Ampeg V4 missing its highs. Help?

[StuartV]

I have an old ('72 - 74 era) V4 (not a V4B). I got it early last Spring. I promptly took it to a local tech who is really good with vintage tube stuff, but mostly works on guitar amps and has not had that much experience with Ampeg heads in general.

Anyway, he serviced the head, replaced some blown caps, etc., and pronounced it good to go. It appears to have all original tubes, or at least American made tubes from that time period. When he gave it back to me, he did comment that he particularly liked it because it didn't have the shrill scream (or words to that effect) that a lot of high powered guitar amps have. While he had it, I did have him modify Channel 2 to V4B specs. That just involves replacing 2 caps, so it was cheap and easy.

Since them, I have also acquired a '72 Traynor YBA-3 Custom Special head, which is an all-tube bass amp only rated for a little bit more power (135 vs 100 in the Ampeg). Comparing them, I noticed, and a guitarist friend commented, that the Traynor sounded warmer and we both like it better.

Also, it seems like the Traynor would play a fair bit louder before starting to distort. The Traynor is rated for a little more power, but it seems like they should be a lot closer in output level (at the point of distortion onset) than they are.

And, with the Ampeg, I have had it where the Midrange control was turned up to the 3:00 position, the Bass rolled off to 9:00, and the Treble boosted slightly, just to get a sound I thought was okay. But, it still seemed to be lacking some midrange cut (cut, as in presence in the mix).

So, yesterday, I took it over to my new amp tech's place. He has a V4B from the same era hooked up to an Ampeg 810 cab. We tried playing both amps, back to back, through the same cab, using the same P bass. Using Channel 2 on my V4 should have resulted in identical sound (in theory, of course). In reality, the bottom notes sounded about the same. But, if I played up around frets 10 - 12 on the G string, on his V4B, it sounded good. And on my V4, it sounded like I was playing with several wet blankets thrown over the cab.

I left the amp with the tech (a new guy - not the one that worked on it 6 months ago) to check out. But, I would really like to know what some of you experts here think might be the problem.

Thanks for any help!

[nashvillebill]

Well, here's what I'd recommend:

-Replace EVERY electrolytic capacitor in there.
-Replace ALL coupling caps
-Replace all plate resistors with metal film
-Clean and retighten all tube sockets
-Make sure all tubes are correct

And lastly...I'd undo any mods done earlier.

Then check all voltages to see if they match the schematic. And then I'd put it on the scope.

[anderbass]

Stu, was ch-1 any brighter sounding than ch-2? (if so, mighta just been a tube/component issue in ch-2)

You coulda also patched most any external pre-amp into the ext. amp jack on the back to experiment/discover if the poweramp section was giving the dark-sound or not...

And/or patched the ext. amp jack out to another poweramp to kinda help confirm the dark sound was being caused somewhere before your V4's poweramp stage. (but of course you'd still wanna keep a cab connected to the V4 for safety/ohm-load reasons)

[StuartV]

Thanks for the suggestions. When I've tried Channel 1 in the past, I thought it sounded almost identical to 2. But, I didn't directly A/B it this week.

At this point my tech has it and he said he suspects a weak preamp tube. So I think he's going to start by just swapping tubes from his V4B to see if he can identify one or 2 that make the difference.

[nashvillebill]

A weak preamp tube is possible of course, but if he were a decent tech, he'd pull the top off and quickly check a few plate voltages....IMHO.

"Shotgunning" by replacing parts is hardly an efficient troubleshooting method. And frankly I don't buy the "weak preamp tube" line. Preamp tubes can go bad, don't get me wrong, but they are surprisingly robust.

He may be pulling your perfectly good old preamp tubes and selling you brand new Russian tubes--then reselling your (still perfectly good) tubes. This is a well-known ploy by some unscrupulous techs.

The Ampeg V4 schematic is readily available and it clearly lists all plate and power supply voltages. THAT is where the troubleshooting should start, again IMHO.

[beans-on-toast]

What Bill is saying makes good sense. I'm wondering if the better sounding amp is stock. Maybe some mods have been applied to make it sound better. If the power supply caps were changed, check if the values are correct or if larger values were installed. Even if the caps have the same value, some cap cans have a -10%, +50% tolerance. This could make the two amps sound different.

Having said this, more than likely a coupling cap or component is out of spec and causing the problem.

Testing the tubes, checking the tube socket tension, cleaning all the connections with deoxit, and ensuring that the power tubes are properly biased is a good place to start. Taking care of the easy stuff first ensures that the basics are covered. Checking the voltages in the amp against the schematic comes next.

The tech can go through the amp by applying an input signal, stage by stage, and looking for a high frequency rolloff. This will take time. It is sometimes quicker and more cost effective to test components in the signal path to see if they are within spec or some prefer to simply start changing components using an educated guess.

I would ask your tech for a game plan and see if what he is proposing makes sense. If he doesn't have a lot of experience with this type of amp, sometimes the cost can get out of hand.

[StuartV]

Thanks for all the suggestions.

I'm not that worried about my tech ripping me off. He knows that I had the amp recently serviced by someone else and that I know what tubes are in there. If he tells me that replacing one or two preamp tubes fixed it - and I can hear that it is fixed - well, NOS preamp tubes just aren't that expensive. So, as I said, I'm not too worried about him ripping me off.

Also, I'm pretty confident that the last guy that worked on it would have done a good job on the basics like tube socket tension and cleaning. He just doesn't know Ampegs that well, so he didn't realize that it doesn't sound like it should. And I say "should" based on, one, my own thinking that it didn't sound "right" and, two, how it compared to Shane's V4B - which I believe to be stock, other than tubes. Between the two, it's not that mine sounded good and his better. It was that his sounded "right" and mine did not. Unless I hear about 5 more old V4 or V4B amps that sound just like mine (muffled high end), I will not be convinced that mine is how it's supposed to sound and his just sounds that much better because of tweaking.

Shane did (I think) a darn good job on my SVT. Based on what he told me found and fixed, I believe that, one, he did what he said he did, and that it had several issues - which are all now fixed. He said he had a total of about 4.5 hours in it. Based on knowing that it had no tubes in it and had sat for, literally, 10 years with no tubes in it, I thought 4.5 hours for all he had to do was not bad.

Anyway, I'll follow up once I hear a diagnosis from him.

Thanks again for the help!

[StuartV]

Well, here it is almost 2 months later and I got my amp back today, all fixed. And the culprit was.... tube sockets not clean enough. A different tech went through the amp and cleaned all the sockets when I got this amp (last Spring). But, I guess he didn't do a good enough job. My new tech told me which specific tube socket it was that made the difference, but I don't remember which one it was now. The important thing (to me) is, I paid him for one hour of bench time, all he did was clean the sockets really well, and now it sounds like the V4B he has. No more wet blanket sound.

Yaay!

[two fingers]

Wow. Cool. Sometimes it's the simple things I guess.

[beans-on-toast]

Good that you got is sorted and are enjoying the amp.

Gunk can cause all sorts of strange problems. Not just on the sockets. It's common to see a lot if dirt built up inside amps. On circuit boards, dirt can have a capacitive effect between traces. A capacitor is basically two electrons with an insulator between them.

NOS tubes can come with oxidization on the tube pins. Oxidization can act as an insulator as well. It helps to clean the pins before the tubes are installed.

So keep those sockets, pins, and circuit boards clean. Regular maintenance is important.