Ampeg V4 eating tube s

[Marc]

I have this 1970 V4 on my bench that likes to eat power tubes.
It has been mostly recapped there is one original can cap left.
It has been through three quads in the past three years. The tubes seem to short before they have much time on them.

there are no fly back diodes on the ot -would adding them help?
The board has a spot under the 470 ohm resistor feeding the screens that has seen some significant heat, not burnt but crispy.
The circuit looks to be pretty much stock other than the caps that have been replaced.

So what are the best (most durable) 6L6's. JJ's and winged C's didn't survive....
Should I go with the Sovtek 5881's? I know the owner is tired of buying new tubes after a couple of gigs....


Thanks,
Marc

[Prairie Dawg]

I've got one I'm refurbishing for a guy. Right now it puts out about 525v on the plates. I'm figuring on dropping that about 50v with a zener diode just because I want the tubes to last for a while-I do not think it will lose anything in the way of clean tone. This one has the flyback diodes installed although it is a bodger of a job-they'll be replaced with some new ones but done right.

[Marc]

yep this one runs 525V too...I was also considering changing the bias circuit to one that is adjustable so I can keep the current draw down.
What tubes are you using?

[Chuck H]

Why not put some KT88's in there, or at least 6550's???

[Prairie Dawg]

I'm using Sovtek 6L6GCs that the customer provided. I'm also going to install an adjustable bias as well. The amp came with mismatched JJ 7027s which are nothing more than their 6L6GC with two extra pins for heat transfer-the Ampeg does not use pins one and six. KT88s and 6550s are a little spendy.

[Chuck H]

+++
Been there.
Maybe you could totem the screen filter node and reduce the screens a little.

[g1]

Odd for me to hear of these amps biased too hot in stock form, I thought they only got by with the high plate volts from being biased way cold. Before changing to adjustable bias make sure the bias circuit is up to snuff (stock resistor values?, bias filter caps, coupling caps from phase splitter, and the .047 bias AC feed cap).
What is the -DC voltage on the grids? With modern AC line I see about 545 plate volts on mine so the 525V you see does agree with high idle current.
I have been using JAN 6L6WGB with no problems (my other one still has the old 7027's which could be original for all I know http://music-electronics-forum.com/i...es/biggrin.png). I would have thought the winged C or JJ would work fine in the stock circuit.
FWIW he V2 schem. shows plates around 585 with -62 rather than -52VDC on grids.

[melvin]

I've always wondered about that bias supply. It seems like it bails if there is any current draw from the grids. So, if you bias it hot at all, given a decent input signal, it will go into runaway or you'll experience blocking. I think this is why your screens are cooked. The V4 has a super low distortion preamp and then the signal gets really amped up right at the PI. If you want to calm it down a bit there is a voltage divider you can tweak right at the "preamp out" point. You can also increase the cathode resistor in the PI, I guess. But I wonder if biasing it colder will solve the problem.

Know of any better designs for a negative bias supply being tapped off of a bridge rectifier? One that can source a little current.

[Gtr_tech]

I'd replace the bias supply cap (the .047/630v) and the rest of the associated components. Its not even an option to make it adjustable, its a necessity. With elevated plate supply voltage, it becomes increasingly important to keep idle current in check. I'm guessing that you're running the output tubes right on the edge of redplate, and that tears 'em up fast.

How are you using the amp? Running it hard and often? Expect to replace output tubes as needed if used this way.

[1ampman]

What is your current draw per tube at idle?

[Ronsonic]

It ain't the tube choice.

The bias supply comes through a film cap off the PT HV secondary. It fails in ways that cannot be measured by the equipment on my bench (Ohms up to 30M, ESR, Capacitance all look good). Replace it. Also replace everything else in the bias ckt, but that cap (C18?) is the biggy. Use an "X" cap, like for a line power filter. Also do that weird diode in the ground path.

Tell the owner to either let the amp warm for over 3 minutes before hitting standby or to not use the standby switch at all. Either is safer for these amps than the usual, power on for 30 seconds then hitting the standby.

A V4 came with flyback diodes so it should have them.

[Marc]

Finally called the owner to get more info on the complaint with the amp. Turns out the problem this time was that the sound dropped out. they pulled the tubes and put them in another amp. I found the problem related to the screen supply resistor, hard to miss the 470 ohm sandblock that had scorched the circuit board when it over heated. I replaced it with a 10W 1K resistor and the amp seemed to be fine with that. I did add the flyback diodes in for good measure. The bias circuit was reworked a couple of years ago when the amp was recapped.
Thanks for everybody's ideas...

[Chuck H]

I found the problem related to the screen supply resistor, hard to miss the 470 ohm sandblock that had scorched the circuit board when it over heated. I replaced it with a 10W 1K resistor and the amp seemed to be fine with that.

Uh... If there's a screen drawing enough current to blow a 5 watt sand block resistor I'd say there is a root cause that needs addressing. Flyback diodes are a bandaid at best but the actual problem still hasn't been fixed.

[melvin]

...and if so, replacing it with a 10 watter probably isn't such a good idea. I think a fried screen resistor and the tubes 'shorting' once a year may be a sign of blocking/runaway. Bias the amp cold. The V4 seems to be a fragile balance due to the high voltages and weak bias supply. It seems to tear itself up easily. I think old 7027A's were just able to handle it. The spec's for the current production ones are quite different. It seems some folks have problems, some have none.

i find ronsonic's info on the standby switch very interesting... what a strange config.

[Ronsonic]

I have beat my head bloody on these things. I actually own almost a dozen myself and have worked on many more for customers and when I first started seeing bias problems with these on startup it was grim. The symptom was that the tubes would run away if you hit the standby switch at the wrong time.

Anyway, changing the spec on the screen doesn't sound like an answer to me.