Ampeg V4 questions on cap value

[DrGonz78]

So I have an Ampeg V4 on the bench and have been researching getting ready to order some much needed parts. The amp came to me with a pair of 6V6 on one side. There was an original Magnavox 7027A and an EL34 on the other side as far as output tubes. Completely crazy finding all these different tubes on the outputs. I was told the amp had passed signal in this state and after pulling it all apart, was rather surprised. The hum pot is completely fried and pin 3 on the socket that had the EL34 was melted. So I am replacing the socket and hum pot.

There are two caps C303 and C305 that the customer wants to replace and definitely time to do so. My question involves the C305 cap which is a 20-20 450v Mallory type. I have attached a picture and it has two positive terminals. The schematic lists the part simply as 20uf/450v. If both terminals are 20uf soldered directly together wouldn't that make it a 40uf value? I have found a good 20-20/450v replacement on fliptops and I am pretty sure this will be a great fit in this amp. Please help make sure I am ordering the right part and reassure my thinking is not flawed.

I plan on ordering a quad of JJ-6L6GC tubes, since the 7027A NOS are not economical. From what I understand the JJ-7027A tubes have the same voltage ratings as JJ-6L6GC, and the only difference are the pinouts have differences. Since the V4 is wired with pin 1 and 6 not connected then why pay more for the 7027A's. From what I have read these amps are wired up already to take 6L6's with no mods of the tube sockets pin wirings.

Lastly I have read about how it would be wise to replace the biasing circuit resistors. R215 & R216 and even caps, but I will test the circuit before replacing anything. Of course I want to do a full cap job on the amp, but that is proving to be over budget for this repair.

Thanks for any advice to my questions and thoughts! I have attached some pics since amp guts are fun! Also, here is the schematic that best matches this model V4 version.


http://www.ampegv4.com/images/VT22_Schem2.jpg

[nashvillebill]

Have you registered (free) with Loud Technologies https://service.loudtechinc.com/loud/#/category/1 and gone to their schematics section? They may have a schematic which better matches (yes they do have the older ones there).

I've got the older V4 and tried to "research" the 7027a versus 6L6 debate. Lord Valve claims they are the exact same tube except for the pin. Tube curves look similar so he surmises they are the same tube. I'm no expert but I'm guessing the old 7027's may have been just *slightly* different to get the higher rating, whereas with today's tubes who knows.

Nevertheless, I stuck Sovtek 7027's in mine instead of the 6L6. Difference in price wasn't that bad.

But I would definitely do the bias supply caps. Lose bias supply and those new tubes are toast.

[Leo_Gnardo]

Someone here was making the point @ 2 years ago, JJ's 7027 and 6L6 are identical except for pinout. Definitely good idea to replace bias filters and you may want to put an adjustment trimpot in while you're at it. Ampeg must have been lucky/smart to install hi voltage filter caps that often are still going strong 40 years later. Change 'em out later when there's a budget for it. Anything else that looks stressed, swap it out.

Something that always puzzled me is the gigantic 0.33 uF caps delivering signal to output tube grids. 0.1 uV / 630V is plenty large for the purpose, and one fewer source of headaches if you do swap those out.

One thing that's often overlooked - output impedance selector switch. These slide switches often get corroded and intermittent, but they do clean up nicely with a squirt of solvent cleaner spray.

[g1]

Can't find C305 on the schematic, feeling duh!
You are correct about new manufacture 7027's they are 6L6's with the extra pins wired which these amps don't need. So save your $.
The one thing I don't like about this version is the bias fuse. If not bypassed, make darn sure it's connections are nice & clean.

[Diablo]

Yours looks like a very early model V4 with the 6AN8A tubes and a bias tap on the transformer. I've owned a V4 since new in 1971, and read everything I could find out about 7027A tubes. Any 6L6 tube will work in a V4 from a pin wiring standpoint. The question is will the tube take the 560 volts on the plate. For my peace of mind, I stuck with NOS 7027A. I have spares, but I'm still using the original output tubes in the amp.

[g1]

Ok, I found C305 for the plate of 1st half of V5. I think you are correct going with what you found there (looks stock), rather than the schematic.

The question is will the tube take the 560 volts on the plate.

I think the key to this is the cold factory bias, and also in this version, the low screen voltage. Installing adjustable bias and trying to run it at 70% dissipation is going to cause problems. I've usually found stock bias to be closer to the 50% range.
Also, for these "distortion" models, with the different OT and lower screen voltage, 6550's are optional as a plug&play upgrade. (see note #6 on schematic).

[DrGonz78]

Thanks so much everyone here!! As always it is nice to have this community online to talk shop. Without the help I can tend to overlook some detail that someone here notices.

Great replies to all and thanks for sharing. I have pulled the trigger to order the parts and will be going forward on this repair. Figuring this amp is such a beast I might have other issues with the repair, so I might have more questions. From what I have gathered about these amps is the bias being too hot or absent is what kills the tubes. Of course the stock setting is probabaly cool bias around 50% as G-One stated. Still perhaps a bias pot is a good upgrade in case you want to try other types of tubes, but this one I am keep it simple for now. Just going to make sure the bias line supply is solid and check other test points in the power circuit.

On aside, I have a dumb question... okay here it goes... This is one of those questions that I thought about asking but never did over the years. When running a tube amp you always want a load on the output, this I know. However, lets say you run the amp without power tubes. Do you still need a load in those situations? Just one of those questions that seems silly but still a good question. Thanks again!

[g1]

With output tubes removed you do not need a load. That's the only time you can get away with it.
However, your B+ may rise quite a bit, so make sure you do not exceed the voltage rating on any of the caps whenever you are operating with tubes removed.

[drewl]

The crazy filtering on these amps always struck me as odd.

I try to get people to go for 6550's on these amps, but the Sovtek 5881's hold up pretty well and are a cheaper alternative.

[Leo_Gnardo]

I try to get people to go for 6550's on these amps, but the Sovtek 5881's hold up pretty well and are a cheaper alternative.

I expect KT88 and/or 6550 would be a treat on V4 amps but I find the glass on these fatties is slap up to the side of the output transformer. Practically a guarantee of perfectly good (expensive) tubes getting smashed. FWIW I did try KT77 but did not like what they sounded like pushed past clip.

[swingarm]

I have a v4 output transformer. I was planning to build a bass amp in the near future. What is voltage/power rating of the PT in this amp?(If you don't mind) It is kind of a narrow chassis and my tube spacing is not the greatest so I would imagine I should go with 6l6? Hope you don't mind the quick question, I don't mean to hijack the thread.

[DrGonz78]

I do have at least two 6550A's in my shop and I might even have a matching quad if memory serves correct. Gotta check that out as a possible option. Only thing is that I have no tube tester and no way of really knowing if those tubes are good. Perhaps I can test out the amp with the 6550's, but I worry about something going south!! yikes! I am thinking on it for now and holding off on the 6L6's for the moment. I can also go check out my reclamation shop that has rows and rows of old tubes to see if I find any vintage 7027's. That's probably a long shot though.

As far as power rating of the transformer it says 545v with the 300725-1. If I read this schematic correctly, the 300726-1 is higher at 594v. I could be misunderstanding, but this is how I am reading it. My amp here has the 300725-1 model transformer.

[g1]

As far as power rating of the transformer it says 545v with the 300725-1. If I read this schematic correctly, the 300726-1 is higher at 594v. I could be misunderstanding, but this is how I am reading it. My amp here has the 300725-1 model transformer.

The schematic is for V4/VT40 (100W) and V2/VT22 (50W). Items with asterisk are for V4. The V2 runs a higher B+. So the 300726-1 would put out more voltage, but less overall power as it is for a 50W amp.

I'm a little confused about swingarms question as he said he has a V4 OT but asked about PT specs.

[swingarm]

I'm a little confused about swingarms question as he said he has a V4 OT but asked about PT specs.

I was just curious what the voltage was originally in the amp. I will be using a plate transformer that is 450-0-450 and 340-0-340 with a separate filament transformer.

[nsubulysses]

V4/VT22s that I have seen typically have voltages between 530V and 540V for B+

I think V2/VT40 is in the 570-590V range for B+

is that what you mean??