Volume Drop in '73ish V4B

[neddyboy]

Hello all, and pardon me in advance for stupid newbie questions. I've been repairing my own amps for years with a 90% success rate, but my V4B is WAY more complicated and confusing than anything I've ever worked on.

I've had it for ten or fifteen years, and it's been in storage for the last few years. All the preamp tubes (except the 6K11) were bad, so I replaced all of them with JJs. The power tubes are EH KT88s with about 5 hours on them. I had a tech look at the amp years ago to make sure I was OK when I switched to KT88s. At that time he replaced the can cap near the PT with individual Sprague caps. I've played it a lot since and it sounded great right up to when I put it in storage.

So, I put in all new tubes, cleaned and tensioned the sockets, and...it sounds terrible! The low channel sounds like it's putting out about 1-3 watts, very fizzy, no high frequencies (even for the low channel). The high channel sounds better, louder, but still only half power. The volume drops more on the Ultra Low setting and rises on the Ultra High setting.

I swapped out the power tubes, no difference. Tried a SS 60 watt amp into the same cab, no problem and louder than the V4B. No problems with the power tubes, glowing, etc., no heat coming off the output transformer. I still need to do some preamp tube swapping to completely rule out the tubes, but it sounded just as bad with the old tubes.

Can anyone here suggest a possible cause of this problem? The issue affects both channels but not in the same way, it doesn't SEEM to involve a bad preamp tube, and doesn't seem to involve the power section. I'm pulling my hair out here and I have little enough already.

[Liquids]

Just for starters - make sure you are connecting your cabinet to the SPEAKER jack, and *not* the EXT speaker jack. That's common.

If that fails...plug your guitar directly into the EXT AMP jack and see if you get a big loud sound or not...plugging into it bypasses most of the preamp--which doesn't sound like your issue, but will narrow it down. If it still has the same problem, you know it is somewhere in the circuit after that and not that part of the circuit (and/or the power supply etc.)

If same, I'd next check/test the flyback diodes, and/or remove them (given that you know how to work on amps from your post?), as that may be the issue.

If none of that does it (or anything anyone else recommends as a starting point), let us know.

PS - I had an EH KT88 blow without much use, sadly. I'm back to running cheap/rugged/generic 6L6GCs.

[neddyboy]

Hi Liquids, thanks for the reply. Flyback diodes would be a first for me. Would those be D7-D10? All the amps I've worked on have been simple 2-tube PP Fender-ish designs -- this V4B is pretty intimidating! FYI definitely not plugged into the ext.speaker jack, and I swapped a set of KT66s in to rule out the KT88s as the problem. That's a drag to hear about the EH KT88s. Did the blown tube take out any amp components?

[Liquids]

Hi Liquids, thanks for the reply. Flyback diodes would be a first for me. Would those be D7-D10? All the amps I've worked on have been simple 2-tube PP Fender-ish designs -- this V4B is pretty intimidating! FYI definitely not plugged into the ext.speaker jack, and I swapped a set of KT66s in to rule out the KT88s as the problem. That's a drag to hear about the EH KT88s. Did the blown tube take out any amp components?

Yeah, D7-D10 are the flybacks. You can simply disconnect them carefully and audition if there is a difference.

I wouldn't mess with KT66s! I don't think most (any?) can spec out to handle the V-series plate voltages--and that's just for starters there!

While you are in there, make sure your screen grid resistors are at least 1K since you are using KT88s and such--that's not a bad move here even if using 6L6GC types.

When the KT88 went, it only took out the fuse given the resultant current draw. When I replaced the fuse I switched it on very cautiously with an eye on the power tubes, and switched it back off immediately as I saw the intense glow from that KT88--quick enough to save even that new fuse from blowing.

Everything else was fine, and I've overhauled the amp, so I would have been miffed if virtually anything was taken down because of that tube. I only used the KT88s in there as I had gotten them for another project I was going to do and never did. I've had the most luck with quality 6L6GC type tubes...even cheap, but rugged unbranded Chinese 6L6GCs have held up better then most of the other non-6L6GC tubes I've messed with on a limited basis.

[neddyboy]

Dang, all I have at the moment is the set of EH KT88s and a set of Groove Tubes KT66s. You think the 66s would be a better choice until I can fork out the dough for a quartet of 6L6s? I used this set of 66s a while before I was given the KT88s and had no problems. I also have a quartet of GE 6550s but they're in my stereo and I don't want to risk them in the Ampeg.

I'll try clipping the flyback diodes after work and keep you posted.

---

I couldn't wait for after work. I clipped out the diodes and it works! I would've thought this problem would've caused a complete failure, not something as complex as my issue. Thanks for the tip.

Something odd is still going on though. The low channel is awesome again, but the high channel isn't super loud. Sounds like the volume is set on 4 or 5. But when I turn on the Ultra High switch it gets MUCH louder. I'm going to clean the switches. Any other ideas?

[Hangman]

The infamous flyback diodes!

I always suggest clipping those out, and leaving them out, because they seem to cause more problems than they prevent. (this is my opinion anyway)

I have seen a rash of problems with the 72-74 era V4s that have resistors that go way out of tolerance (always higher resistance) The symptom of this is often low overall volume, or low volume on one channel.

hard to say if this is the problem here... but It could be the case. and pretty easy to check. first try swapping V1 and V2 to rule out tubes.

[neddyboy]

Which resistors are you talking about? I'll check 'em. I've already swapped tubes around to no effect. I cleaned the crap out of the three rocker switches too, but no change.

[Hangman]

unfortunately its never the same one. I have just seen a large number of resistors that show no sign of stress, that measure 4-10 times their labeled value.

if you can isolate the problem to channel 2 then just check the resistors in circuit and see what they measure. note that in circuit they will not necessarily measure correctly, so if you get one thats way off... you can compare to the in circuit measurement for the same resistor in channel 1 (the circuits are identical). if they are the same, its likely ok.

[neddyboy]

Cool, thanks Hangman. I'll take a look.