Ampeg V4 redplating

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ckeene
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Ampeg V4 redplating

Post by ckeene » Fri Oct 23, 2009 12:53 pm

I recently got an Ampeg V4 that was virtually DOA, and probably didn?t have much service over its 40 year lifespan.

I replaced the old 7027 tubes with Svetlana 6L6GCs, put in new power supply capacitors, did a little other cleanup, and it?s basically working. I didn?t do much to the preamp section or bias section, except I changed the bias to run a little cold, just to be on the safe side.

Basically, it runs ok now, but only when I let the amp warm up for a while on standby beforehand. If I rush into turning standby off (after a cold start), the power tubes will glow red. My theories as to why this could happen are:

A) The bias circuit needs a long time to warm up
B) There is a positive DC voltage leaking onto the grid, which effectively defeats the negative bias. Perhaps it?s through a capacitor that only leaks intermittently?

However, the bias voltage looks fine when the amp is on standby and gets down to the -70 volts very quickly. Is it possible there is something else wrong in the bias circuit?

Also, I disconnected some coupling caps and the above redplating still occurred with a ?fast start?, so it seems like that theory is ruled out, too.

So before I do a shotgun replacement, are there any other suspect areas I should investigate? I still need to look at plate and screen voltages/resistors. The V4, incidentally, is a very high voltage amp that puts 540VDC onto the plates. I think this is high for a 6L6, but I?ve been told by an Ampeg guy the Svetlanas can handle it.

Thanks!

-Chris

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Post by lacquer_monkey » Thu Oct 29, 2009 12:04 pm

Take a close look at the way the ampeg sockets are wired. The 6L6 and 7027A pinouts are not exactly the same. The 7027A has both the screen and control grids available on two pins each. 6L6 only one each. Maybe there is an issue there.
uh... what??

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Post by RodC » Thu Oct 29, 2009 3:04 pm

I havnt seen any issues replacing 7027s with 6L6s before, but interesting theory.

Are all of the tubes red plating? Check the tube sockets, its unlikely they are all bad, but that model seems to have some issues with them.

We have a V4 coming back this week that has a single tube red plating. Its really becoming a pia. New sockets, new screen grid resistors.

From what we have seen on line replace the 470ohm screen resistors with 1K and double the wattage.

I'm begining to worry that most modern 6l6s will have problems with 540 (the one we have been working on has 560 at the plate!)
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Post by RodC » Thu Oct 29, 2009 3:08 pm

Forgot to mention, the Svetlana SV6L6GC is only rated at 520

From 2001
http://www.drtube.com/datasheets/6l6gc-sed2001.pdf
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Post by RodC » Thu Oct 29, 2009 3:12 pm

I also ran into this:

http://www.triodeel.com/7027.htm

Now, about them Ampegs...

Most likely you ran across this page after picking up an old Ampeg PortaFlex or V-series head or combo in a flea market or pawnshop. You liked the tone, in the case of the V amps, loved the cool tone controls, in the case of the Portaflex they look way cool, too.Then you looked under the hood, and ACCKKKK! The output tubes were 7027's! You called every guitar store & electronic shop in town: nobody has any.Someone said you might use 6550's, but they weren't sure..

Fear not, gentle reader, for the kind folks at Ampeg not onlyforesaw the problem and provided a simple solution, they made your Ampeg one of the most versatile amplifiers on Earth in regards to adapting to different output tubes! Years before Mesa & Crate even thought about it. You can pop in *any* good matched pair of 6L6-GC, (except skinny Chinese & Russian ones), 7581, 5881, 6L6-WGB, or KT66 without any modifications,
because the socket is actually wired for 6L6's.
6550, KT88, or KT90 may be used without modification, in most cases...(biggest problem is that the base clips usually won't fit KT88 or 6550) and by installing a simple jumper, both 6CA7 (fat bulb ) & EL34 (skinny bulb) tubes, too!
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Post by ckeene » Fri Oct 30, 2009 1:49 pm

thanks lacquer monkey and Rod.

As far as I can tell the redplating isn't happening exclusively on any single tube or single socket. What appears to be happening is when one replates others are getting too hot as well, but I put the amp into standby before they all go red.

I'm going to replace the screen grid resistors with the 1Ks, as rod mentioned, and I'm also going to replace the plate resistors ad rebuild the bias circuit and replace the flyback diodes.

I believe the pinout on my amp is ok, but I will double check. I think the issue was to make sure nothing was tied to pins 1 or 6, which are the missing pins on a 6L6.

I talked to Dennis Kager and he recommended using the Svetlana tubes, and I also saw the link you posted, Rod. Still, I feel that if I were to retube again, I might go with 6550s.

As far as the screen voltage is concerned, is there an operation issue if the screens aren't running at a certain voltage below the plates? Because as it is, they are at the same voltage as the grids: 540V.

Thanks!

-Chris

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Post by fazeka » Sat Oct 31, 2009 12:26 am

ckeene wrote: rebuild the bias circuit
A very good idea.

From Len Flint's now-defunct geocities website on the Ampeg VT-22 (similar to the V-4):

"The Ampeg circuit is a bit weird... because there is no center tap on the trannie, there can be no reference to get a negative voltage so instead, the AC power is taken thru a capacitor and rectified into negative DC like an AM radio detector!! This is considered by some to be a possible source of trouble - if any one of the componants goes - the bias goes!!"
ckeene wrote: As far as the screen voltage is concerned, is there an operation issue if the screens aren't running at a certain voltage below the plates? Because as it is, they are at the same voltage as the grids: 540V.
I don't think this is a problem, I thought I saw this on my amp when I had it opened up last month and there was no redplating in that amp... it's the *control* grid negative voltages you want to check.

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Post by fazeka » Sat Oct 31, 2009 12:39 am

On NOS tubes, sometimes plate voltages can run a little high because the more important thing is the dissipation rating of the tube not being exceeded. For example, if you are running an NOS 6L6GC at 540VDC and the current draw is 34mA, the total dissipation is 18 watts which is below the 30 watt max. dissipation rating. The tube in this case will probably be fine.

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Post by RodC » Sat Oct 31, 2009 5:58 am

fazeka wrote:On NOS tubes, sometimes plate voltages can run a little high because the more important thing is the dissipation rating of the tube not being exceeded. For example, if you are running an NOS 6L6GC at 540VDC and the current draw is 34mA, the total dissipation is 18 watts which is below the 30 watt max. dissipation rating. The tube in this case will probably be fine.
The dissipation for the unit we worked on was very low imo, sowhere around 13W. The first thing we did was totaly replace everything in the bias circuit.

Current draw and dissipation are well within limits, but i still worry about plate voltage being over the Max. Keep in mind the PS will prob spike higher when its turned on, makes you worry that the voltage will be so high that there may be an arch iternaly.
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Post by Mudcloth » Sat Oct 31, 2009 10:50 am

A little OT.
I bought a V4 on craigslist a couple of months ago for real cheap because it needed a little work and had no tubes. I finally got around to checking it out this week.
The guy said the standby switch had fallen into the chassis. In actuality, someone hard wired the standby on, which explains what happened to the tubes!
Hopefully the power supply is still okay, though.
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Post by fazeka » Sat Oct 31, 2009 1:41 pm

RodC wrote:Current draw and dissipation are well within limits, but i still worry about plate voltage being over the Max
Well, that's why I mentioned NOS tubes. Manufacturers back then sometimes slightly underrated their max values. I don't use anything other than Sylvania 7027A in my V-4B, but then again that tube is rated to 600 max.

So if you really want to be within specification on paper, you really only have two options: NOS 7027A as came stock in the amp (but you better have deep pockets) or convert to 6550s. Regarding the latter, you'll have to remove the "bear traps" on the power tube sockets so they'll fit in the V4 (-B).

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Post by RodC » Sat Oct 31, 2009 6:49 pm

fazeka wrote: or convert to 6550s. Regarding the latter, you'll have to remove the "bear traps" on the power tube sockets so they'll fit in the V4 (-B).
If it was our amp that's the route we would have taken, I really like 6550s much better. This is a coustomers amp, they insited on 6L6s.

Hopefully it just a bad tube, he did get a couple of months out of it. I'll keep you posted if we discover anything new.
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Post by ckeene » Sat Dec 05, 2009 4:44 pm

So here's the update: after replacing the plate resitrors, grid resistors and bias supply, the issue is still there. However, if I take out the 6L6s and use the original 7027s the amp will not redplate. So I think either I have a bad 6L6, or they're just running outside their specs and can't handle the voltage.

When the redplating happens, the AC voltage going to bias (same point as one leg of the PT secondary) drops from 335VAC down to like 300VAC. Am I seeing a voltage sag because the heat from redplating changes internal tube impedences? And could it be that as the PT secondary voltage drops and the bias also gets lets negative and the tubes heat up more, the effect is sort of a vicious cycle? And maybe due to the operating point, there just happens to be a very fine line whether the redplating will or will not occur?

Ultimately I think 6550s are the answer. Any particular brand you'd recomend?

Thanks,

-Chris

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Post by ckeene » Wed Jan 13, 2010 2:02 pm

If anyone needs any closure on this, I put in KT-88s and it's up and running. The overhaul of this amp included:

New faceplate
Replacing all filter caps
Rebuilding Bias
Replace flyback resistors
Replace page and screen resistors
Install new tube sockets
New tubes
Cut and drill the chassis in order to shift the power xformer 1/2" to accommodate larger tubes

[/list]

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Post by RodC » Thu Jan 14, 2010 5:29 am

Just an update on the V4 we had in the shop.

We could not get this amp to redplate, after some discussion with the owner we discovered he did not allow the amp to warm up before he took it out of stand by.

With this amp the bias voltage would go nuts if you didnt allow 15 sec for warm up (We always recomend at least 30 sec.) The bias circuit was completely rebuilt among many other things. The amp has been running fine, as long as you give it time to warm up.

We prefer Tung Sol 6550s. They seem to sound the best, and hold up real well. We have had a couple that experienced small cracks in the glass, but this was with an SVT that was heavly toured and went from extreme temps a couple of times.
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