Early 70s 50w Rebuild Issues

[PeterJ_22]

Hi Guys, I've had an early 70s 50w 1987 for years that when I purchased was in a bad state of repair so it has been rebuilt many times as a test amp for mods. Anyways I decided to put it back to stock spec last night.

However I am having some strange issues (this amp always used to have the same weird issues when the ppimv was installed and I put it past 3/4).

Basically If it try turn the amp on without an attenuator plugged in, regardless of volume setting, it just squeals at me, kinda like oscillation but the lead dress looks fine to my eyes.

When using an attenuator it stops making the noise, however if I let notes ring out there's this strange farty sort of noise that comes and goes as the note rings out. Also the amp is distorting at any volume, aka NO clean head room, yet still heaps of volume.

I have checked the voltages and most seem fine, the PT has a VERY LOW B+ voltage of around 360-370 however this has always been the case with this amp, the first preamp seems to be running a bit hot with 220 on pin 1.

The Power tubes are biased to .045 and .041 (due to lower b+)

Voltage chart:

TUBE PIN 1 PIN 2 PIN 3 PIN 4 PIN 5 PIN 6 PIN 7 PIN8 PIN 9
1(12AX7) 220 0 0.6 3.1 3.1 240 0 1.6 3.1
2(12AX7) 192 0 1 3.1 3.1 315 191 191 3.1
3(12AX7) 239 26 44 3.1 3.1 217 28 42 3.1
4 (EL34) GND 3.1 363 358 - 31 363 3.1 GND -
5 (EL34) GND 3.1 365 358 - 31 365 3.1 GND -


Any thoughts would be great as I'm scratching my head at this stage

[PeterJ_22]

I have uploaded some photos and the proper voltage chart as for some reason it doesn't like me typing it out

http://s1289.photobucket.com/user/Peter ... t=3&page=1

[stef]

I cannot see the PPIMV.
Did you use no name black silver mica caps?

[PeterJ_22]

PPIMV has been removed, using silver mica caps from valvestorm so whatever brand they are.

[PeterJ_22]

Had another good look over and checked all resistor values, nothing is jumping out at me. Solder joints all look good and nothing is touching where it shouldn't as far as I can see.

Hmm very strange.

[stef]

Preamp tubes, maybe power amp tubes too, caps (especially mica), OT, speaker and instrument cables, speakers...

[neikeel]

Afraid the lead dress is pretty off the mark! The preamp tube wires are far too long and loopy, even one looped round a mounting nut ? Asking for cross talk and PO, shorten them ASAP!

Wonder if you might have transposed the grid wires for the output tubes (normally the green and orange wires from the 220k resistors, although your wiring colours are a bit more freestyle ) that can have the same result as getting the OT primaries reversed.

Shorten your preamp tube wires properly first and post back

http://forum.metroamp.com/viewtopic.php?f=7&t=43180

[PeterJ_22]

Hi guys thanks for the responses. I have just shortened every wire going to each preamp tube as much as I can, also the grid wires from the 220k resistors are wired up correctly (double checked). Just about to fire up the amp and see if its made much of a difference, will post results later!

[PeterJ_22]

Ok so lead dress didn't seem to make a difference, tried full new set of preamp and power tubes, also no difference.

plenty of volume, clicking attenuator back from -16 to -8 to -4 increases volume as expected but once I go to -0 or no attenuation the amp kills the guitar signal and makes a high pitched squealing sound.

Still total lack of clean headroom, the gain increases as the volume dial is turned but even on 1 there's quite a bit of extra distortion, and notes ring out unevenly with a pulsing sort of sound, low frequency humming / grinding / nasty overtones (hard to describe)

Hmm any other thoughts?

[neikeel]

plenty of volume, clicking attenuator back from -16 to -8 to -4 increases volume as expected but once I go to -0 or no attenuation the amp kills the guitar signal and makes a high pitched squealing sound.

Absolutely classical parascitic oscillation. There is cross talk when you turn up the gain (ie the volume on a NMV amp) you are getting interference. We should also try to see why you are getting more gain (nasty grind and no headroom) than normal. Odd that you are getting a high plate voltage on V1 when the rest of the voltages are lowish. Are the first (preamp) filter can wires arranged correctly under the board - is the ground wire secure?
Does it do the PO on both hi and lo inputs of both channels equally.

Hmm any other thoughts?

Double check your component values, your pics are a little grainy on magnification but I cannot see any particular errors.
There are a couple of other house keeping points. Suggest you move that yellow screens feed wire that is going over the top of your OT secondaries and lay it on the chassis floor, under the output tube grid wires. Suggest you swap your fuses holders round (or get another short one so that you do not have to use tape around the HT wires where it is so close to the board)

Have you internally jumpered your inputs (that green 'grid' wire between the input jacks - another potential antenna and if wired incorrectly will cascade your preamp channels).

Lots to explore and subtle tidy ups to do.

There are tricks to suppress PO in a high gain amp but a 1987 is not really that!

[stef]

getting the OT primaries reversed.

[PeterJ_22]

Just ordered a whole bunch of parts from valvestorm including a small fuse holder for that very issue
Component values all check out .

The oscillation happens when no attenuator is used even when the volume pots are on zero. However when the attenuator is used I can have the pots on 10 and the attenuator on -4db and it doesn't oscillate.

Oscillation happens on both inputs and they are not internally linked.

Preamp filter can is a 16x16uf F&T with its own dedicated ground under the board. I might raise the value slightly to 32 but again shouldn't make huge issue.

I'll move around the screens wires a bit later today and see if that makes a difference,

The OT primary and secondary wires are bundled together if that makes a difference?

[neikeel]

The oscillation happens when no attenuator is used even when the volume pots are on zero. However when the attenuator is used I can have the pots on 10 and the attenuator on -4db and it doesn't oscillate.

Oscillation happens on both inputs and they are not internally linked.

Preamp filter can is a 16x16uf F&T with its own dedicated ground under the board. I might raise the value slightly to 32 but again shouldn't make huge issue.

I'll move around the screens wires a bit later today and see if that makes a difference,

The OT primary and secondary wires are bundled together if that makes a difference?

Best if they are bundled separately, ie the pin 3 wires twisted together and laid on the chassis floor, you can do that same time as the screens wire.

Presume you have tested it inside the head cab with its aluminium shield? (the presence wire should be chopsticked too).

If these things dont work then 4k7 or 5k6 swamp resistors on the grids (pin 5) of the output valves may help - last resort tho'

[PeterJ_22]

They are twisted individually but both bundles are then cable tied together,
Hard amp as half of it is original early 70s and half new parts - seems like a lot more messing around than a fresh build.

I'll do what you suggested when I'm home later and see if that helps

[PeterJ_22]

Ok so:

re-did the input jacks and shortened the wires from the jacks to the 68k resistors.

Re did the grid wires

Separated the primary and secondary wires from the OT

5k6 swamp resistors on pin 5 of power tube sockets

Installed smaller HT fuse holder

Upped the preamp filtering to 32x32 and double checked the wiring.

(this is on top of the complete preamp tube lead dress overhaul from yesterday)

2 hours work later and no difference.

Starting to bug me now haha.