Blues DeVille reissue squealling preamp

[terrykvan]

Clean ch. is ok and I have tried many other tubes with only slight improvment. Can't turn gain up past 6 or so without setting off high-pitched oscillation which is present at output of V1-A. This can be done with or without inst. plugged in. tone controls affect sensitivity and freq. of squeal. Input load also affects, less load-less squeal. Nothing plugged in- very sensitive. Insert plug to open input contacts- much less sensitive. Input resistance cks ok, in1-1meg, in2-135k and no signal present. Plate voltage is 259, cathode is 2.06 for all 3 stages. Also, can't seem to find a schematic for the reissue. Thanks for any help. TKV

[Jazz P Bass]

Blues Deville RI

Here is the schematic.
What is the history of the amp?
Was anyone in there before?
Quote:"Input resistance cks ok, in1-1meg, in2-135k and no signal present"
What was the reference point for these readings.
I would measure V1 grid to ground.
Insert a cable into each input jack & jumper tip to ring.
This will effectively ground the grid.
If the noise goes away, then you have a bad input jack(s).

[terrykvan]

Hey, thanks for your help. No sign of previous work. V1 grid to ground with nothing plugged in is 34K (the 2-68k input resistors in parallel). When I plug in a cord and short tip to ring, the squeal is worse. The scope shows there to be no signal at the input during the squeal, just everywhere after the first stage.The schematic I'm looking for is for the reissue version of this amp and there appears to be more than a few changes. Like grid resistors and the like. Don't know if it's bad design or not, that's way beyond my scope. Thanks again and have a Happy New Year! TKV

[frankeg]

Just adding a suggestion here. Sometimes ocillations can occur from un-wanted feed-back between stages. Did you try pulling V1 and then turning the volume up?

[terrykvan]

Thank you for your suggestion. Yes, I have tried that and it removes all sound whatsoever as it removes the first 2 stages leaving only the gain control and the 3rd and final "distortion" stage. (V2-B is not used). But I certainly agree with you about the feedback between stages, I mean that's what it is. I just don't know how diagnose the cause. All cathodes have same gnd. potential, caps "look" ok when I put an ohm meter across them. They charge until they reach the 1.5kohms of the cath. resistors. I must be barking up the wrong tree. Or is it the nature of the beast? This is a friends amp who said it just started doing this. I don't want to start pulling parts off of it until I can find good reason. Appreciate your help, Happy New Year, TKV

[Chuck H]

I'm going with V1 being microphonic

[terrykvan]

tried many tubes in both positions. Low gain Sovtek wa's and wxt+, brand new JJ's. none were microphonic as far as tapping on them to produce noise. Those tubes worked well in other high gain apps.

[frankeg]

"Or is it the nature of the beast?" Yes, and an ugly beast it is. Sometimes you will never find the root cause and have to deal with (fix) the symptoms. I know you checked the cathode gnds, did you check all gnds? Check that all tubes are tight in their sockets. Pull the tubes and carefully inspect the sockets - You'll need lots of light and a magnifier glass. Look for any signs of arching (carbon) or cracks. If nothing there, then chop-stick it (an o'scope would be nice to monitor the output as you do this). Do all leads, tube pins, and components. Physically touch the preamp tubes and wiggle them a little. If nothing shows up, you may have to alter the amp to fix it. From my Gerald Weber Tech book: Some Fender circuit boards can become conductive over time. He doesn't say how to check it but I guess you'd use an ohmmeter. Another trick is to make a cap probe: 500pf (approx) cap with leads attached and clips (mini clips/ball clips are best) make sure all conductive surfaces are shrink wrapped or taped. Connect the probe 1. across the preamp plate resistors 2. from preamp plate to grid 3. from preamp plate to cathode 4. Pwr tube grid to gnd. Listen to what changes. If you can eliminate the problem with the cap probe, you may need to make it perminate. Good luck! I hope you find it.

[terrykvan]

Good morning Frankeg, I guess you couldn't sleep in either. I've done the chop stick thing to all tubes and components, wiggled the tubes etc. and everything's ok there, have'nt visually checked sockets. Can't say I've checked ALL grounds, just the all the ones in the preamp section. Which Gerald Weber book had the cap probe. I have "A Desktop Reference..." and "All About Tube Guitar Amps" and may have forgotten reading about that. Thanks for the tips, I'll give 'em a try. TKV

[Jazz P Bass]

Beast

Do any of the controls change the frequency of the squeal? Tone, Volume.

[terrykvan]

Yes, the tone controls do and the bright sw. especially. The mas. vol. can be set to "0" and you can still hear it but it does'nt bring it on any earlier if it's turned up so only the eq. and gain affect it.

[Jazz P Bass]

C2

Can you check the grid side of C2 (0.01uf/400V)for Vdc leaking through from V1 plate?
Also check the grid of V2 for any Vdc.
Does On Drive make it worse?

[terrykvan]

No DC on V2 grid. I'll lift C2 up get back to ya.

[terrykvan]

Sure wish I had a schematic for this "Reissue" version. Lot a differences. First stage coup. cap. is C18. C2 is .047 3rd stg. coup. cap which was .022 on the original. Everywhere I look there's a design or value change. Well I checked that coup. cap in circuit for dc and found none(I know thats not the right way) but I have'nt pulled the PCB yet and don't think that merits it at this point.

[frankeg]

Which Gerald Weber book had the cap probe. The title is "Tube Amp Talk for the Guitarist and Tech" I enjoyed reading it. The cap probe is my idea based upon His suggestions for fixing hi freq oscillations.