1966 Princeton Reverb bass pot not much responce (FIXED)

[punkykatt]

What would cause this? Pot is within spec and wiper shows no waver on a  analog meter,(replaced pot also, no change) tone stack caps  250p  reads 254p, 0.1 reads 0.098, .047 reads .045, slope resistor reads 113k, 6.8k on pot reads 7k to ground. Tried different speakers,  Tried different tubes preamp and power.  All "E" caps have been replaced (filter , bias, and bypass).  All coupling caps have been replaced except one that still read the proper value.  Did a turns ratio test on the OPT = 8.6k/8 ohms.  All voltages  look normal just a tad high but with today`s voltages that`s to be expected.  Everything on the amp works well(sounds a bit thin) the bass pot does not add very much bass as the other PR`s I have played .   Any thoughts or ideas are much appreciated.
Thanks in advance  Punky

[HotBluePlates]

Are the preamp tube cathode bypass caps aged to very small values?

[punkykatt]

All the preamp  tube cathode bypass caps were replace with 22uf/50v.

[HotBluePlates]

Well, I was thinking if the amp's overall voice was too thin, maybe the Bass pot had little to act on.

The Bass pot range is typically impacted by the Treble pot setting (you can verify this with Tone Stack Calculator). Are both Treble & Bass pots audio taper? What happens to the Bass pot response when you turn the Treble pot?

[eleventeen]

Maybe it's the particular speaker you're playing through?


Sounds like you've pretty much covered the waterfront.

[punkykatt]

The schematic calls for bass and treble pots to be 250kA (audio) The bass pot has little effect even if the treble pot is turned down/off.

I tried different speaker cabs with no change.  I have a AA1164 PR build I use at gigs. I tried this  build through the 1966 PR cabinet/speaker and  there is  plenty of bass available . So it is not the speaker.

[SILVERGUN]

I would have to try lifting the 6.8K from ground just to see the effect... basically, a tone stack lift

See if there's bass there to begin with...it'll give you a point to look forward or backward from there.

[punkykatt]

Thanks Silvergun,  lifting the 6.8k from ground increased the volume a little.  Not much bass in this amp at all.  Usually stock PR`s have more bass than you ever need.  Partially shorted eyelet board maybe?  Something is sucking the bass out of this amp.

[HotBluePlates]

I saw you measured the tone stack caps; are all the coupling caps close to correct value?

[punkykatt]

All the coupling caps have been replaced but one that still  read the proper value.

[Fresh_Start]

I'm going to assume that you've recorded voltages on each tube plus bias voltage and they're all "normal".  Apologies for the shotgun approach below. A listening amp would help find the location of the problem.

Have you tried different preamp tubes?  Power tubes?  I assume you have but must ask.

Have you tested current and bias of power tubes? (I'm guessing "yes" based on having measured the OT primary winding)

Lifting the 6.8K Mid resistor should jack up the voltage gain a lot. Is the volume what you would expect?


Is there DC voltage anywhere there shouldn't be?  A leaky coupling cap can cause problems, especially a tone stack cap.


If the grounding is done with the original brass plate, could there be a mechanical/physical problem with grounding?  A jumper between the brass plate and ground would create a ground loop but might help ID the problem.


I'm just throwing out ideas to see if anything sticks to the wall. Troubleshooting can be so frustrating and I've certainly missed checking something that I thought I'd checked already.


Respectfully,
Chip

[punkykatt]

Chip, thank you for the reply. Yes I have done all you mentioned including using jumpers to bypass the brass grounding plate and the listening amp tool. When performing the listening tool procedure I could not find any location from input jacks to power tubes that the bass pot would increase bass tone.  I also think the amp should have a bit more volume.  All the voltages through out the amp are a bit high but nothing way out of wack.  V1 pin 1= 175v,   pin 3= 1.3v, pin 6= 178v,   pin 8= 1.17v.   6V6`s  pin 3= 431v,   pin 4= 422v,   pin 5= -34.5v,   pin 8= 22.1 mA.    When I lifted the 6.8k from there was some gain but not a lot.  Tone stack caps have been tested for leaking DCv , All other coupling caps have been replaced except for one that was the proper value. I don`t remember if I tested that one for leaking, its the .02 just before  the 500pf going to the reverb. If it were leaking wouldnt the 3.3m/10pf block and DC?

[SILVERGUN]

Punky, since you're having a rough time, I'll just throw this out there.
It's a stab at something that stood out to me on the schematic and got me once before...(I'm looking at an AA1164 schemo)
....and I could be off base here, but it's worth a stab
 
I would check the reverb pot and its ground connection...I would also be inclined at this point to remove/bypass the reverb circuit and see the result, although it will show a volume increase because it is inherently lossy, and might mislead you into thinking that you found the problem.
 
Just throwing ideas at you,
Good luck Sir

[punkykatt]

Silvergun, you might be onto something.  When the customer dropped off the amp (he just purchased it at a trade show) I found a pot in back of the amp  in a plastic bag marked "original reverb "  I will check the pot the last tech installed.  Hopefully that it.  Thanks

[punkykatt]

Reverb pot connections and pot are good, connection from pot wiper to connection to 470k and 3.3m/10pf are good.  Jumper across 3.3m/10pf results in good size boost.  Amp gets a bit noisy with 6" bypass jumper installed. Bass control still the same.  Dang itall      Thanks Silvergun for the suggestion.

[SILVERGUN]

Crap....I had my fingers crossed for you.
..and it's been a while since I earned one of sluckey's gold stars.
Maybe next time.
SG

Here's a visual guide to my typical trouble shooting/repair process
 
 
 
 
 
 
 
 
 
 
 
  ......you'll get here eventually

[HotBluePlates]

Since you've ruled out almost everything, I'm hoping it's not some obscure issue with the fiberboard itself.

[punkykatt]

I`m hoping there is no issue with the board too.  I need a break from this amp for  a couple of days.  Just got a 1980 Marshall head in for some mods.  Thanks for all the input Guys.

[punkykatt]

Found the problem.  Removed all the pots and input jacks, pulled up the brass plate, lifted the fiber board, pulled the fiber insulator board. I could not believe the crap that had built up between the chassis and both the boards.  Im thinking someone must have sprayed  a cleaner under there that had a lubricant in it and it, over time collected all the the dust and dirt?  Cleaned up both boards and the chassis.  Now it functions like a Princeton Reverb should.  I lucked out!!!!    Thanks to everyone who posted.  Punky

[HotBluePlates]

Wow!

Was there gunk between the fiberboard & insulator board? Or primarily insulator & chassis?

I follow how the brass plate may not have made good-quality grounding.

[punkykatt]

HBP, there was gunk between  both insulator/chassis , insulator/ eyelet bd. about 50% each.  Previously  I used short alligator clip leads for testing grounds to the pots/ input jacks to chassis  bypassing the brass plate with no results.

[SILVERGUN]

 
 
 

[Willabe]

That's weird. 

They must have really drowned the amp inside. 

[HotBluePlates]

Well, now we have a new thing to check in vintage amps... Good to know & glad you got it resolved!

[Fresh_Start]

Thanks so much for coming back and explaining the exact problem and solution.

Really glad you figured it out.

Chip

[eleventeen]

Thanks for reporting what you found and what finally fixed it. I grew up with these things since the 70's...but we are talking about stuff that is 50-60 years old now.

[p2pAmps]

I often gut older Fender amp and 99.9% of the time that brass plate is corroded pretty bad.  I find all kinds of surprises under boards LOL...  Glad it's working better for ya now.

Cheers

[floyd]

Woweeeee,, I'm really happy for you!! Did i say Woweee ?

[punkykatt]

Your welcome Guy`s, that is what this forum is all about, asking for help and sharing your success and experience.