V4B and V4, pair of power tubes, slow red plating

[Mitch]

So I experimented with those cap values this evening. Installing 0.1uf's improved the red plating problem significantly but did not resolve it. I jumped to 0.015uf's and the red plating was non existent. I then swapped in 0.022's with still, no issue. So it seems as if that solved the problem. The amp still sounds great but I'm just wondering how much of those "Ampeg Lows" I actually cut.

Monday I plan to experiment with a few more cap values, working as close to the original 0.33uf mark as I can before these JJ 7027's start to red plate again. When I settle on a value that works I'm going to attempt to do a spectral analysis to see how much low end I have lost as compared to what the circuit does with the original 0.33uf values. I'll be sure to post my results and upload some traces if I can.

I'm curious if it is normal to see a blue haloing effect in a pair of tubes on V4's while playing. It seems as if my "problem" pair of tubes glows blue as I dig into a chord, while the other pair does not exhibit this glow until the amp is idle.

AMPREPAIRJN, thanks for spending so much time on this thread, I really appreciate it! Aside from NOS 7027A's, what do you recommend as output valves for these amps?

[AMPREPAIRNJ]

That’s excellent to hear, Mitch! The exact value will depend on your power tube tolerences, of course, so happy experimenting! The lows those caps pass are well beyond human hearing range, so at least your local dogs might thank you for the cap swap, lol. To make sure the lows you “do” hear aren’t cut, try to stick as close to “.1’s” as possible, try “.047’s” even. What really matters in Ampegs, especially the V-series, is that amazing preamp and the output transformer. All Ampeg output transformers, up until a certain era, were designed to output down to 40hz. All of their RMS ratings are based on measuring at 40hz. This quick and easy cap mod will also save your speakers from blowing prematurely, as those same unheard frequencies plague your speakers integrity as well.

As far as recommending tubes, I’ve never had good experiences with new 7027’s. Failure after failure a few years ago, haven’t tried or recommended them since. Of the nine V-4’s that I personally own, I think three or four of them have either groove tube or svetlana 6L6’s, the rest have USA 7027’s. Plus I was able to set aside about five USA 7027 quads for future needs ; ) Just avoid 5881’s or any 6L6 that’s actually a 5881 and chinese tubes and you should be ok. The Russian 6L6 (any brand) seems to hold up ok in there with a typical tube failure as often as a Marshall with EL34’s. Just be sure to adjust R49 appropriately. I usually just put a trim pot in series with the 75k to make biasing easier. The Tung-Sol reissue 6L6 is my next test for V-4’s, I’ll let you know how that turns out.

I’m glad I was able to help! You deserve some credit yourself, Mitch! You stuck to the thread with good observations and ultimately helped to clear a few things up. That type of communication serves this forum justice.

[jjmt]

Hi folks! Sorry, I've been away and so I'm a bit late to this discussion. Since the problem is resolved, I have a question that I'll ask just for academic reasons: I can see how the changing the .33 coupling caps will restrict the passage of low frequencies. What I do not understand is what this implies about the issue as it was initially presented by the OP as red platting on *one pair* of power tubes (one side). Is the implication that the PI is simply imbalanced? Or is the something about the PI that is passing more low frequency on one side than the other? Are we talking about 60hz heater noise here (maybe leaking to the cathode)? If so, wouldn't it be possible to solve the problem by simply using a quieter tube, or maybe a different brand?

I realize that the theory is that this is super-low frequency, well below 60hz. I guess I'm just not sure I understand where that comes from.

[AMPREPAIRNJ]

Hi folks! Sorry, I've been away and so I'm a bit late to this discussion. Since the problem is resolved, I have a question that I'll ask just for academic reasons: I can see how the changing the .33 coupling caps will restrict the passage of low frequencies. What I do not understand is what this implies about the issue as it was initially presented by the OP as red platting on *one pair* of power tubes (one side). Is the implication that the PI is simply imbalanced? Or is the something about the PI that is passing more low frequency on one side than the other? Are we talking about 60hz heater noise here (maybe leaking to the cathode)? If so, wouldn't it be possible to solve the problem by simply using a quieter tube, or maybe a different brand?

I realize that the theory is that this is super-low frequency, well below 60hz. I guess I'm just not sure I understand where that comes from.

If you read the entire thread, you'll see that the condition in order to induce the red plating is extreme, not normal and with newer tubes most likely not in spec with the original 7027's. That extreme setting greatly offsets the voltage across the power tubes, more so than the PI, leaving them vulnerable to red plating at which point the PI circuit most likely just determines which side will constantly red plate due to signal/frequency strength from one side over the other. One side of the PI always has less steps to pass/amplify signal than the other, so there comes a point, in extreme settings, that the PI is not balanced. But the unbalanced PI is not technically what's causing the red plating, but how hard the power tubes are working with such offset voltages. What changing the .33 caps does is take the stress off of all of the power tubes equally, resulting in the tubes not running smashed to the wall trying to amplify unnecessary frequencies, taking enough of a load off the tubes and supply to keep the power tubes from running away. Playing with the phase inverter at these extreme settings might buy you something, but given the 12AU7 and PI circuit does a great job at normal settings, well balanced, is it really worth modifying it to find a middle ground between normal and extreme settings and being happy with both results? I'm not sure. The .33 caps are just letting too much low freq through regardless of the red plating, let alone in extreme settings. I would love to know what results you come up with should you start experimenting though!