Ampeg 7027A Power Tubes Replacement Blues

[telemoxy]

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. 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?

[Enzo]

And you are not considering just using current production 7027s because...?

6L6 will drop right into a 7027 socket anyway.

I would check bias with ANY tube I installed, and make adjustments accordingly. if they fall outside of range, the adjustment circuits are simple enough to alter.

[telemoxy]

And you are not considering just using current production 7027s because...?

6L6 will drop right into a 7027 socket anyway.

I would check bias with ANY tube I installed, and make adjustments accordingly. if they fall outside of range, the adjustment circuits are simple enough to alter.

Thank you for your prompt reply. Your biasing comment is good advice.

The short answer to your first question is I am considering current production 7027As.

The long version is that when I started investigating power tube replacement NOS 7027As were the only option. Much time has passed and the original Ampeg branded Sylvania tubes have proved very durable.

I figured I was pressing my luck so awhile ago, to be prudent, I checked with a couple of local techs and they were uneasy at that time with the Sovteks and the JJs and NOS 7027A were too expensive. Both recommended rugged current production 6L6GCs or NOS 7027As and an adjustment to the bias circuit and rebiasing.

I had never heard of using 6L6GCs so I started investigating on my own, recording what I found and tucking it away until I needed to act on it, which brings me to the present.

The decision regarding the replacement power tube fro the 7027A is contentious and seems to revolve around the sound I are looking for and the plate voltages the VT40 is capable of dishing out. The one thing everyone seems to agree on is not the sound. It is that the 585 volts this amp is capable of means carefully considering the construction of whatever tube is chosen. Ampeg hasn't helped by wiring the sockets for 6L6GCs yet suggested 6550 as a replacement tube. These are very different tubes so this serves to muddy the waters.

As you can see from my post, I have collected many options and many opinions on each option so I thought before I take the plunge I would see what other people had actually installed and how it worked out for them.

As a mentioned to a friend this fun but annoying. Ever onward and upward.

[J Martin]

I use the JJ 6L6GC in all of those Ampegs. They seem to hold up particularly well in those amps. The 7027 tubes out there now are just repinned 6L6 tubes as far as I can tell. Ampeg did not use those extra connections. The extra cost of the modern 7027's is just for for the rewiring. The 6550 can be used in the black faced amps because they have a much lower screen voltage than the silver faced, early 70's amps. The 70's amps exceed the 6550 screen voltage. Hardly a week goes by when I don't have one of the V4 series amps in my shop. Check the screen voltage in your amp to make sure it is less than 400v if you want to use 6550 tubes. EL34 tubes will work also, but expect a much earlier break up.

[telemoxy]

I use the JJ 6L6GC in all of those Ampegs. They seem to hold up particularly well in those amps. The 7027 tubes out there now are just repinned 6L6 tubes as far as I can tell. Ampeg did not use those extra connections. The extra cost of the modern 7027's is just for for the rewiring. The 6550 can be used in the black faced amps because they have a much lower screen voltage than the silver faced, early 70's amps. The 70's amps exceed the 6550 screen voltage. Hardly a week goes by when I don't have one of the V4 series amps in my shop. Check the screen voltage in your amp to make sure it is less than 400v if you want to use 6550 tubes. EL34 tubes will work also, but expect a much earlier break up.

Thanks for your response. Great information. I looked up the spec sheet on the JJ 7027 and the JJ 6L6GC and I agree with you they have repinned the 6L6G. No shame in that. It bails out people with amp that use the extra pins. It makes sense too from a production standpoint. Yes, the VT40 is wired like the 6L6 family of tubes so I believe the 6L6GC would be fine. Do you have any experience with Winged C SED 6L6GCs. They have been recommended to me from a couple of local sources.

Good tip on that screen voltage if I want to try the 6550. I heard there is a way to wire the sockets so the amp will take 6L6GCs, EL34s or 6550 with the necessary biasing. Something about using a jumper to allow the socket's pin configuration to accept any of these three tubes in pairs. Are you familiar with this?

[J Martin]

The only tube that needs a jumper is the EL34. Pins 1 - 8, but don't use a real 7027 after that. I never used the SED 6L6's in the V series Ampegs, but I use them in other things. They would probably be ok. I had stopped using them a few years ago because some of them became noisy. I got some more in a few months ago, and sure enough one dropped out of the first quad I used. They generally sound good, though. I would like to hear if anyone else is still having this problem. I stock nine different 6L6 types and their variants in my shop. There is no one best tube for all amps, but there is a best tube for a particular amp.

[JJGross]

I have 2 identical 1971 Ampeg VT 40 4x10 combos (both top-mount control units).

One runs the original Sylvania 7027a power tubes, the other came to me with Groove Tube 6L6 GC tubes.
The original 7027s are still chugging away fine after all these years and sound great.
The GT 6L6GC did this in about 6 months of casual use:



Red Plate Special anyone?
Notice the label on the base of the bottom tube is shrunk from the heat.

I talked to Bob at EuroTubes and he claims that his 6L6 will handle the almost 600v on the tubes just fine.
In fact, he says he uses them in an Ampeg himself.

Cheers,
- JJ

[Enzo]

It's not the voltage, it's the current - dissipation kills.

Mechanical failures in the tube kill them often as well.

[J Martin]

Groove Tubes sells several 6L6 versions, but it seems no one pays much attention. They can add all the testing they want, but they can't add durability. If anything the JJ tube exceeds the 6L6 spec and its one reason why it makes a good 7027 replacement.

So many people just stick in power tubes without a thought about bias. They bring the amps into my shop surprised the new tubes have failed already. When I ask them about it I get the blank stare. I offer to show anyone how to set their bias, but when they hear that their Ampeg has 590 volts on the plates they usually decline.

[Diablo]

My Ampeg V4 is still running it's original power tubes. I know the Vt40 runs even higher voltage than the V4 (585 vs 560V). I assume Ampeg did that with the VT40 to get more power out of the two tubes. If it were me, and I was on a budget, I'd get the JJ 6L6 tubes. The boys from Triode electronics have been using the JJ tubes for quite a while in the VT40 with good success.

I'd like to know how you set the bias on the VT40 or V4. There is no bias pot.

[J Martin]

There is usually a schematic pasted inside so you can locate the bias setting resistor. You can install a pot if there is room to mount it, but its got to be somewhere it can't be accidentally bumped. Those original tubes were of really good quality and last a long time. In my experiance, they tend to fail suddenly and take other stuff with them. I always change the small electrolytics, especially the bias filters because there is a lot of heat in that chassis. Another tip is to change the screen resistors to 1K 3W wirewound in the older Ampegs. Its not an issue with the later MTI models.

[Diablo]

Thanks J Martin. I have the schematic for the amp. I've replaced every electrolytic capacitor and nearly every other capacitor. The bias circuit has a 75K and 56K ohm resistor. The screen resistors are 470 ohm - 2Watt in the V4. What's the advantage of going to a 1K ohm - 3Watt screen resistor? I've check all the resistor values in the amp and they are right on the money.

[Enzo]

Hi Diablo, welcome to the forum. May I suggest if you want to discuss your amp's bias that you start a new thread for it, instead of hijacking telemoxy's thread. I am getting old and can be unsure which amps we are talking about when more than one is in a thread.

[J Martin]

The 1K screen resistor is easier on the newer tubes. It has been a point of contention for some, but I have done so many of these amps by now I have statistics and customer feedback on my side. The part I use is Mouser 72-RWM410-1K-5. I use the same series in Fender power supplies and the like.

[telemoxy]

I did it!

Sorry for the delay. 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. We replaced the filter capacitor cans with individual caps, fashioning brackets from nylon coated galvanized strapping. IWeleft the original Caps in place and used their ground points.

We replaced the 470 ohm 2 W screen resistors with 1K 2W. The Tube Amp Book 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. We 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.