issue about a month ago.

I'd gone over everything in there, and checked component values with my meter. Today I switched to the low voltage taps, and changed a couple resistors in the bias supply to get that right. Works, and there's no more redplating. Seems to be missing something though (besides red plates).

I emailed Heyboer and they said they'd make an OT with any primary impedance I wanted. I should have asked them how much it would cost. Mercury Magnetics already has one for a plexi with a 3k primary, but it's not cheap. I might get a new transformer, but at least this amp is functioning again.

[Roe]

redplating has almost disappeared now. Running RTFs at 60% diss, 10k tail resistor 100k/82k plate resistors, there is just slight redplating on v4-5 when running flat out for some minutes. the 82k iskra even reads 83k5. I'll try to lower it a tad and bias v4-5 colder [edit: didn't really help]. I'll try to find enough old mullards for this amp. I have a few lying around, but I doubt they are matched. I'll try using the dual bias circuit again.

everything suggests that the redplating was caused by the dual larmar master volume or the dual bias circuit. to me, the former sounds more probably than the latter

[VelvetGeorge]

Dual Lar/Mar......yep. What value pots did you use? I'm sure this was all answered way back in the thread, sorry.

So if the total effective "bias resistor" resistance is high enough (around 500k with two dual 250k pots) it could limit current and drop bias voltage under heavy load. Even though bias current is typically very low, this could be your issue.

george

[ThunderOne]

...This is EXACTLY why I've been against amp kits and DIY biasing all along...

I'm sure that George was delighted to read this on his forum section which deals with his kits.

[Roe]

Dual Lar/Mar......yep. What value pots did you use? I'm sure this was all answered way back in the thread, sorry.

So if the total effective "bias resistor" resistance is high enough (around 500k with two dual 250k pots) it could limit current and drop bias voltage under heavy load. Even though bias current is typically very low, this could be your issue.

george

I used one dual pot which read 220k and another which read 148k - I used a layout found in the larmar thread. each plate of the PI was connected to two caps - one went to one master volume and one to another master volume. so master volume 1 controlled v4 and v7 while master volume 2 controlled v5 and v6

[VelvetGeorge]

That's a little high for my taste. I'd try dual 100k's for around 200k total. Excellent wiring and lead dress would be mandatory as well.

george

[Roe]

27k tail resistor gives no redplating; 20k gives very slight redplating on v4-5 when slamming the amp; 10k gives quite noticeable redplating when running everything on 10 for a while

this is with a 470k in parallel with the 82k resistor for a total of roughly 70k

[AJW]

I am going to order a new OT from Heyboer. Does a 2.7k primary sound like a good choice?

[Roe]

2.7 sounds high - it will change the sound. try upping the tail resistor first. it helps a lot on my amp

[Midnite Zone]

Hold on, why are you saying 2.7 is high?? With voltages as high as 470v -480v, 2.7k sounds perfect. That's what the EL34 datasheets are saying.

The sound may change some, but it shouldn't be a bad change. You're only changing the load to the one that the output stage wants to see.

Edit: changing the tail resistor changes the sound too. I played around with it in my Ampeg Reverberocket a few years ago... It was sort of like the effect of messing with the NFB resistor IIRC.

One last thing - the fact that Mercury Magnetics is making a plexi OT with a 3K primary is good evidence that it won't sound bad!