The time has come. The beloved power tubes in my VT40 Ampeg are going south. Can't complain they are the original Ampeg labeled variety.

I have been considering suitable replacements for some time as I knew that power tube replacement, like death and taxes are inevitable.

I have researched the replacements; the 6L6 family of tubes (6L6, 7027, 5881, 7581A, KT66 etc.), EL34 family of tubes (EL34, KT77, 6CA7) and 6550 family of tubes (6550, KT88, KT90. Yikes, Ampeg certainly planned ahead.

I ruled out the EL34 due to changes needed to the circuit. I thought that would simplify things. Ha!

The 6550 family of tubes are a consideration and are the Ampeg recommended replacement but that would produce 80 watts and the current 60 watts is a handful and a loud one at that.

I haven't ruled 6550s out but my main focus is the 6L6 family of tubes the 7027A is part of i.e. NOS 7027As, NOS 7581As or new 6L6GCs from Winged C SED, JJ and Sovtek.

My main consideration is sound. Volume is less a consideration. Mostly it's the tone and smooth transitions from clean, to overdrive to distortion. I play everything from crystal, warm cleans, through bluesy overdrive to full on distortion all of which the VT40, in top shape, does well if you can get the volume up without making your ears bleed. I use a Marshall Power Brake (what irony) for this purpose and it is excellent.

Also, do I just plug them in and go or do I make changes to the circuit (bias etc.) to correct for wear / inherent weaknesses like the capacitors in the bias circuit, filter caps etc.?

Yikes what a pain. Is the fun in an annoying kind of way or annoying in a fun kind of way? Pass the aspirin.

But what an education. These forums are great for trading info.

My question, and I do have one, is this. Has anyone personally converted their V Series Ampeg (V2, VT40, V4, VT22) from NOS 7027As to current production 7027As, 6L6GCs or 6550s?

What brand did you use? Did you modify the bias circuit or make any other modifications? Did the change in power tubes result in other repairs having to be made. What was the end result?

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

Good reading.

I'd play it safe and take it to a good tech.

-or-

buy JJ 7027a's from www.tubesandmore.com
enter "7027a" in their search engine.

No worry with this amp. You can use the current production 6L6GC tubes as a direct replacement.

The 7027A was just a 6L6GC with a few extra pin-outs available and slightly better performance spec. The Ampeg amps do not use these extra pin connection so you can just plug any decent 6L6GC in there and it should work fine.

I would just have a tech make sure it's biased properly for the new tubes.

I have had good luck, in my Ampegs, using Svetlana 6L6GC, Sovtek 7027 and JJ 6L6GC.

I use mine for bass so I go from max volume before distortion... but for a sweeter tone maybe try the EH 6L6 in middle to low IP (plate current) rating. you can specify this when ordering them.

I have one early 70's V4 and used to have a second one. I was surprised to find that there was no bias adjustment. It's locked into a specific bias setting, which really doesn't make any sense. It's a really, really easy mod to make the bias adjustable. I'd say it's worth doing the mod and setting the bias properly for whatever tubes you end up putting in there.

I'd say the 5881's would be a good choice for what you've said you're looking for out of it, but unfortunately I don't think they're really an option as their max plate and screen voltages are much lower than what you've got in your amp. The 6L6GC/7027 options are probably the way to go.

I have had the same set of 7581s in my '65 Bassman for 12 years and it still kicks ass. I love this tube in this amp. If you're not topping out the plate voltage, there's no reason they wouldn't pull the weight in your Ampeg.

Max plate voltage for the 7581 is spec'd at 500VDC, and screen is 450VDC. In an old Ampeg, this may or may not be cool. They ran high voltages in a lot of cases. I'd guess that if anything, the screen is more likely to be the problem. Easy enough to check, if you're comfortable working with dangerous high voltages and consider it easy getting the chassis pulled out.

Oh and make sure to have the 470 ohm screen resistors changed to 1k.

This will help with your max ratings... since the B+ on this amp will be well over 574v with today's higher voltages. A tech should be able to easily put a bias adjustment pot in there... I found a little mini box-type that I stuck right on the board.

scott wrote:I have one early 70's V4 and used to have a second one. I was surprised to find that there was no bias adjustment. It's locked into a specific bias setting, which really doesn't make any sense. It's a really, really easy mod to make the bias adjustable. I'd say it's worth doing the mod and setting the bias properly for whatever tubes you end up putting in there.

I'd say the 5881's would be a good choice for what you've said you're looking for out of it, but unfortunately I don't think they're really an option as their max plate and screen voltages are much lower than what you've got in your amp. The 6L6GC/7027 options are probably the way to go.

actaully you can use one 5881. Here are a couple of posts about using the Sovtek 5881/6L6WGC in place of 7027As.

http://www.jt30.com/jt30page/micKtubes/ ... specs.html
http://www.jt30.com/jt30page/micKtubes/ ... -7027.html

If this post is accurate they will take higher plate voltages than the 7027As and the price is right.

Should be fine. The 5881 Sovteks are pretty beefy... Just make sure your tech biases the amp right and makes the appropriate resistor upgrades.