Ampeg V2 bias question

[yaryaryar]

Hello all,

I have an old Ampeg V2 that works, sounds fine, but seems to be biased very cold. I just put a pair of new JJ 7027s in there and tested with an Amp-Head bias tester. It gives me 12mA on each tube. That seems very low to me. The closest I can find by googling is a mention in a forum somewhere (might have been this one?) where somebody quoted Bruce from fliptops.net saying it should be something like 20-25mA for this amp.

I'm thinking of doing an adjustable bias mod (which was discussed in another thread) but haven't gotten around to that yet. For right now it's stock.

Here are my voltages at the power tubes (both tubes were just about the same):

1: -
2: (filament)
3: 594v
4: 590v
5: -74v
6: -
7: (filament)
8: -1mv

Pin 8 seems off to me, but I can't see what could be adjusted there. That's just connected to ground, but if I'm understanding this right, that's the cathode, and should be quite a bit higher, right? It's also entirely possible that I'm just not understanding that right... so can anyone offer some insight?

Schematic is attached. Thank you for the help!

[Leo_Gnardo]

First, 1 mV offset between cathode & whatever ground point you put your meter on is 1: negligible and B: possibly a tiny measurement error, not worth worrying about.

I'm surprised to not see the same voltage on pin 1 and pin 4. They're internally connected in 7027, and I see a jumper wired onto the sockets in old Ampegs I've worked on, unless they've been modified for use with EL34 or 6550 for instance.

Yes 12 mA is insufficient bias current. I'd say 25 mA is the low end of the good zone, and I'd cap it at no more than 35 mA, should be plenty enough. What I usually do is change out R49, the 75K resistor in the bias supply, for a 56K with 20K trimpot in series. Wire the trimpot as a variable resistor, wiper to one end or t'other, then follow ordinary bias setting procedure. Nice to have a scope & load resistor & sine wave generator setup, then you can watch the crossover notch disappear as you approach proper bias current.

[Enzo]

However...

I have an old Ampeg V2 that works, sounds fine

What part of that do you want to change?

peavey 5150 heads come from the factory with something like 11ma to 15ma tube current. I surely would not be modifying an amp that works good and sounds good just because the idle current seems lower than what you see all over.


That 70% number you see bandied about all over the internet is WAY hotter than an amp needs to be to get rid of crossover distortion.

[J M Fahey]

70% idle dissipation "Internet knowledge" is NONSENSE, only justified in so called "Class A" amplifiers, which are really hot biased AB2 types.
VOX AC** anybody?

Most other high power amps: Marshall/Fender/Ampeg and their "derivatives"use enough idle current to overcome crossover distortion (that's the point of biasing) and nothing else, because it's a waste and actually reduces available power (if roof height is fixed and you rise floor height, you have less space available, as simple as that).

[yaryaryar]

You know, sometimes when I post here and get an answer from you guys, I end up smacking myself in the forehead. I'm definitely a noob at this stuff, and have silly questions. But you guys always give thoughtful and helpful answers, and I appreciate that.

With respect to pin 8: For some reason I was expecting to see a larger number there. But if pin 8 is connected to ground, and I'm measuring voltage respective to ground, it's going to be zero (or very close). YEESH. Here's where my brain was: I was reading about how to calculate what the bias should be, and part of that involves plate voltage minus cathode voltage. So I was thinking "ok, measurement at pin 3, minus measurement at pin 8....... wait a minute, there's nothing at pin 8...." Now I see why.

But so the plate minus cathode thing - how do I measure that? Not that it's a big deal, I'm just curious.

As for the pin 1 reading: I was looking at a 6L6 pinout, which I thought was the same as a 7027. And there was nothing wired to it, so I just skipped it for the measurements. But you're right, if there's something wired internally to pin 1, I'm sure it would read the same as pin 4.

To Enzo/JM's question: You're right. It does sound good, so maybe the bias is OK. I was definitely shooting for that generally accepted "70%" number (or even 50% - mine is significantly lower.). I think this is the first time I've read anything contrary to that 70% rule, so it's good to know. I think I'll still do the adjustable bias mod - I bought this amp dirt cheap, and the primary goal here is to experiment and learn from it. I've already had to do a ton of work to get it up and running, so I don't think "originality" or whatever is a concern here, and I'd like to know what a slightly hotter bias sounds like (if there's much difference at all). But knowing that that low of a bias is sometimes acceptable certainly makes me feel better about the amp.

Again, thanks for the help!

[g1]

Amps that are cathode biased will have a voltage on the cathode. This is where you subtract the cathode voltage when calculating idle dissipation.
In a fixed bias amp like this, the plate voltage will be the full voltage across the tube.
I completely agree with the others about the cold bias. However, the -74V at the grids sounds a little high to me, I thought it would be closer to around -65V in these amps. Is R49 good, and with good solder at both ends?

[Leo_Gnardo]

the -74V at the grids sounds a little high to me, I thought it would be closer to around -65V in these amps. Is R49 good, and with good solder at both ends?

C16, the 0.047/600 film cap that passes AC to be rectified into bias supply may also be at the hi end of its spec. Given there's at least a +/- 10% tolerance on that cap, it's common enough to find the bias voltage riding high in some of those old Ampegs.

But so the plate minus cathode thing - how do I measure that?

Your cathodes are at ground, zero volts. So Vplate minus Vcathode = Vplate, that's all. Multiply bias current x Vplate to calculate Pp, your plate dissipation. Even if you dial bias current up to 35 mA you're still well under 70% of the plate max 35 watts spec'd for 7027. And if your ears are happy with 12 mA, so be it.

[Enzo]

Tube dissipation is voltage across the tube times current through the tube. the tube has no idea if it has a grounded cathode or not. So if you have 300v on the plate and have to subtract the cathode voltage, and that cathode voltage is zero, then what is the result? What is 300 minus zero? I get 300 on my calculator.

When the amp works, stop fixing it.

[Jazz P Bass]

When the amp works, stop fixing it.

Git along little dogies.

[g1]

When the amp works, stop fixing it.

Can we add the provision "correctly"? I'm sure you'd agree an amp can still "work" with various faults.
I'm probably one of the last people on this forum who would advocate adding adjustable bias (or hotter bias), but I don't want to let that blind me to the possibility there could still be a bias issue here.
If R49 and it's connections are fine, then yes, move on. I thought it would be good to check it as I've never seen a 6L6 type run with bias as cold as -75V, even if 600V at the plate. A simple check.

[yaryaryar]

Agreed on all points. The amp does sound fine to my novice ears, but I wanted to make sure I wasn't doing any damage with a (seemingly) very cold bias like that. Also, as mentioned, this amp was a junker brought back to life, so it's primarily a learning platform for me - I'm really enjoying the tinkering process. So thanks very much for explaining all that to me.

When I have a chance, I'll check R49 as suggested. If everything's all good there, I'll probably leave the bias circuit alone. Thanks again for the help!

[nsubulysses]

I almost always add a bias trimpot to these amps and V4s. The bias is usually in a bad place (Edit: when new tubes are installed) and adjusting it to idle between 45-55% max plate dissipation has been good results for me. In the early years I bought two broken V4s to learn on. I biased at 70% for a "hot" sound. The only thing that got hot was the amp, then it broke down regularly.

The amp is designed to have a bias setting with tubes and wall voltages from ~1972. Modern tubes vary quite a bit in current draw and in my experience it's pretty rare to get a pair or quad that works good with the stock V2/V4 bias resistor values. This is where the resistance substitution box comes in.

Does the amp put out full power ? 60W or so at clipping. Does it have a crossover notch? This is where the scope comes in.

.012A x 594Va = 7.1W plate dissipation at idle for a JJ 30W 7027 -- 24% plate dissipation. I would assume there is a big crossover notch if this were to be viewed on a scope. I bet it works ok but I bet it don't work like it's supposed to, or as well as it could.

[Leo_Gnardo]

File under: "For comparison."

Just by coincidence, had a VT40 across the bench over the last day. Same amp as the V2, in fact its owner now has it housed in a head box. In almost-new shape, still had the original output tubes, a pair of Magnavox-labeled 7027A's. Bias voltage measured -63.9V, bias current was 29 milliamps on one tube, 23 mA on the other. I saw no crossover distortion & left well enough alone. Turns out 55 watts at onset of clipping, as expected. B+ 570V, with no signal passing thru amp.

[Eggman]

I also have a v2 that 1 power tube socket allows the bias to increase to -75 and is only doing about 5-7 watts output.1 socket stays at -65 . The only difference is the main b+ is around 540 volts. All other voltage’s remain consistent. Caps were replaced and the 2 10uf@100v were misplaced and I put them back where they belonged. The amp is a Selmer built looking like it’s from the 70s. Any help for clue’s would be greatly appreciated.

[Enzo]

Hi Egg, please start a new thread for your amp. Working on more than one repair in a thread gets confusing.