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Vintage Ampeg V4 6K11 Tube Problems

Thomasr098

Thomasr098

Jul 27, 2018
32
22
#1
Hey guys!

I have an early 70's Ampeg V4 that I got a couple of months ago and after playing it for a while, I deduced that the 6K11 tube might be microphonic so I ordered a NOS tube from eBay. I popped that NOS tube in this morning and my amp was instantly much louder and distorts much earlier. I am able to turn the volume up to 12 o'clock and get light-medium breakup with the old 6K11, but if I turn the volume past 9 o'clock with the new tube installed, my signal is distorted beyond use. Any idea on why there might be such a drastic difference between the new tube and old tube? I'm almost tempted to put the old tube back in to make the amp more usable.

Also, what exactly does the 6K11 tube do in the amp? Are there any tubes I can use as a substitute for it?
 
#2
Not quite sure why there are no replies here. Interesting. Online companies like Fliptops, etc., folks that specialize in vintage equipment would be the people I would seek out. Might be worth the research in the long run. Just a thought.
 
#3
That tube is a "compactron" which has 3 separate triode sections similar to 2 x 12AX7 and 1 x 12AT7. I've never used one so I don't know their particular characteristics or quirks, but this would be a good question to ask a tech (probably an older tech, even older than me) who has REAL WORLD experience with this device.

Since this is a guitar amp, the sensitivity will be quite a bit higher than a typical bass amp, so try turning your bass down to about 1/2 and see if that makes a difference. If it does, it's simply more gain that you can use in the amp.
 
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#4
agedhorse said:
That tube is a "compactron" which has 3 separate triode sections similar to 2 x 12AX7 and 1 x 12AT7. I've never used one so I don't know their particular characteristics or quirks, but this would be a good question to ask a tech (probably an older tech, even older than me) who has REAL WORLD experience with this device.
Click to expand...
See........Ask and you shall receive, advice to to OP. Excellent advice has been given here. Amen X 2!
 
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#5
Well I reexamined the old tube and it’s actually a 6U10 which can be used as a low gain replacement for a 6K11. I actually had an old 6U10 laying around (my grandpa used to be a TV repairman and was into amateur radio) so I tried popping it in but the new 6U10 blew almost immediately. I’m planning to order a NOS 6U10 to see how that sounds and hopefully it doesn’t blow, and if it does, I guess I’ll have to do some more research. I’m guess the other 6U10 I tried was just a bad tube since my original 6U10 still works perfectly.
 
#6
This might be a good time to seek the help of a qualified and experienced tech... before possibly causing damage or additional damage. A little analysis and diagnostics can go a long way.
 
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#7
I am actually old enough to remember when you could go to the local Sun Drug store, test you tubes, and get replacements. This is like the Silvertone 1548 ? Era. 1x15. Head stored to the side of the speaker cab, if I remember correctly. Yikes! Ancient history! One thing I have learned over the years is that Tube Amp’s in general are like a group of small children, in that if left unattended, bad things happens. So if you make that sharp left turn down tube amp boulevard, find a good tech to take with you.
 
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#8
Not I tube that I would trust buying on ebay. You never know what you'll get in terms of meeting spec. Try Fliptops Home of Parts for Ampeg Lovers or Invalid Link Removed, RCA are good. You can also use a 6Q11, not exactly the same. In that amp, the 6K11 readings are never as indicated on the schematic.

On that amp, reflowing the solder joints can help. After years of heat, they take a beating. It isn't unusual to find gunk under the tube sockets. This can affect performance.
 
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#9
agedhorse said:
This might be a good time to seek the help of a qualified and experienced tech... before possibly causing damage or additional damage. A little analysis and diagnostics can go a long way.
Click to expand...
I guess I should’ve mentioned this earlier but I did actually take my amp to get checked out by a tech about a month ago. He went through and checked all of my caps, resistors, and tubes and replaced whatever was bad. I trust this tech’s work as he was able to explain everything he did on the amp to me and it all seemed logical (based on my limited knowledge of circuits from the couple electrical engineering classes I took in college).
 
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#10
Thomasr098 said:
I guess I should’ve mentioned this earlier but I did actually take my amp to get checked out by a tech about a month ago. He went through and checked all of my caps, resistors, and tubes and replaced whatever was bad. I trust this tech’s work as he was able to explain everything he did on the amp to me and it all seemed logical (based on my limited knowledge of circuits from the couple electrical engineering classes I took in college).
Click to expand...
The tech was also old enough to be alive when these amps first came out, if that adds anything to his credibility
 
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#11
Maybe leaving well enough alone might be considered as well. If your tech didn't see the need to address this, maybe it's not a problem?
 
#13
As a general comment, may of the problems that i see in the shop are the result of "tinkering", or the problems were made worse because of this. That said, there is an entire class of people who call themselves techs who fall into the tinkerer category. I cringe when I see the damage these guys cause, and the additional costs that their customers are charged to correct the damage.

Often, leaving well enough alone, or seeking the help from the most qualified tech that you can find are better avenues in general.
 
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#15
Note that the V4 has a lot more gain using the 6k11 tube as it was originally designed for guitar. In general, guitar levels are about 12dB - 20dB lower than bass, especially in the days pre-distortion, pre high output pickups, etc.
 
#16
Thomasr098 said:
Hey guys!

I have an early 70's Ampeg V4 that I got a couple of months ago and after playing it for a while, I deduced that the 6K11 tube might be microphonic so I ordered a NOS tube from eBay. I popped that NOS tube in this morning and my amp was instantly much louder and distorts much earlier. I am able to turn the volume up to 12 o'clock and get light-medium breakup with the old 6K11, but if I turn the volume past 9 o'clock with the new tube installed, my signal is distorted beyond use. Any idea on why there might be such a drastic difference between the new tube and old tube? I'm almost tempted to put the old tube back in to make the amp more usable.

Also, what exactly does the 6K11 tube do in the amp? Are there any tubes I can use as a substitute for it?
Click to expand...
The 6K11 is a 3 triode compactron. The V4/VT-22/VT-40/V2 used the same preamp circuit, just different output. 100w vs 55w. The 6K11 was used as the midrange control frequency select and baxandall control. The 6U10 obviously was changing the gain structure of this section. Additionally, if you using this amp for bass rather than a V4B, you will have very different gain structure throughout the as well a frequency response. The V/VT series was based on SVT technology, and the difference between a V4 and V4B will be in the area of bypass capacitors frequency response and range. Basically, the V4B is for bass frequencies, and V4 was for guitar. You can use a V4 for bass, but expect a completely different EQ curve, and the V4 will suffer from inability to handle the harmonic content of the lower frequencies produced by bass guitar.
 
#18
FYI, the schematic reference voltages for the 6K11 are not all correct. They make an error and it gets carried over on subsequent revisions. If the tech changed resistors to match the readings on the schematic, that could be a problem.

Buying tubes on ebay is very risky. Resellers buytubes, test them and pass the bad ones along. The worse of the worst winds up on eBay. Not all ebay tubes are like this but it can be the lowest rung in selling out of spec tubes that nobody else wants. You never know what you’ll get. It could be some other tube relabeled.

Even if it costs a bit more, for the 6K11, it’s best to buy a name brand NOS from a trusted reseller.
 
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