Schematic wanted

However, 75 *8* 1, as Meat has now corrected this to, and 6L6, same pinouts ...

Arfa

Yes I read that too, along with his incorrect statement that 7591 and 7981 pinouts are the same. They are not. I am correcting him here as he was accusing me of being in error. I am not.

I think this should now be the end of it.

Don't bother, Gareth is a pedantic pisswit playing some kind of idiotic game.

Oh and I did find out what was causing the B+ fuse to blow. Seems I have a dodgy SJ-6465 tube driver transistor. Wouldn't blow the fuse on low power since the B+ was only 262VDC. But as soon as I switched to high power it drove two tubes into max conduction. The transistor wasn't shorted but very leaky. I'm going to replace both drivers (each responsible for two tubes) with a couple MJ15024 I had in the parts bin. They are about double the watts/current/volts. I used them all the time for the old Soundcraftsmen amps I used to repair as outputs.

One odd thing about this amp is it doesn't seem to be class I recognize (not that it ever hampered repairs not identifying them).

Pin 4 or each tube pair driven by the SJ6465 connects to the primary of the audio OT, one tube in the pair uses a 100 ohm 5 watt resistor, the other does not. Pin 5 of all tubes fed 15VDC through a 100 ohm resistor then a 1.5K ohm resistor on pin 5 of each tube. Pins 1,8 are direct coupled to the collectors of the two SJ-6465 drivers. Does this ring any bells?

That sounds an odd configuration. Fixed positive bias on the control grids (pin 5) and driven cathodes (pin 8) ? Screen grids to (taps on ?? ) the output tranny ? Presumably, the anodes (pin 3) also go to the tranny ? There is a configuration called 'ultra linear' where the screen grids are fed from taps some distance down each limb of the OPT primary.

Some variation of that, maybe ?

Arfa

Yeh somewhat similar. Here have a look for yourself.

I've owned Peavey amps for 30 years. First was an Artist 115 and I still own it albeit I've recapped it and fixed a couple problems caused by age. It has a more traditional final stage similar to Fender

100 watt finals. Second Peavey I owned was a 130 watt SS 1x12 which looks identical to this MX. I used to play through both direct in bypassing the tone stack using a Peavey Tube Fex. Blew the Scorpion 12 in the SS amp and bought a replacement on Ebay. The Black Widow12 in the MX is indestructible however it is not the most versatile speaker sound-wise. I have a Peavey 4x12 cabinet loaded with 4 JBL's that has a better response range. Going to be working on my Laney AOR Pro Tube 50 today. I've fitted it with a new Fender audio tranny I had ordered as a spare. Wont have the impedence taps as the original did but it will do the job at 4ohms.

Wheeee ! That really *is* an odd configuration, and one that I don't recall ever having seen before ... With the screen grids tapped down the tranny below the anodes, I guess that it has got to be a variant of the basic 'Ultra Linear' scheme, but why do two of the tubes have a bloody great power resistor in series with the screen, and two not ? Cathodes are commoned, as are the control grids, so you would expect the operating conditions of all of the tubes to be the same, except that they won't be (quite) with those resistors there. I suppose that the actual physical circuit really is like that ? Not just that the schematic has been drawn wrongly, and the resistors should be shown north and south of the screen grid junction points, so each one feeding both screens of one tube pair ?

The drive scheme is also odd. With +15v on the grids, the cathodes must be up at what ? +50v maybe ? Can't remember what the typical bias level for a

6L6 in AB is, but around -35v I would guess ? So why use a low Z cathode drive, I wonder ? Seems a complicated way of going about what is a fundamentally a straightforward task, when tackled textbook, like in a Vox or Fender or Marshall, or just about any other two or four tube tetrode / pentode output stage. I can quite see how a failed or leaky transistor such as you had, would lead to a massive shift in the bias in totally the wrong (well, undesirable, anyway) direction, driving a tube pair to full saturation. That alone makes it a questionable design to me, but I'm sure we must be missing some clever bit of design philosophy here.

Anyone else got any thoughts or insights into the reasoning behind this odd design ?

Arfa

First of all the Laney AOR 50 absolutely screams with the Fender iron at the output. Two Sovtek EL34's at 400vdc plate and biased around

-39vdc. I used my Fender Twin as a speaker cabinet since it is back in my work area and has two Weber Californian vintage style 12s. I swear the Laney is just as loud as the 100 Twin amp.

Back to the Peavey MX VTX. Yes the schematic is correct. I didn't take any voltage measurements since there were very few on the print. But I did verify the plate and the 15vdc and whatever else was in the drive stage per the print.

The philosophy is to lightly fuse the B+ :) I suppose the design has something to do with the fact that the low power setting drops just the plates to 262vdc so anything besides linear or cathode biasing wouldn't track the drop from 525 to 262 unlike placing a variac at the mains where everything drops.

Yep I'd like to be educated on this desing too. Don't like repairing things that I haven't a clue as to the operation.

I can see how they might be using that sort of drive arrangement to ensure that the bias remains correctly tracked for the reduction in plate voltage, but it seems a complicated way to go about it. I'm sure that you could have a negative grid bias arrangement which tracked the B+, implemented in a simpler way than this.

Arfa