Ampeg 7027A Power Tubes Replacement Blues

[telemoxy]

First let me apologize in advance for the lengthy tome but this amp is important to me.

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

My question, and I do have one, is this. I think I have heard from everyone else in the relevant threads here, but 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?

[willhutch]

I popped a new set of Groove Tubes 7027s in my V4. Worked fine witout making any changesto the circuit. Regular old 6L6s worked too. I'll admit I did not check the bias, but everything worked and the tubes didn't redplate.

[Blue Strat]

Using redplating as an indicator of anything but imminent demise is a VERY bad idea. Tubes don't redplate until you hit about 200% of maximum dissipation and they shouldn't be set any higher than 70%.

7581As or 7027s would be your best bet. Typically, you can just rebias but there's always a possibility of getting a particular pair of tubes that will be out of range for your bias circuit. If this happens, you'll have to change one resistor in the bias circuit.

[telemoxy]

Using redplating as an indicator of anything but imminent demise is a VERY bad idea. Tubes don't redplate until you hit about 200% of maximum dissipation and they shouldn't be set any higher than 70%.

7581As or 7027s would be your best bet. Typically, you can just rebias but there's always a possibility of getting a particular pair of tubes that will be out of range for your bias circuit. If this happens, you'll have to change one resistor in the bias circuit.

I popped a new set of Groove Tubes 7027s in my V4. Worked fine without making any changes to the circuit. Regular old 6L6s worked too. I'll admit I did not check the bias, but everything worked and the tubes didn't redplate.

Thank you both for your replies.

willhutch Which 6L6s did you use?

Blue Strat Good advice on biasing. I am going to convert to a variable bias using external trim pot and bias points to check the bias with the new tubes in and then periodically after that. Where can you get 7581As these days without taking out a mortgage to pay for them. Are any of the 6L6GCs up to the task?

[Swarty]

Cheapo Sovtec 5881s hold up very well in V series Ampegs. I have them in my V4.

[Dave C]

I put a quad of 7581a in a VT-22 a few years ago when they weren't quite so pricey , the Sovtec 5881 as Swarty suggested can handle the plate voltage of an Ampeg. I would expect any "good" 6L6GC to tolerate it as well....ymmv

[Blue Strat]

At this point, NOS 7027s are less expensive than NOS 7581As.

[soulohio]

yea, go with 7027's if you can't find 7027a's. NOS ob course.

[TimmyP]

Ask the folks at Groove Tubes. I expect they can tell you just what to do. (Also do a web search - I've seen info on this somewhere.)

[Blue Strat]

Ask the folks at Groove Tubes. I expect they can tell you just what to do. (Also do a web search - I've seen info on this somewhere.)

Believe it or not, most calls like this to Groove Tubes end with a recommendation to call me.

[telemoxy]

Cheapo Sovtec 5881s hold up very well in V series Ampegs. I have them in my V4.

So I see. I found these references attesting to the performance of the Sovtek 5881/6L6WGC as a substitute for both the 7027 and the 7581A.

http://www.jt30.com/jt30page/micKtubes/Sovtek-5881-for-7027.html
http://www.jt30.com/jt30page/micKtubes/Sovtek-5881-specs.html

My friend has these in his new Traynor Custom 50 and that amp sounds fantastic. I was wondering how Yorkville was getting 50 watts out of a 5881. Now I have the answer.

Thanks for the tip Swarty.You learn something new every day.

[telemoxy]

Believe it or not, most calls like this to Groove Tubes end with a recommendation to call me.

That saves me a call to Groove Tubes.

[telemoxy]

Thank you all for your input. Here is what I did.

With the help of a knowledgeable friend I replaced all the electrolytic capacitors in the amp. I replaced the filter capacitor cans with individual caps, fashioning brackets from nylon coated galvanized strapping. I left the original Caps in place and used their ground points.

I took the advice in The Tube Amp book and I replaced the 470 ohm 2 W screen resistors with 1K 2W. They suggest replacing the 47K bias feed resistors with 5.6K if you want to use EL34s or 6550s but we can't see why so I didn't do it. Does anyone know why they suggest this?

We modified the bias circuit. I replaced the 75K bias resistor with a 47K resistor and a 100K LIN locking pot in series installed in a new hole I drilled in the back under the fuse. This way the bias is adjustable allowing me to change between different power tubes.

We did add a jumper between pin 1 and pin 8 so I can use pentodes like EL34s. I will not be able to go back to 7027As but I believe their sound can be replaced by 6550s or 6L6GCs if I want the original tone. If anyone knows how to use EL34s without the jumper pin 1 to pin 8 I would love to know as I could continue to use 7027As as well.

We replaced the hum balance pot with a much heftier 250 ohm pot. Everyone I read felt it would be shot and it was.

We replaced a crumbling 180K resistor in the standby light circuit so the light no longer flickers annoyingly.

We built a bias probe that allows me to read the cathode and the plate current for accurate biasing. The plate current feed was my idea and as luck would have it it is very handy. Wish I had an oscilloscope.

I replaced the 7027As with a pair of used NOS GE EL34s from a friend.

The amp sounds like a Marshall, not surprisingly. And is it quiet in fact it's a bit spooky frankly but who cares it doesn't hum.

Unfortunately one of those new 1K 2W screen resistors cooked. The only culprit we can think of is the tube so I bought a pair of Winged C SED 6L6GCs replaced the crispy screen resistor, installed the tubes and biased the amp. It has plate voltage of 585 volts and I biased at various settings from 50% to 70% setting at 65% or 33 miliamps. Next I want to try new EL34s, 6550s and perhaps Sovtek 5881/6L6GCs.

Anyway, I left the amp on for awhile to let the new caps settle in then tried it out. Man can this amp wail. Unfortunately I blew the speaker in my enthusiasm.

No problem I have a couple of spares of the speaker, an Altec Lansing 417Cs from the 70s, kicking around. Or so I thought.

What I actually have is one Altec Lansing 617C and a Celestion 12G-80.

I installed the Celestion and the amp sounds fantastic. And incredibly loud. Fortunately I have a Marshall power brake to tame it and get great power tube distortion.

Next I need to address the preamp tubes. Any suggestions.