Loud hissing from new build/conversion.

[Stratz]

Hi all. I just finished a 2x 6L6/2x 12AX7 PA to guitar amp conversion (No schematic as I dont have any info on the PA make/ model) and while everything works great I get a loud hiss with no cord plugged in and with the volume on zero. It get louder as I turn up the volume. Sounds like a dimed JCM800 but with the PA amp's volume on zero.
There is another issue which may be related. The guitar's volume control must be all of the way up or I get hardly any volume.

It's set up with a AA964 Blackface Princeton/Blackface Champ tone stack (Vol, Treble and Bass) and the rest is wired just like the AA864 Bassman. Long tail pair PI and adjustable bias.

All resistors are new carbon film. Filament wires have an artificial center tap. I have a 1 meg input resistor wired to the jack and a 33k grid stopper which is 1/2 of 68k as there's only one input/one channel.

I'm hoping someone here has run across this issue before. I've checked and triple checked my work many times and can see nothing out of line.
I've attached both the Princeton and Bassman schematics that I used.
Any help is greatly appreciated.
Thanks
Rob.

Princeton AA964 Schematic:


Bassman AA864 Schematic:

[xtian]

Please say how hiss changes when you remove V1, and then V2.

[Stratz]

Xtian, My apology's for the late response. I had posted my question the night before I went away for a few days.
I just fired up the amp. I pulled V1 (12AX7) and it gets a lot quieter, still a bit of hiss/white noise. If I put V1 back in and pull either V2 (12AX7) or V3 (12AT7 PI) it goes dead quiet.

It hisses loudly even with the volume all the way down and with no cord plugged in.

I checked my V1/V2 voltages and they're good according to Fender specs. ~180-200V on the plates and 1.6- 1.8v or so on the cathodes.
Thank you for your reply.
Rob.

[Stratz]

Forgot one thing. The guitar's volume control must be all of the way up or I get much, much less volume than I should and a very thin tone.
Thanks.

[eschertron]

Something around stage 2 (2nd half of V1) is not right. The tube is amplifying something, but not the guitar signal. I know you checked and rechecked, but a wire, component value, or solder connection is to blame.

Shunt to ground the grid of stage 2. Still noisy? Follow onward. Once you identify the area where the hiss (or the worst of the hiss) is coming from, go over that section again. check for connections that may be 'weak', not making good continuity between associated components. Make sure your supplier didn't send you 220M resistors when you asked for 220k. Been known to happen.

You made it a single-channel amp? Be sure to omit the mixing resistor. That's noise you don't need to add to the signal.

[Stratz]

Something around stage 2 (2nd half of V1) is not right. The tube is amplifying something, but not the guitar signal. I know you checked and rechecked, but a wire, component value, or solder connection is to blame.

Shunt to ground the grid of stage 2. Still noisy? Follow onward. Once you identify the area where the hiss (or the worst of the hiss) is coming from, go over that section again. check for connections that may be 'weak', not making good continuity between associated components. Make sure your supplier didn't send you 220M resistors when you asked for 220k. Been known to happen.

You made it a single-channel amp? Be sure to omit the mixing resistor. That's noise you don't need to add to the signal.

Thanks for the reply.
Sorry but I dont know which is the mixing resistor...Could you help me out please?

I just now noticed that the 100k and 82k resistors on the 12AY7 PI are not connected together (the Bassman schematic is in my original post) so I'm wondering if this is causing some other issue that I dont know about yet.

As soon as I find out where the mixing resistor is I'll check to see if I used one or not.
Many thanks for the reply's.

[Justin Thomas]

The mixing resistors on the Bassman are the 220ks coming off the plates, going into the PI. Should be after the 3rd stage on the Bass Channel and the 2nd stage on the Normal channel. Question - did you use negative feedback? Because switched plate leads doesn't ALWAYS lead to runaway howling & sfreeching... Also, how did you build ot - point-to-poijt, eyelets/turret board? Since it drops a lot when you pull V1 yet there is still a lot of hiss, you may have more than one source. But as was said - it's amplifying SOMEthing. Hiss is usually a sign of too much gain.

Justin

[Stratz]

Is the mixing resistor a 220k R that goes from the PI plate (pin 6) to the Normal channel 7025 plate (pin 6) and also to the 500PF cap into the grid of the Phase Inverter?

[Stratz]

Whoops! I was posting as you were answering my question.
It's point to point. The long tail pair is a task to build when there's no board to work with so I used a 3 lug terminal strip.

I have the 6.8k NFB resistor in place. Should I simply remove it or should I swap the plate feeds from the OT or both???

Thanks Justin.

[Stratz]

By the way, I should have mentioned that I built the bass channel only but with the Princeton AA964 tone stack.

[Justin Thomas]

The Princeton uses a 2.2K NFB resistor, the Bassman an 820R. Only 6.8k I saw was the Mid resistor in the Bassman's normal channel. Is the 6.8k something you picked? Anyway, if your NFB is wired correctly, and you lift one end of the resistor, you'll have zero NFB, and noise and volume should go UP, along with some nicencrunchy breakup earlier on the volume knob. If you switch the plate leads, and the noise and volume go DOWN, then you had them reversed and were getting Positive Feedback. But if the noise and volume go UP when switched, you had them right.

Hoping you don't take any of this as "talking down to you," because I don't mean it to be that way. Just some general help I've gotten in my own PTP PA & organ conversions...

PTP can be incredibly fun, incredibly challenging, and extremely frustrating. Four builds, only one circuit board, because it was the only one where the chassis allowed it. I'm willing to bet that you're picking up something somewhere very early on, and it's getting amplified. There are a bunch of remedies, but you have to be slow, methodical, and patient when you don't have a proven layout to work from. At the same time, I think that inconsistency in builds helps give my amps a special edge, an uncertainty that sets them apart from others. Kinda like a Trainwreck. Yeah, I'll go there...

Quintuple-check your wiring, and maybe pick up or download some copies of the ARRL handbook or others like it that are geared toward completely home-made EVERYTHING, and glean from them for your wiring guidance. They're heavy on principle but they know most home builders don't work from a road map. And a fraction of an inch can be the difference between a whiny screeching mosquito-tone and a rampaging Godzilla-spawn toxic-waste spewing mini-behemoth of sonic destruction. Even though you have the two schems that you were inspired by, an actual schem of YOUR amp would be good to have, even if for your own reference. I've caught myself making wiring errors in the process of tracing my builds in order to make a hand-drawn schem. And some pictures might help point out some potential construction pitfalls, too. Stick with it - PTP is friggin' awesome.

Justin

I should add: my third build, I had to break almost every grounding "rule" in the book, because of a set transformer layout, and a PT that threw a WICKED hum no matter what official "remedy" I tried. Eventually I had to make what should have been a 3" wore about a foot long and route it in a most convoluted fashion around the edge of the chassis. Again - PTP = WTH?

[Stratz]

No offense taken at all. I believe it was all great advice. I built a few amp kits this spring because I wanted to know what makes them tick. The first amp kit I built was a Mojo Deluxe Reverb AB763 kit, then a 5E3 Deluxe and then an 18 watt Marshall TMB with the JCM 800 master volume but I really dint learn much working with kits. It was like paint by numbers.
I've done about 6 PA/Hi-Fi conversions and a few came out great but I'm still trying to work the bugs out of a few including this one but I've learned 100X more modding the PA's and Hi-Fi's than I did building the kit amps. I agree. They are a blast to convert. Especially when you wind up with a great sounding amp for free or $50 max..

One very important thing that I forgot to mention is that EVERY tube I put in the V1 slot comes up micro-phonic. The tubes are fine when moved to another slot or to another amp. I believe that's a side effect of my initial error/problem.

I'll post a gut shot in a bit. It "was" nice and pretty looking before the trouble shooting started then neatness went out the door.
Thanks Justin.

[Stratz]

Here are 2 pics. I realize it will be very tough to tell whats going on with the point to point mess.
The first is the full gut shot and the second is V1, V2, and the PI.
Thanks.

[Stratz]

Anyway, if your NFB is wired correctly, and you lift one end of the resistor, you'll have zero NFB, and noise and volume should go UP, along with some nice crunchy breakup earlier on the volume knob. If you switch the plate leads, and the noise and volume go DOWN, then you had them reversed and were getting Positive Feedback. But if the noise and volume go UP when switched, you had them right.

Justin. I removed the NFB resistor (it was a 680 ohm ) and the hiss went AWAY. This is a good start.
I put my decade box in place of the NFB resistor and if I dial anything over 4 or 5 ohms the hiss comes back.

Do I have my plate wires hooked up backwards? It's a bit difficult for me to understand your quoted post.

They're wired the way they were when I got the amp but I did a lot of work to it including the long tail pair PI and a negative voltage setup with a pot for grid bias.
Thanks
Rob.

[Justin Thomas]

Progress! Since the hiss stopped with the resistor lifted, try reversing the plate leads. Switching the plate leads is the easiest way to reverse the phase of the fed-back signal.

Basically, applying negative feedback in reasonable amounts in a correctly wired amp will cancel out any peaks and transients, and make for cleaner operation. Lifting the loop allows for all of the signal to pass through, no corrections or transient suppression => crunch, noise, more fun. So with no feedback, everything is amplified more - noise, too. If your plate leads are switched, any intended negative feedback becomes POSITIVE feedback, meaning all of your signal and noise are amplified even further => hiss, or worse - squealing, etc. You may still have a hiss source earlier in the amp, but fixing this problem should make it easier to find, if at all.

Justin