Replacement suggestions for 7027A

Tron Pesto

Member
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818
Tron, if I were closer, I would make you a great deal on an Ampeg 4X12 V2 cab I have had for over 25 years.
That would be awesome - but yeah, what beasts, they're only good for "local pickup"! I have a couple of latter-day Marshall 4x12 cabs that I mainly use for work bench space (no joke) that I'll use once I get it fired up. In the meantime, I've finished the Combo-to-Head conversion.

It was a labor of love/hate. I cut it clean just below the braces and mounted a solid wood bottom. Once I removed the tolex, I ended up have to wire-brush the remaining cement and crud. It was pretty bad. I was hoping to re-use the tolex - and I even cleaned it all with soap and water - what a mess as the backing just disintegrated. It ended up being unusable.

I also chopped down the baffle and cloth to use it on the front - and had to reposition the Ampeg logo.

I'm very pleased with how it came out! Here are some chronological pics:

Full Cab - amp removed:


Full Cab, tolex removed and wire-brushing commenced:


Cleaning original tolex in a failed attempt to re-use it:


Post-chopping off bottom of cab (I cut out the bottom of the original baffle to allow ventilation) - loose parts being re-glued prior to mounting new bottom:


New bottom attached:


Time to re-tolex (black bronco):


Finished - front:


Finished - back:
 

Tron Pesto

Member
Messages
818
For any one who commented and made great suggestions, the amp is up and running with Sovtech 6L6WXT+ tubes.

After doing some research and having discussions with my friend I installed a quad of Sovtek 6L6WXT+ tubes. These have become my "go to" 6L6GC type tube, in most cases and I had several quads on hand.

I biased them conservatively and the owner played that amp regularly until he sold it at a local guitar show last year.

The Sovtek 6L6WXT+ output tubes performed flawlessly in that Ampeg---- It had similar Plate and Screen voltage levels as your amp.

https://www.tubesandmore.com/products/6l6wxt-sovtek

When the old USA 7027A's give out in my '75 Ampeg V4B I'll use the Sovtek 6L6WXT+ tubes without hesitation.
Have you considered the Tung-Sol 6L6GC-STR? They'll handle (essentially) anything you throw at them, even above max voltage ratings. Other than that, Sovtek 6L6WXT+/EH 6L6 (same tube). JJs could probably handle it, but really you never know with them.
Although I planned to (and eventually did) install an adjustable bias, I dropped in the original GE 7072As with the nearly dead-on nominal hard-wired bias of -62.4V (nominal is -62V). However, in addition to that bias being very cold - the tubes were poorly matched. Namely 29%, 32%, 40% and 52%. While it's possible they might "even up" a little better at higher current draw, I did not think as a quad, they would really run the amp well.

I installed an adjustable bias, dropped in the new Sovtech 6L6WXT+s and bang, I was able to dial them up to the low sixties all within a few points of each other. Sound great - and they have a great blue glow to boot. The amp is quiet at idle and monstrous when played.

I'm about to recap the second VT-22 and have another set of Sovtechs on the way.

I am very pleased!
 
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VICOwner

Silver Supporting Member
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1,692
The background is a sad story - I've had a master volume Ampeg VT-22 (circa 1978) on the back burner for a while. It was a playground for all sorts of vermin and I had it quarantined in my garage. I had the pulled tubes and put them in a separate box.

I just completed the cleanup and as I was putting things back together, I realized that I threw out the box with the tubes last weekend when I did a spring cleaning in the garage!!!! Gone are the original and strong 7027A tubes - they were GEs with clear tops. I'm completely gutted - the bigger loss actually was the 6K11 - thankfully I have a spare.

The tube sockets are wired to accept any 6L6 type of tube (no connections on pins 1 and 6), so that provides the option of going with a host of tubes.

The amp nominally pushes 545V on the plates and 535V on the screens, so I want something that is built to handle that. Current 7027A tubes (JJ and Tung Sol for instance) seem simply to be exactly the same as their 6L6GC tubes with "7027A" connections to pins 1 and 6 (screens and grids respectively).

I'm intrigued by the Sovtech 5881WXT which claims to handle higher plate and screen voltages well and is touted as a good replacement for 7027A. However, the datasheet is SHITE and I can't seem to find what the max plate dissipation is - is it the typical 25W for a "regular" 5881? Or is it higher/equivalent to a 6L6GC (30W)?

I am going to install an adjustable bias circuit - I could set it up with a wide enough range to accept KT77s. 6550s and KT88s are out of the running as they have nearly twice the filament current draw. What about 7581As - or are we once again pushing over the plate and screen voltage limits too much?

Maybe I'm over-concerned with the 545V on the plates (and even moreso) the 535V on the screens...

Any thoughts, experiences, suggestions?
The Sovtek 5881 is a Reflector brand Russian 6P3S or 6P3S-E tube made for the Russian military. It is not a 5881. If you look at the data sheets for 6P3S you will notice the amazing resemblance of Sovtek’s datasheet. These are good tubes. I don’t think they are built to handle 500+ volts. I have and use 6P3S-E tubes. I bought a bunch for $3.00 each back before they got popular. Just so you know what’s out there.
 

Tron Pesto

Member
Messages
818
The Sovtek 5881 is a Reflector brand Russian 6P3S or 6P3S-E tube made for the Russian military. It is not a 5881. If you look at the data sheets for 6P3S you will notice the amazing resemblance of Sovtek’s datasheet. These are good tubes. I don’t think they are built to handle 500+ volts. I have and use 6P3S-E tubes. I bought a bunch for $3.00 each back before they got popular. Just so you know what’s out there.
Well that's my issue with modern tubes that are called 5881s - what are the really? At the end of the day, if a tube is called an 5881, then the max plate dissipation is 23W. So if a tube is labeled as a 5881, then I am only comfortable biasing according to 23W for which I'm looking for about a 15W or so plate dissipation. But if the tube is actually something else relabeled as a 5881, if it's closer to a 6L6GC in pd, then not only am I'm leaving a lot on the floor at my 15W bias target, I'm betting the amp isn't sounding as good as it should.

I just don't understand why tubes aren't labeled exactly what they are and a true data sheet is provided. I've bought 6π14π tubes that have datasheets that show how they compare to EL84 and 7189 tubes. I can deal with that, but it bugs the crap out of me what's going on with modern 5881/6L6/7027A/7581 - it looks to me like it's all about marketing and the truth is all but occluded.
 

VICOwner

Silver Supporting Member
Messages
1,692
I have used the JJ 6L6GC and say that I like them. I’m not sure about running them at 500+ volts though.
There are still some nos 7027A’s out there but they are expensive.
I would not recommend running a 5881 at anything like that voltage or biasing them over 16 watts either.
The 6P3S stuff mostly says 250 volts and I think I saw 1 sheet with 400 volts max. People say they will take more but I get skeptical pushing something that far from the chart.
 
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oneblackened

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1,097
I have used the JJ 6L6GC and say that I like them. I’m not sure about running them at 500+ volts though.
There are still some nos 7027A’s out there but they are expensive.
I would not recommend running a 5881 at anything like that voltage or biasing them over 16 watts either.
The 6P3S stuff mostly says 250 volts and I think I saw 1 sheet with 400 volts max. People say they will take more but I get skeptical pushing something that far from the chart.
The 6P3S-E and 6P3S-EV are pretty well known to do 500 volts fwiw.
 

Rockinrob86

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3,687
I stressed out about this big time with my V4 - ended up putting in JJ 6l6Gcs and it ran for 3 years on bass duty for weekly or more rehearsals and the occasional gig.

At 3 years, one of the tubes got a little cherry on the plates, so I replaced them. Those original tubes drifted a bit, but will be fine in another amp, so on with the JJ's we go!

I've also stressed out that the standby switch is rated for 250vdc and it has 550vdc or whatever on it. I put a now infamous sticker (to people at the studio) below the switches that says "No, No, YES!"

I wish there was one big tube that would work in all these high screen amps - I had a traynor YBA-1A that was the biggest PITA.
 

Tron Pesto

Member
Messages
818
I stressed out about this big time with my V4 - ended up putting in JJ 6l6Gcs and it ran for 3 years on bass duty for weekly or more rehearsals and the occasional gig.
That's a good testimony.

also stressed out that the standby switch is rated for 250vdc and it has 550vdc or whatever on it. I put a now infamous sticker (to people at the studio) below the switches that says "No, No, YES!"
I like that! On the VT-22/V-4, the standby is actually a ground lift - I wonder if they did that to keep the stress off of the switch. I don't like that setup either. And damn, it is poppy.

wish there was one big tube that would work in all these high screen amps - I had a traynor YBA-1A that was the biggest PITA.
Had one of those at one point too - a finicky beast that just never stayed happy. And I hated the fan - I don't need an amp that requires a fan.
 

Tron Pesto

Member
Messages
818
But Tron, think about it. What's the voltage across the switch when it's in the open (standby) position?
Hmmm - I guess it is floating - since they use this in a bridge rectifier, technically the voltage at either "point" (one point being the first node, and the other being the eventual ground connection) is exactly the same high voltage, just opposite in polarity! When tripped, the standby switch gets that same jolt as it establishes the ground reference!

So, yeah, thanks for making me think through that!! LOL. :)

So time to put "NO!! NO!!* YES!!!" stickers on the switches!

*I defeated the function of the middle switch on the two I have (polarity), so I guess instead of "NO!!" I could label it "DO WHAT THE HELL YOU WANT, THIS SWITCH DOESN'T DO ANYTHING". That probably won't fit, so maybe "DWTHYWTSDDA" in small type could work.
 

tymish

Member
Messages
237
Ok now that is TOO COOL! A Vt-22 was my first real amp as a teen in the mid 80s. We joked about it being the amp the caused the big quake. "Amporius the doomer!" It was so incredibly heavy and freakin LOUD!! The looks on those cartoon faces were exactly what we imagined! After it I went to a silverface Fender Twin for portabiltiy LOL!! Great amps though, some of my favorite live Rolling Stones tracks they were running a massive Ampeg backline and when cranked these things rocked insanely and had a midrange that would not quit.
 

Blue Strat

Member
Messages
30,065
And the current TS wouldn't hold up here, you think?
No idea, but quite often modern tubes with numbers that suggest that they have upgraded specs (like this one) don't even meet the standard (6L6GC) specs...so I wouldn't trust it just because of the part number.
 
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5,674
No idea, but quite often modern tubes with numbers that suggest that they have upgraded specs (like this one) don't even meet the standard (6L6GC) specs...so I wouldn't trust it just because of the part number.
This fellow is supposed to be though, the previously released 7581 having been met with massive critisism for not adhering to A standard and subsequently just being another 6L6GC but with a different, more obscure name.

Not tried it myself though.
 

Blue Strat

Member
Messages
30,065
This fellow is supposed to be though, the previously released 7581 having been met with massive critisism for not adhering to A standard and subsequently just being another 6L6GC but with a different, more obscure name.

Not tried it myself though.
Have you seen an actual spec sheet for these that isn't just a copy of the old/NOS spec sheet?
 




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