Help...my amp turned into a fuzzbox...!

[Blindog]

So I'm playing my favorite build, a '68 Traynor YBA-1 that I rewired to 1987 Plexi specs, and all of a sudden it crackles, then a volume drop, and now where my sweet spot of about 5-6 on the volume control gave me that awesome crunch, it gets splatty, fuzzy, and worse as you turn up.

As fyi, I added a split/shared cathode switch for V1, a passive effects loop ala Dumble, and a PPIMV.

Things I've tried:

Switched out all tubes
New output tube sockets
Hit every solder joint
Poked and prodded everywhere with a chopstick
Took out the PPIMV and tacked back in the 220k's

At this point, I'm thinking a failed coupling cap. Anyone elso care to chime in? I haven't been here in a while due to a relocation last year..must have lost my amp skills in the hiatus!

Many thanks, this has been my go to live amp and I miss it!


Mark

[ampgeek]

Bummer! But...I highly doubt you have lost any skills!

Do you measure any DC voltage downstream of the coupling caps and/or hear any scratching on any pots? That might direct you to an ailing cap. I have seen wasted coupling caps that caused effects similar to what you are describing.

Consider building a stethoscope if you have another known to be good, clean amp. Near the bottom of the page here:

http://www.el34world.com/Hoffman/tools.htm

Mine looks like this.

I have used it to identify many a less then optimum cap/other component very quickly.

Good luck,
Dave O.

[cyprusx]

Sounds like either a tube or the OT. Most likely a tube. Replace each one with a known good one until it either works or you eliminate the tubes as being a possible cause.

[Randy Magee]

It could also be a failing plate load resistor, usually on the PI tube, but not necessarily...

[Blindog]

Thanks for the input guys. I went ahead and cleaned/re-tensioned the pre amp sockets and popped in 3 brand new tubes...same condition. However, the presence pot is making noise when I adjust it, and it seems to effect the fuzziness as well, so a leaky cap could be the culprit. I'll check around the coupling caps for DC voltage downstream and report back...Thanks!

Mark

[Tillydog]

fyi, I added .... a passive effects loop...

Are you using the contacts on the jack sockets to bypass the FX loop when nothing is plugged in? If so, try jumpering the contacts temporarily and see if the problem persists.

... the presence pot is making noise when I adjust it....

I'm not sure how your presence pot is wired, but if it is part of the PI tail then it *will* sound scratchy as it is adjusted.

(Lots of dogs round here )

Andy

[Blindog]

I've jumpered the passive loop previously..still the same issue. Thanks for reminding me tho!

The presence pot never made noise when the amp worked without any issues, so I think it's a good place to start checking.

Regards,

Mark

[Blindog]

Well, I'm beginning to suspect a faulty OT. I've pretty much ruled out faulty coupling caps, all tube voltages are in spec..bummer, that old Hammond OT is a beast! So I guess I'll insert the preamp out from my old LAB L5 to the return jack and see if it acts the same..should confirm or deny..

[Blindog]

Swapped out the OT...same problem. This thing is driving me nuts..

Any suggestions?

Thanks in advance,

Mark

[Tillydog]

.. I'll insert the preamp out from my old LAB L5 to the return jack and see if it acts the same..should confirm or deny..

What happened?

Andy

[desu boi]

Measure voltages to ground AC and DC on all pins on all tubes with no signal in and post here.

[Blindog]

Here's my voltages:

V1: P1-138, P2-0, P3-.98, P6-190, P7-0, P8-1.7

V2: P1-162, P2-0, P3-.92, P6-271, P7-161, P8-163

V3: P1-209. P2-26, P3-35, P6-209, P7-27, P8-35

EL34'3: P3-440, P4-438, P5- -37

I'm thinking the problem lies in the PI. Should I be reading ANY DC on pins 2 & 7?

If I run a cable from the loop send to a clean solid state power amp, ir sounds fine. I ran a cable from my Lab preamp out to the loop return and I get the same low power output, fuzzy situation.

Regards,

Mark

[Tillydog]

Should I be reading ANY DC on pins 2 & 7?

If V3 is a LTP PI, then yes, you should: a few volts -ve of the cathodes (pin 3 + 8 ). Your meter will drag the voltage down a bit, so it's difficult to get an accurate reading. There does seem to be quite a big difference though: - 9V

If I run a cable from the loop send to a clean solid state power amp, ir sounds fine. I ran a cable from my Lab preamp out to the loop return and I get the same low power output, fuzzy situation.

Do you have a schematic showing how the loop is wired in?

As a shot in the dark: Make sure you have a coupling cap between the PI and the FX loop return (if that's where you've put it).

Andy

[Blindog]

I'm using this circuit, with some tweaks to component values. I've inserted the Send/Return jacks from the wiper of the Treble pot where it feeds the PI, before the coupling cap. Just a simple loop that I use my D-later external buffered loop with. It's worked great this way for about a year now. Can't figure this one out!

Mark

[Tillydog]

What is voltage is the B+ for the PI?

{Edit} - I figured it out - 320V ?

Did you do anything with the NFB when you fitted the PPIMV?

Are all the resistor values in the PI as per the schematic?

Andy

{Edit} Your PI seems to check out apart from the voltages on the grids (which I'm not sure you can measure accurately).

What voltage to ground do you get between the junction of the 470R / 10k / 1M / 1M resistors in the PI tail? (Guess: 33.5V)

What voltage do you get between the point where the 4 resistors join & pin 2 & 6 respectively? (Note, not the voltage to ground)? (*should* be 0.00)

What voltage at the top of the presence pot (junction of 10k & 47k NFB resistor? (Guess 11.5V)

/edit