too much preamp gain in my ampeg v2?

[antieatingactivist]

I have this ampeg v2 that have been giving me problems. I walked away from it for a while, but I decided to give it another go. Basically, my power tubes are driven way too hard. To the point of the grids glowing almost white on one side of the power amp. The phase inverter is putting out 70 to 100 VAC on one side, and 50 or so on the other. The 2nd DW7 half is putting out 50 vac with a normal guitar signal on the input, which I think is driving the PI crazy. I probed for AC voltage on all of the preamp stages. It looks like on average, a stage takes 4 volts or so on the grid, amplifies it to about 40, then throws most of it away before it gets to the next preamp grid. I get about 4 volts at the "from preamp" point on the schematic. The stage turns it into 50 volts, and the only place for the signal to bleed off is the 1M resistor before the PI grid. Is this normal? Does the PI want this much voltage? I'm not sure I quite understand what signal levels in the preamp should be. It would be helpful if someone could point me to a resource to learn about this. I feel like if I had another V2 or V4, I could compare the two and solve this puzzle.

http://www.schematicheaven.com/ampegamps/v2powamp.pdf

[bigdaddy]

I really wish I could help you, all I can do is make an observation. That is that those voltages were so high then I could not imagine what they are now with 120-125 volts coming from the wall these days. They were designed for probably 115 volts. 5-10 volts doesn't seem like a lot until you realize the transformer is a step up and multiplies it so every volt that goes in comes out multiplied not added, it's mind boggling.

I would think the amp is running at max the way it was designed and this over voltage situation could be really ruining the sound of the amp.

Since that amp was from the early 70s and ran such high voltages I can bet almost all those carbon resistors are way off and all the electroltyics need replacing.

The first thing I would do is start measuring the values of all your resistors and write them down. If they are more than 10% out then replace them and replace ALL your electrolyitic caps they are 40 years old. In your case with that amp I think that's SOP. Good luck.

[jjasilli]

Checkout the voltage chart in the Service Manual:  http://www.el34world.com/charts/Schematics/ampeg/AmpegV4_V22.Service%20Manual.pdf

Does that help?

[antieatingactivist]

Thank you so much for that resource. I appreciate it.

Well, I think I have it running well. I really learned my lesson with not modding a broken amp. I was trying to run the thing with EL34's (changed screen resistors). The tube on one side was glowing inside the plate more than the other, so I blamed the weird mismatched signal voltages coming off of the PI. I put some Sovtek 6L6wxt+'s in it that I pulled out of my Soldano before the end of their service life, biased them to 30ma each, and now I'm able to run the amp at full volume with no ill effects. The screens look like they are taking a beating, but no red plates. I am still confused about the PI being so asymmetrical, and I have some funky voltages on the 6K11 plates (replaced it, along with every resistor and cap in the circuit), but the amp is back to being bone stock with all new capacitors. It sounds very good too.

I read about all of these mods for early 70's V4's, not realizing that the Voltages are WAY higher on the v2 than the v4.

I do have a couple of questions though...

Will el34's work in this amp?
Would attenuating the signal going into the PI help things out?
Would it be wise to lower the screen voltage? Would I get any benefit from adding a choke?
What new production tubes are least likely to fail in this amp? Please don't tell me NOS, I play loud and often and would rather spend my money on other things.
Are there any upgrades that will benefit the amp. Things that they had to leave out because of production costs? Kind of like how Yamaha left out the filter choke that Mike Soldano wanted to put in there.

You may ask yourself, "if it sounds good, why do you want to change it?" I am learning, and I want to try new things. They can always be changed back. I have also learned a great deal from this forum, and for that I would like to say thank you.

[bigdaddy]

I really think those amps from that era are what they are. Not that modding one is a waste of time to me it's just not all that necessary IMO. I don't think you can change the sound of one all that much, for the better that is.

As for new power tubes that run well at those high voltages. I would forget about EL-34s and try a set of JJ KT-77 tubes. From what I read about them they are what you might be looking for and sound really good, everybody seems happy with them and from what I read they are designed for high voltages. I have one in one of my SE 10 watt amps and it does sound really good. Other than that a KT-88/6550 is the other option.

I would think adding a choke would always help if the amp is noisy, if it's not then doing so would be a major waste of time and money. If it's not giving you a problem leave it be. IMO there is some weird stuff in that amp that is SO unnecessary to make a good sounding amp. In fact they probably do more to make it sound bad. One thing I wonder is the preamp tubes using such high resistors on the plates. I would have been more inclined to use a dropping resistor and 100K resistors then what they have, a 390K plate resistor? The whole way the preamp is makes me wonder what they were thinking, just some strange unnecessary stuff. But this is a printed circuit board and it's best left to just service the amp as you did, continue the process and use power tubes that sound good and can take that voltage, the new KT-77 and KT-88/6550 tubes are designed for that as far as I have read.

[jjasilli]

Overall I agre with bigdaddy (though I'm a major choke fan, I wouldn't bother with my V4).  The Ampeg V2 family is what it is (great tone).  Additionally they are real hard to work on.  Even so, the reverb in my amp was weak, so I boosted it.  Otherwise my preference is to mod or build other amps.

[antieatingactivist]

Good points. I guess my main concern should be what power tubes I should put in it, since these Sovtek's don't have much life left in them. I think I might try a simple pre-PI master volume just to some of the pressure off of the power tubes.

How concerned should I be with the screen voltage being so high? I read that 6550's are especially sensitive to this.

I also have a problem where one of the power tubes runs away when you don't wait at least 20 seconds to come off standby. I read about other people having issues with this. I suppose it isn't a problem. I just have to be careful not to smoke my tubes.

[The_Gaz]

I had a very similar problem that I fixed today in an Orange OR80! With the gain up past a certain point, one of the screens would flash bright orange, but no red-plating. After measuring voltages, and seeing the screens at nearly 530vdc, I knew something was arye. After some research, I decided to add a resistor after the choke, which brought the screen down to about 500vdc, and solved the problem!

It was a very confusing problem at first, and like you I thought there was something wrong with the PI.

I really wish I could help you, all I can do is make an observation. That is that those voltages were so high then I could not imagine what they are now with 120-125 volts coming from the wall these days. They were designed for probably 115 volts. 5-10 volts doesn't seem like a lot until you realize the transformer is a step up and multiplies it so every volt that goes in comes out multiplied not added, it's mind boggling.

I think that is exactly on the $, here's why using my amp as an example:

530vdc (plate voltage) / 120ac (wall voltage) = 4.4

115 (old tyme wall voltage) * 4.4 = 506vdc... which is the voltage I dropped the screens to in order to stop the flashing.

It's my presumption that the amp was designed to run its tubes at their limits in order to get the most power (80 watts from 2xEL34s in my case!), and that the 20-30 volts was just enough to push them over the edge.

So if I were you, I'd try increasing the resistance of the 'choke resistor', and/or increase the screen resistors to 1k-2.2k to see if that helps. Now both of those solutions might give a little more compression and loss of headroom, which may or may not be desirable or even noticeable. However, if your problem was really caused by wall voltages simply creeping up over the years, then this may be the only pracitical solution besides getting a new PT.

Hope this helps.

-Alex

[antieatingactivist]

I had a very similar problem that I fixed today in an Orange OR80! With the gain up past a certain point, one of the screens would flash bright orange, but no red-plating. After measuring voltages, and seeing the screens at nearly 530vdc, I knew something was arye. After some research, I decided to add a resistor after the choke, which brought the screen down to about 500vdc, and solved the problem!

I have a friend who plays a 120 watt '70-something Matamp with the gain all the way up. He uses a variac to get the voltage down 10 volts or so.

I also have another friend who has an 0R80 that was having problems because of the voltage that I converted to 4 power tubes. This brought the voltage down some and made the tubes a bit happier.

I will try increasing the "choke resistor". I already changed the screen resistors to 1K, which 6l6's seem to be happy with. I haven't measured the power output of this amp yet, but it is definitely louder than any 50watt amp i've heard.

[antieatingactivist]

I just got some winged C's to replace the the old sovteks. One of them still starts to red plate after a few minutes of playing while biased at 17 watts. Seems my strange plate voltages are an issue.

In general, my plate voltages in the preamp are too high. About 20 volts. Same with the phase inverter. The only section that is too low is the 2nd 6k11 section, which is about 25 volts too low according to the schematic. I would blame this on the wall voltage, except that my power tube plate and screen voltage are too low. my plate voltage is about 570. The schematic says 595.

I know that lower plate voltage in THIS amp is a good thing, but I feel like all of this is somehow related to my PI clipping asymmetrically and having too much output, or just more output than my power tubes want in their current state. The inside tube is getting the most amplitude from the PI, so it is the first to turn red.

The amp had a cap job when I got it (a very nice looking one). The only caps that are out of spec are the first filter section (40+20+20) and the 70mfd that are in series. They were replaced with 2 80mfd caps in series.

I've changed all of the plate resistors in the amp. Is there anything else that is worth shotgunning? even if it is in spec? I just recovered the cabinet and grill and am working on restoring the faceplate, so I'm anxious to get this thing working right!