Princeton AA964 clone low volume output and various other issues

[Les s of Paul]

Let me begin by saying i have scanned the fourms in an attempt to solve this issue. and if this is in the wrong place, please let me know, completely new here!

Anyways, onto the amp. I dont have a lot of the voltages right now but i can get them later.

Ok, so the main issue right now is fully cranked, with a Les Paul, and a Eminence legend 1218 speaker, i can play the amp at near bedroom levels, the bass response is nonexistent. the negative feedback loop cannot be connected unless you want an EXTREME ear piercing. it sort of putt putts like a motor boat, but the speed and intensity of the putt putt can be affected by the Tremolo controls. never mind when the amp's volume is off, it picks up local radio stations. Oh, and the tremolo foot switch cannot be turned off, otherwise the amp makes some really strange noises.

There seems to be a massive drop in signal strength between Pin2 and 7 of the 7025 (schematic reference) (from .7V signal strength to 5mv.) So i'm assuming the problem is there. i attempted to short the tone stack by running a jumper from the 250 pf cap across to the tab opposite ground on the volume pot, the first time, a noticeable increase in volume resulted, the second, well it shocked me, literally!

The amp has a Switchable mid pot mod added, and a Switchable negative feedback mod. (both pots switched off for testing) Hammond Deluxe 1760h OT used also. Standby switch added with 50k resistor in paralell, sort of a soft standby (i bought the part before i knew it wasn't really necessary) Bias pot added, 1 ohm bias measuring resistors soldered to ground, and 2 100 ohm resistors for a false heater center tap. Power transformer is a Mojo 760ex, tubes: JJ Gz34 rectifier, JJ 6v6's and JJ 12 ax7's.

i'm a complete newbie to this amp building, although i have a decent understanding of electronics. Probably a poor choice of a first amp to build, considering i bought all the parts seperately.

Thanks in advance for any help you can offer.

EDIT: Whoops, on second look this probably needs to be under "Debugging your build" Sorry about that.

further edit: Layout and schematic closely follow an original princeton layout, it should appear as a post down below shortly. the intensity pot can make it go away at both 0 and 10! and the thin sound is with bass and treble maxed (mid pot not engaged) the treble seems to cause large variations in the amp's overall volume, although that might just be a quirk of the amp

[nickb]

(1) Putt-Putt: try swapping the connection to the output transformer to change the feedback phase - either swap the plate wires or the the secondary wires. Sound like you've got positive instead of negative feedback.

(2) Thin sound and loss of signal- You didn't state what you had the controls set to. It makes a difference! Very carefully check the wiring of all the components around the 7025, especially the tone stack. Also check the values are correct - especially that 250pF - sounds like it might be too low.

(3) Tremelo noise - does it still happen with the intensity set to zero?

(4) Radio interference - might be related to (2) Lets fix the other stuff first. Did you follow a layout or do your own. You might want to post take pictures of your layout.

[Les s of Paul]

Removed double post.

[Les s of Paul]

Well, it would seem the power tubes were out of phase, swapped the OT wires at the plate and the putt putting is gone, as well as the negative feedback squeal, strangely though, and it could be completely unrelated. the output volume is now even lower, and touching the faceplate (which isnt connected to the main chassis except by a ground wire.) causes the signal to pop LOUDLY. i cant take any voltage readings or such at the moment, my DMM is currently unnacessable, as well as a way to discharge the filter caps. since it just got fired up, well i cant really touch it... oh well at least things are headed in the right direction.

The tremolo noise can be turned off at either 0 or 10, and the layout is based around a fender princeton aa964, some changes were made in tube and transformer location, but the circuit board remains 99% the same.

http://ampwares.com/schematics/princeton_aa964.pdf

[Les s of Paul]

Hmm, it would appear my previous post did'nt make it through.

Anyways, the amp is built using the aa964 layout, with a Very slightly changed circuitboard. (just added a couple turrets to make it a little neater) all tubes are at the front of the amp near the tone controls. i have some pics's just need to get out of the camera.... as for the tremolo noise, the intensity pot made it go away at 0 and 10, (Before swapping the OT plate wires. have not tested since)

http://ampwares.com/schematics/princ...erb_aa1164.pdf < Schematic and aproximate layout

[Les s of Paul]

Ugh. Edited my last post and added a link to the schematic, and poof... it vanished.

Anyways, i swapped the plate wires for the OT, the negative feedback squeal is gone and so is the putt putt(at least as far as i can tell) However, it would seem the output volume has dropped again! also tapping the front faceplate, (not connected to the main chassis except by ground wire) causes the amp to pop LOUDLY.

Looks like one step in the right direction. Thanks a bunch!

Oh, before i forget again, the layout is very close to the fender princeton AA964 layout, (cant post the link) its at ampwares under the silverface princeton.

I'll test things a bit further when i get my DMM back later, hopefully things dont go this far, but i should be able to get a-hold of an oscilloscope.

[nickb]

The volume drop is expected - that's the price you pay for the benefit of the feedback. On the noise when you touch the chassis, are you using a three wire cord with the safety ground connected to the chassis?

[Les s of Paul]

Yep, three prong cord. Oh and its worth noting, the volume drop is still there even with the feedback disconnected. maybe i bumped something else inside, but the volume output on the amp is SERIOUSLY lacking... (i did play a vintage silver face before assembling this amp, and it was a LOT louder than what i have now...) plus the radio signal is now 100% permanant. guess i better get those photos and check the pot grounds.

[Les s of Paul]

Maybe these photos will help, maybe they wont. i must admit, the buss on the front faceplate is terrible work, and needs to be redone soon.

So, one problem down, one? to go. progress has been made, i owe you one nickB, i Never would have guessed swapping the OT plate wires would fix that squeal.

[nickb]

It appears that this is constructed in three sections. Not using a single chassis without paying careful thought to wiring is giving you large loops and is the likely the root cause of the interference. Look at the loop caused by the negative feedback as an example; the black wire starts at the speaker socket goes to the feedback switch, then the PI and eventually back through the blue safety ground to the speaker socket. You should run a twisted pair from the speaker socket to the witch and continued to the PI and grounded it there. Also remove the connection from the safety ground to the speaker jack else you will have created a ground loop. The same principle applies to all other wiring.

If you're not really sure about this you might want to consider rebuilding into an aftermarket Princeton chassis and carefully following the layout. Once you have that going you can thoughtfully add your mods one at a time.

[Les s of Paul]

Rebuilding into a different cab isnt an option unfortunately, ground loops... i was afraid there might be those. ah well, looks like i got some research and a LOT of work to do. Thanks for all the help.

[Les s of Paul]

Finished moving the grounds, safety ground hasn't moved or the pot buss ground, all the others are now connected at the bias board lug. the radio stations and hum are reduced. so thats a plus. now if only this volume issue would go away! guitar plugged in with both the amp and guitar dimed, its quieter than my cellphone speakers.... might have to lift the tone stack ground to see if that helps the problem, might at least isolate the cause of the problem.

Edit: lifting the ground significantly increased volume, which is to be expected. but still by no means loud at all. might just have to rip it all apart and try again... (minus the circuit board) Tremolo is completely dysfunctional, with both the footswitch on and off. tubes are not biased hot at all, 20ma or so.

just got done looking at a vintage fender for its grounding scheme. looks like i have a couple small changes to make... should save my bacon though. Hopefully i'll have this thing up and running very soon. and i realized i forgot to ground the lugs on the can cap... although the plate has exposed metal holes, i wonder if thats part of my problem... guess theres only one way to find out!

[nickb]

You might try arranging your wiring something like this: .

If you're certain that it's wired correctly and that components are correct then you should take some DC voltage readings, especially around the 7025 and 12AX7.

[Les s of Paul]

Perfect, that's just the kind of info i needed in that layout! i'll try and get some voltage readings later if nothing works. although i do remember reading 320 volts on the 7025 at the 100k resistors before pins 1 and 6.

once this is all finished and in the cab, ill have to post some celebratory pics.

[nickb]

It's probably not necessary to wire all those pot bodies to safety ground as they have metal bushings which if used with a serrated washer should contact the chassis anyway.

The DC voltages are shown on the schematic for you to compare against.