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#1 2012-03-17 19:55:36

tonight, we ride
New member
From: Bellingham/Seattle
Registered: 2012-03-17
Posts: 6

Severe, Intermittent Redplating

I recently picked up a mid 70's V4-B head and the thing is fucking amazing, but i suspect has some minor issues. Yesterday during a recording session it lost power on me. Upon investigation I found that the back plate was hot to the touch and that the fuse had blown. After tracking down a new fuse and replacing it, 3 of the power tubes immediately began to red plate SEVERELY. Seriously, they looked like they were filled with bright orange lava, i'll post a pic once i get it from a friend. After turning off the amp, we discussed our options, being only a handful of hours into what had been booked as a full day of guitar recording. We tried a few other amps but nothing approached the V4-B head. Against my better judgement i decided we should push forward one song at a time, figuring i would have to fix something anyways so the red plating couldn't do TOO much damage other than to the tubes (someone please correct me if i'm wrong about the assumption that prolonged red plating of tubes could not damage your transformer before the tubes fail). We setup a fan to cool off the back of the amp and I turned it back on. Although the tubes began to fill with an orange glow, originating from the plates they shortly after normalized and proceeded to function like normal tubes for the remainder of the day.

I plan on investigating the cause of this today or tomorrow and although i have experience working on amplifiers (i've done a few cap jobs plenty of retubes, and know my way around electronics theory and a soldering iron) I have never had to deal with redplating tubes. I was hoping someone here could confirm that the potential causes i should look into are:

1)Potentially shorted bias resistor
2)Potentially shorted coupling cap

Anything else? Also I'm curious as to what could cause three of the tubes to redplate but not the fourth? The amp was serviced shortly before i bought it (which was one week ago) and i have the invoice for the work. One filter cap was replaced, along with "balance resistors across the screen supply filter caps." The work is under warranty, but to be honest i'd not only rather do it myself but am a bit skeptical of the work that was done considering i'm having this issue a month after service. Any advice is appreciated.

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#2 2012-03-22 14:03:25

tonight, we ride
New member
From: Bellingham/Seattle
Registered: 2012-03-17
Posts: 6

Re: Severe, Intermittent Redplating

A small update:

Got a chance to check out the amp for a little bit last weekend and I didn't find anything immediate. Checked a few coupling capacitors (C10 -C13 I believe) for shorts and found nothing, but i only had a few minutes to work on it. I did find that the Hum potentiometer is failed short, but from what i've red that shouldn't be causing the tubes to redplate.

After recording again last night, and again having the same problem about 3 hours into the session, i'm begining to suspect a bad solder joint that's shorting the grid of a power tube to the anode connection, but someone please chime in if you have any ideas or recommendations on what to check out. I'll be posting pics on Friday, and hopefully a more detailed report since i'll have some time to actually dig into this thing tonight and tomorrow.

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#3 2012-03-30 21:30:04

athena
Member
From: USA, CA, Solano County
Registered: 2007-12-02
Posts: 56

Re: Severe, Intermittent Redplating

first off.....
YOU have BIG problems if you are describing this correctly.
The caps from the phase inverter DO NOT have to be anywhere near shorted to cause bias problems. You need to test them for DC leakage while they are running in the circuit. But it will be easier to just REPLACE the caps and see what happens.
But that is getting ahead of the game anyway. I would take this back to the tech that did the work before you do anything else.
If not, pull the power tubes and check the negative bias to confirm it is working properly.
Check or replace all the components in the bias circuit, especially the bias cap(s).
If those tubes were red plating and you played the amp for that long of a time, THOSE tubes are done, you need a new quad, but you need to make sure this amp is functioning properly before you install them.
You just need to check everything in the power supply. Have you put an Ohm Meter across the screen resistors.?
Good Luck

Last edited by athena (2012-03-30 21:39:35)

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#4 2012-03-30 22:39:05

tonight, we ride
New member
From: Bellingham/Seattle
Registered: 2012-03-17
Posts: 6

Re: Severe, Intermittent Redplating

Thanks for the response. Yeah, those power tubes lasted a little longer than i thought they would, but are definitely shot. Just got a new quad that is running fine, but i'm keeping a close eye on them. And by that i mean that I'm using the amp as sparingly as possible until i can get the parts to do more repairs (another week or so). To be honest, I'm perfectly happy, and capable, to do a COMPLETE cap job on this thing as well as replace any resistors that are suspect. I did a close inspection, both visual and with my multimeter, of all caps/resistors, but only checked for resistance/capacitance in circuit. Didn't bother to check for DC leakage, since i'm planning on replacing parts anyways. I'm want this amp to be good and reliable for future tour plans, so i'd rather do a minor rebuild and not have to worry about it. In regards to the tech that did the prior work... i'm not even gonna bother. I didn't pay him anything, and if he let an amp out of his shop that is capable of a severe problem (after charging almost $400 for parts and labor) i don't want him touching my amp. That being said, any recommendations on what sounds good in these things is appreciated. Work has been slow this week so I've basically read the entire forum and have picked up the following:

1) People have recommended changing the 5 watt screen resistors from 470 ohm to 1K ohm.

2) Someone (I think it was Hangman) was saying they didn't like the sound of Orange Drop caps in the V4s, and was recommending the Mallory 150s in conjunction with Solen for the coupling caps. Orange drops have always been my standard replacement caps, but my distributor sells the Mallorys and Solens, so any other opinions on this would be appreciated.

3) Whats the deal with the flyback diodes? Those are the guys that are off board right (between the transformer and the board)? I've read on here that i can just take 'em out, but would rather just go on and replace them while i'm doing other overhaul work.

Am i missing anything else that should be replaced while i'm digging around? I really want to get this thing in good shape and am going to change out all capacitors, but am unsure of what resistors, other than the 470 ohm screen resistors to replace since none were burned out/had suspicious resistance readings. Thanks for your help!

Oh, and here's a pic of my tubes just so you know what I was going through...

http://www.flickr.com/photos/hearmeshim … otostream/

Last edited by tonight, we ride (2012-03-30 22:43:52)

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#5 2012-03-31 00:18:04

athena
Member
From: USA, CA, Solano County
Registered: 2007-12-02
Posts: 56

Re: Severe, Intermittent Redplating

Holy Cow.!
Those guys are on fire.!
Did not catch the history of this amp. Maybe your old quad of power tubes was just way past their prime.?
You checked the bias on the new quad right.?
There are better guys to ask about an Ampeg V4B than myself but.....
You are running EL34.? I would say the forum was on mark to suggest upping the screens to 1k. 34's can take A Lot of plate voltage, but their screens are kind of girly.
Caps.....?????
That always seems to end in a verbal fist fight. If the capacitance is the same, I am not at all convinced anybody can tell the difference in an audio circuit. I know people SAY they hear a difference between an Orange Drop (which orange btw, they make SEVERAL different types) and a Sozo. But I SAY, if I change the caps they will not be able to tell me what is what. Use what you want and what is readily available. BTW.....THAT is why a Fender had Blue Molded , and Marshall had Mustard in them. THOSE are what Leo and Jim could get at a decent price and in consistent numbers. Whatever.
If your fly back diodes are good, then they are good. They are kind of a half ass fuse... if the OT spikes, the diodes will short to ground and protect your tubes, and perhaps even further back in the circuit. No harm in replacing them that I know of. They can also be done away with as you say. But why tempt fate, or spit into the wind.?
I am all for gutting older amps and rebuilding them......Maybe consider all the jacks.? Yeah, they work, but everybody and his brother have gotten their 83 cents out of the poor things by now. 2 or 3 dollars a piece for new jacks is pretty cheap liability insurance.
Just stuff like that.
Good Luck

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#6 2012-03-31 00:42:36

tonight, we ride
New member
From: Bellingham/Seattle
Registered: 2012-03-17
Posts: 6

Re: Severe, Intermittent Redplating

There's a good chance that the old set of tubes hadn't seen much use in about 20 years. I recently bought it from a guy that is the second owner and has had the amp for 25+ years, but hasn't played much since the early '90s from what he said. Just to be thorough, I'm running 7027a tubes. It had Sylvanias in it, but I replaced with JJ's (just cause that's what i prefer). Despite it being a V4-B, I'm using this thing for guitar and it sounds AMAZING, in fact i couldn't be happier with the tone, which is why i'm willing to go the extra mile preemptively instead of repairing things as they fail.

Thanks for the heads up about jacks. I always forget about those when they should be the first thing i replace. I usually use Switchcraft, but am open to opinions. Shit, do i need an enclosed jack? I'll have to open it up and check what's in there when i double check cap values. I usually use the Orange Drop 715P or 716P caps, so i may just throw those in. Although, I'm tempted to try out some of those Solens for the coupling caps, but I tend to agree that a lot of the difference is either not really there or for people that have far better ears than I do.

The first thing I did when i put the new tubes in was to check the bias (one tube of each set) and it was running surprisingly cold... at 40% dissipation assuming 30 watts per tube. Plate voltage was something like 546 volts with an idle current of 18 or 19 milliamps. I was honestly surprised and had expected to find a problem with the bias but also had to run right out the door to play a show. It did just fine last night for about a 30 minute set. But I hate to treat gear poorly and am very anxious to get some serious work done on this thing so that I know it's running properly. The last thing I want is another set of tubes that look like they're filled with lava!

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#7 2012-03-31 02:57:29

athena
Member
From: USA, CA, Solano County
Registered: 2007-12-02
Posts: 56

Re: Severe, Intermittent Redplating

I forgot that I have a V4-B. I re-capped it about 2 years ago, and was fighting some hum/noise. I got the amp for free, and NEVER heard it run, so I am not sure what problems it may have had before I inherited it. I had put it aside to work on other things.....
I added a small trim pot for the bias circuit, and pieced together some caps that worked. The style and value of filter caps, as they were originally spec'd, are no longer available. Of course that is true with a lot of amps this age. Component styles change over time. Anyway.....the guys on this forum have kind of dealt with this power supply/filter cap ordeal many times.
Looks like I replaced the two input jacks and the speaker jack with new Switchcraft shorting jacks.
For the time being,I just chose to ignore the preamp out/slave circuit.....or whatever that is.
Your [post has made me realize I need to get back into this amp and get it sold.
Too Bad we are not closer, we could compare nightmares. I am not real fond of these "television era" Ampegs.
Leave the Pot and Switch PCB alone if you can. That whole area is another sorry sack. I am sure there are guys on the forum that are accustom to these things, but to me, they are  another fine example of why hand wired amps are a better way to.
Unless, of course, you do work like Andy Fuchs, Mike Soldano, etc. Those guys are the exception in my book. I do not put Mesa Boogie in the same class as those guys. There is another brand of amps I would just as soon avoid. They sure have some qualified and helpful techs though.
I'm rambling.....sorry.
Take Care

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