Ampeg VT-40

[davidfinch]

I replaced V3 on this amp due to low muddy volume. it greatly helped the tone, but when I checked the bias of the output section I saw it had to come up. (9mA) Now, it just has low distorted volume even with the power tubes in range. (45mA) All voltages on the pins of the power tubes seem normal, but the amp doesn't seem as loud as it should. With plate volts around 540V, I feel like a pair of 7027A's should be quite louder. Anybody have any info on these amps? Are they designed to be as loud and heavy as V4's etc.?
Thanks,
-Dave

[rf7]

Should put out 50 to 60 watts. How did you change the bias? Was the amp modded with a bias pot? You're running those tubes at 81% of full power, that's kind of hot. I would back it down to around 2/3 of full power.

And yes, it should be pretty loud.

[nashvillebill]

This is the combo version of the V2 so it was a nominal 50 watts or so; compared to the 100 watts of the V4 with their quartet of 7027A's.

Nevertheless, I agree it should be quite loud.

[Enzo]

And don't equate power with loudness. If the amp is not as loud as before, it can easily be because the preamp is sending the output tubes less signal. It was a preamp tube you were wroking with, eh?

[Chuck H]

Maybe clean and retension the tube sockets. Clean the pots, jacks, jack switches, switches and all the tube pins. Then re-bias the amp to 70% dissapation. The problem may magically go away.

[g1]

Just in case it was overlooked, you have verified it's not an issue with the speakers? Like Enzo said, "don't equate power with loudness".

[davidfinch]

Thanks guys, there ended up being a concentric stacked pot that I assumed was the bias. I'm still not sure what it was, as I took it out. Maybe, a balance for the output? Either way, I reverted it back to stock, put some new tubes in and just put a resistor in the bias circuit to get them in range. Definitely loud now. Does anybody here have an opinion on 6L6's in place of 7027's? At a plate voltage only 40V higher than they're rated, I figured it was safe (and affordable) to put JJ 6L6's in.

[g1]

Does anybody here have an opinion on 6L6's in place of 7027's? At a plate voltage only 40V higher than they're rated, I figured it was safe (and affordable) to put JJ 6L6's in.

There is no reason NOT to use 6L6's in this amp. I will side with uncle Ned on this:
Help, My Amp Has 7027's!
7027's have extra pins connected to g1 & g2, those extra pins are not connected in 6L6's and not required for Ampegs. There is no other difference between a 6L6 and a 7027. The plate and transfer curves are the same. The old datasheets said 7027A's could handle 600V but there is no evidence to refute that this was a marketing gimmick/juggling statistics.
Back in 1999 I asked for a spec sheet for the newly available Sovtek 7027. It was identical to their 5881 datasheet but with the extra pins connected.
I have attached the JJ datasheets for their 6L6 and 7027's. They are also identical except for the extra pins. Their 6L6 data sheet even has 7027 in brackets in the title. They do show a difference in the Ca/g1 capacitance which should be due to the extra pin connected to g1.
So the question is: if 7027's could handle 600 volts, how can you sell a tube rated for 500 volts as a 7027 ? Aren't JJ worried their 500 volt 7027's are going to blow up in circuits with more than 500 plate volts? My answer would be that uncle Ned is correct, it was just playing with numbers. Why wouldn't they have sold a 6L6 pinned version as a high performance 6L6 if it existed?
Aside from all of this, let's not forget there are Musicman and other amps out there putting more voltage to 6L6's than Ampeg does.
Bottom line, use good quality 6L6's for any Ampeg calling for 7027's. For amps other than Ampeg that need the extra pins, rewire the sockets to use 6L6's.

[Enzo]

g-one, PESENT DAY production 7027s are just repackaged 6L6s. But look at the data for the two types in an RCA book. The 7027 was a higher voltage higher power tube. Now considering presently made tubes, it is correct that the 6L6 has the same specs. But in the old days, or if you want to use NOS tubes, the amp would be happier with 7027s. RCA was not going to repackage 6L6s and quote higher specs just to sell the same tube under two model names. The tube guide was intended for engineers, and they would have seen through that in a heartbeat.

600v isn't going to blow up the tube, it is excess dissipation that destroys tubes.

[g1]

Enzo: I appreciate your skepticism of my skepticism . I realize the 7027A spec. sheet states 600V max. and 35W max. The Ned Carlson quote from the link above seems to be dealing with appearances, nothing is said about materials involved or having performed an "autopsy":
"OK: I have a Sylvania 7027, 6L6-GC, and 6L6-WGB and a GE 7027 (made by GE,not an RCA relabel) and a GE 6L6-GC. The internal parts of the Sylvania 7027, the 6L6-GC, and the 6L6-WGB are exactly the same, except the 7027A and the 6L6-WGB have a grid cooling fin on the top. GE 7027 and 6L6-GC look exactly the same. IOW: all of them have the same effective plate dissipation."
Thrown into the mix we also have the 7581, essentially a 6L6GC with 500V max. but 35W rating. I haven't read anything about anyone examining those plate structures, perhaps they also look the same as the others.
So we have 2 possibilities (excluding the 7581's for now)
a) the plate structures were built with the same tooling & assembly lines but used different materials. For me to accept the spec sheets I would insist that both the 6L6GC and 7027A were put through all the same tests and the 7027A came out on top.
b) the conspiracy theory option, the 6L6GC were never tested to see if they could handle the 600V and 35W max. ratings, why would anyone bother?
Aside from this, JJ asserts their 6L6GC and 7027 (definately a re-pinned 6l6GC) will both handle any circuit calling for original spec 7027A or 7581, even though they are only rated for 500V and 30W max. Did no one build amps that really needed the 600V or 35W spec?

[Enzo]

I will very quickly exhaust my knowledge of tube construction here, but you mention one tube has a grid colling fin. I am willing to believe that grid heating may be a limiting factor in tube performance, and by adding extra radiating surfacce they extend that. Its possible...

Data sheets are not arbiters of just how much a tube can handle. They are instead the limits of what the manufacturer will stand behind and recommend. For example, Leo Fender exceeded 6V6 B+ "limits" by 100 volts for decades. Tubes don't have to look different to be different. Plates are hunks of metal and can stand getting hot a lot better than tiny grid wires, which can sag or melt through. Cathodes can be doped with different formulations. The heater inside it can be configured differently. Cathode's ability to emit electrons is just as important as the plates ability to receive them. Moving the grid spacing a fraction of an inch can affect tube performance. That construction is similar just means to me that they used an existing proven design to make more than one model. I can't accept "IOW they all have the same plate dissipation" without knowing a lot more about the tubes themselves. There is more to it than just the metal.

And there are things tube makers knew that I will never know. How about thermal stability of support pieces? Those mica discs for example? MAybe those extra support leads to the base pins actually make a difference. Possible.

Ever go through the drawer of 100k or 220k or whatever, and measure a few, selecting one closes to value? There is nothing at all wrong with a 10% part being 9% off. But you selected the closer one anyway. I do it. Within possibility that certain tube runs may have the same inner structure, but some are selected for higher specs. Just how they'd do that, I have no knowledge. But the higher specs are characteristics the tube maker will stand by. These 6L6s meet all 6L6 specs. These other tubes are 7027s and we stand behind them that they will make the stated specs. They may be special 6L6s inside, but that doesn't mean all 6L6s would meet those specs.

[MstrB]

I had good luck changing my V4B to straight bottle 6550's.