Trailing distortion on Ampeg V4

[Sansteeth]

Hello everyone,
it's been a while I haven't posted on here, I guess it's good, it means I've been able to fix the stuff I was given 

This one is pretty subtle but I feel stumped:

A customer of mine has an old Ampeg V4 that he has brought to me a bunch of times for a check-up, then a recap of the power supply and a biasing after he got his hands on a quad of RCA 7027A.
He plays the amp through his loadbox (Two-Notes Torpedo Reload + Cab simulator into headphones) and he has been complaining about a distorted noise that sits on the trail of the note when he plays. We tried on a 4x12 at my shop, to rule out the whole loadbox set-up as the culprit, and sure enough, that noise is there too.
It's subtle but you can hear it here https://soundcloud.com/smokestackthermionics/untitledampeg-v4-distortion

So far this is what I've done:
- recap (prior to the issue)
- swap all the preamp tubes one by one
- swapped the 7027A with some 6L6GC
- swapped output transformer
None of that solved the issue.

I would be curious to know what you folks think this could be.
Thanks a lot!

greg

[shooter]

tried jacking the pre into another PA amp to 1/2 split the problem?

[Sansteeth]

No, not yet, good point!
I'll try that tomorrow or the day after
Oh and I'd like to add, I removed the reverb tubes and I still got the trailing distortion.

[Willabe]

A customer of mine has an old Ampeg V4 that he has brought to me a bunch of times for a check-up, then a recap of the power supply and a biasing after he got his hands on a quad of RCA 7027A.
He plays the amp through his loadbox (Two-Notes Torpedo Reload + Cab simulator into headphones) and he has been complaining about a distorted noise that sits on the trail of the note when he plays.

He put in real RCA 7027A's to play through a loadbox, Two-Notes Torpedo Reload + Cab simulator into headphones?
 

[Sansteeth]

Honestly...
After opening up some of those Two-Note Captor and Reload loadbox rated at 100W, it's not even the RCA 7027A I feel bad about. It's the output transformers of those collectible amps they are diming into those loadbox...
It's litterally just a "small" 8R resistor rated at 100W...
On a heatsink with a fan, we are meeting the requirement to dissipate the 100W it's rated at but that's not how you choose a load resistor for a vintage amp that reaches 100W clean at 9 o'clock on the volume pot...

[dogburn]

Have you done the biasing yet? When I built a single channel AB763, when working out the initial bugs, the sound was not right when playing guitar - it was partially clean, partially distorted. If I played softly, it was okay, but strumming harder brought out a buzz. Too a while to figure out that the two 6V6s were way off from each other and that one was biased way far from the other. Switching tubes around to find a more closely matched pair and then a suitable biasing took care of it. But that may not be the case if you already tried switching out the tubes with 6L6s.

[Sansteeth]

Have you done the biasing yet?

Bias was my thinking too but the Ampeg V4 doesn't have a bias pot so I temporarily installed one and I was not getting significant change as regards to the trailing distortion.

There is an Ext. Amp input/output on those so I plugged the V4 preamp into the return of a Peavey 5150 I had lying around in the shop. The nature of the distortion changed (more like a haze in the background (as if you had a mix control on a distortion pedal: clean signal on top a distorted one in the background) but I didn't find that test conclusive either, the ext-amp takes the signal before the second half of the 12DW7 and obviously before the PI.

I tried to play the entire V4 into one of my cabs to check if it's not the whole loadbox setup that messes up with the signal (hate working with so many parameters...) and then, plot twist, I got massive hum.
Checked the B+ on the plates of the 7027A, one pair was getting none, my fears were confirmed when I checked for continuity on the primary windings of this ORIGINAL MAGNAVOX AMPEG V4 OUTPUT TRANSFORMER 
The output transformer is dead, which was to be expected considering how that amp was being run and it was a long time coming (can't say I didn't warn the guy).
Now my customer is going to put the repair on hold so he can gather himself (emotionally and financially) understandably.
Either it was the dying OPT that was the culprit all along or not, I'll have to wait to troubleshoot some more until he pulls the trigger on a new transformer somehow. To be continued!
Thanks @dogburn and @willabe for the pointers!

[sevenicecubes]

I had to wait for confirmation to make a reply here but just wanna make a few general disclaimers about these that may or may not help you. I'm just a hobbyist but I love ampegs and have pulled my hair out over quite a few of these.

1) In general the ground scheme is all floating off the chassis and connected at the input jack only, this must stay the same. There's a great youtube video where the gentleman walks you through the ground scheme.
2) There is an issue which I've experienced and also read of others experiencing where only one half of the output will redplate at higher volumes. This seems to me to be due to an imbalanced PI, I add a 68k in series with the coupling cap on the red-plating side. This one really drove me nuts. I bought my first o-scope over this.
3) The diode off the standby switch will fail and give you some strange issues and voltages.
4) You have to ground new filter caps the same way that they were grounded originally. One of the caps has B+ on the can because it's isolated from chassis. You can not ground any of the filter caps directly to the chassis like you might in a Fender, and you need to use insulated caps if you're using cans which can be touched externally.
5) The output speaker jacks must also be isolated from chassis. If you put star locking jacks here you will suffer parasitic oscillation.
6) Lots of the components fail from heat or age. I would put some pressure on each component with a chopstick and see if anything pops up.
7) The .047 bias feed cap is another known failure point which could be your issue.
8) I haven't experienced this but if you replaced the screen grid r's, I've been told that it's best to use wirewound over ceramic as something about the inductance makes them happy (I'm not smart enough to explain this)

I have a friend with a good V4 OT for sale if that truly is the issue. I would not run these amps into one of those two-notes things. I would just run the line out.

Edit: Fixed #7. Willabe

[Willabe]

Thank you for posting this information. 

7) The .047 film bias feed resistor is another known failure point which could be your issue.

You must mean .047 capacitor not resistor.

[Sansteeth]

@sevenicecubes: this is extremely interesting information! I did notice the particular ground scheme. I'm not smarter than an Ampeg engineer, I followed scrupulously their wiring scheme for the recap. However I'm not sure that was done by previous techs on the rest of the amp, like the jacks and such. I can't check now, the customer took the amp back home and yes, I told him to use the ext. Amp output to go straight into a power Amp/cab simulator.
I was also suspecting the diode off the standby switch, changed it for peace of mind, but the original one was good and there was no noticeable improvement after the change.

However I might really be interested in the V4 output transformer from your friend, we are based in France though. My customer was going to bite the bullet and order a replacement from the US but I think he'd be interested in getting the real deal.

[sevenicecubes]

Thank you for posting this information. 

7) The .047 film bias feed resistor is another known failure point which could be your issue.

You must mean .047 capacitor not resistor.

yes thank you

However I might really be interested in the V4 output transformer from your friend, we are based in France though. My customer was going to bite the bullet and order a replacement from the US but I think he'd be interested in getting the real deal.

Report back if you need it, or if you fix the amp otherwise!

edited: brain fart

[Willabe]

Thank you for posting this information. 

7) The .047 film bias feed resistor is another known failure point which could be your issue.

You must mean .047 capacitor not resistor.

yes thank you

I fixed the typo.

[Sansteeth]

Report back if you need it, or if you fix the amp otherwise!

I sent you a private message