My Amp Tech

Bummer,

The mods should work. The mods can be undone. Depending how they were implemented. It might not be worth it to undo them. Like some of the wires and things
being covered in epoxy, would be expensive to dig out.

You tech doesn't sound like he knows or has the desire to to it, is what is going on. Four months for tubes and a bias if the amp can be (haven't checked the schematic), but still.

Time for another tech and get a second opinion.

If there are new holes cut/drilled in the chassis, it will
look ****** with the switches and pots removed.
And just keeping them in but with nothing attached
or removed is kinda, heck, you can fool them at jams!

Take some close ups of the front and back.
Pull the chassis and take some pics of the circuit.

There are plenty of folks here who can get you
in the ball park, worth it etc.

Seeing it is step 2.

 

Very interesting... the second one of these I had, I extensively modded - I also fitted a separate volume for the left-hand channel (instead of the reverb knob), moved the reverb knob round the back and made the reverb common to both channels, added a presence control, half power switch and fitted Celestion G12-65 speakers. At least I think that's what I did, I might be wrong on a couple of those things, it's a long time ago! The idea was to use it as a channel-switching amp with a right-hand channel for clean and the left-hand one for dirt.

Given the severity of the mods, I actually agree with your tech - putting it back would be a nightmare and probably impossible, certainly to anything close to original condition. You'd be better (and maybe even cheaper) just to buy another one.

On mine I altered the circuit board as well as drilling a couple of extra holes in the back, to the point where it could never be completely put back (not to the sort of standard I'd like, anyway), and it was nowhere near as heavily modded as yours. I kind of wish I hadn't done it now - I certainly wouldn't even consider doing anything like that now - but it was fifteen years ago, and those amps were pretty worthless (still are, sadly). It was the second one I had and it just didn't sound as good as the first, so I thought what the hell... try and make it into something that it was never designed to be! Actually this is one of the mods that convinced me that this sort of thing was a bad idea.


I do think your man should have done more in four months than retube and bias it - although did you tell him there was 'no hurry' or anything like that? I have a few repairs like that lying around, they just don't get the priority when other people having working amps that they need back in a matter of days.

 

That is what I thought. I assumed who ever has worked
on these would know. AND on all things british it just
doesn't get much better then John Phillips.

Shameless plug John....please don't hold it against me,
as I'll have a question for you shortly.

Just about 98 percent what John says I agree with,
the other two percent...I'm still waiting to send John
money for the Marshall Screen Resistors next time he
heads down to his supplier.

Oh the pain of non communication.

 

I was unable to get the pics downloaded last night but while I have the smart amp people looking at this thread, let me list the mods:

Line out with a volume control (haven't tried to use it yet)

1/2-full power switch that blows tubes unless it's in the down position (and I don't know if down is full or half power) ....... that switch changes the DC feeds to the screens from the plates OR from the B+ downstream of the choke

A switch that reads pentode/triode??? ....... see the ax84 november

then there is the channel switching input (me likes that)

a switch that reads 100/50 watts (set to 100) ..... that must lift the outer tubes out of the circuit

the presence control (nice)

Oh and there is also a sharpie written note under the ohms selector that reads 4 ohms two tubes and 8 ohms four tubes??? ........... when you run an amp from 4 to 2 tubes you have to double the OT impedance

If any of you can shed some light on what any of that means, I would appreciate it. I'm really starting to get into this amp. It just loves pedals and has so many different dimensions.

Thanks

 

The impedance is marked correctly in the sense that when you use two tubes you have to set the amp to half the impedance of the speakers. But unless I'm wrong, the stock speakers are a 4-ohm pair anyway aren't they? (Two 8-ohm speakers in parallel.) Since there's no 2-ohm setting on the amp, you can't do that.

The triode/pentode switch is the one that swaps the screen feeds between the plate connections and the output of the choke. If I remember rightly, when I did that to mine it had a tendency to blow tubes too (in triode connection, with them connected to the plates)... I think the voltages may be too high for triode operation, but I didn't know that at the time. I think I changed it so it was a 1/4 power switch (like the Fender Red-Knob Twin), which involves re-routing the OT B+ feed to the middle of the filter cap stack and reducing the bias voltage too...

This really takes me back! Shame my memory isn't quite what it used to be... it was 1989 or 90 I think. I hadn't been working on amps all that long, and it was a bit (or a lot) of a learning process at the same time... way before the Net or anything like that. I'm certain that this isn't the same amp that somehow got over there and got further modded though, since I did definitely change out the speakers, and I'm pretty sure I put the reverb control on the back panel at the right-hand end (looking from the back).

 

Did you try to set it to 50W and half power to see if that .. ehh keeps tubes alive?

 

plug some headphones in the line out. I think that should work ... see if the signal is noisy.

 

Without seeing inside, it's almost impossible to know for sure what the switches do. Even from a pic it could be difficult to trace wiring and follow what has been done.

There is more than one way of reducing the power - you can turn two tubes off, operate the tubes in triode mode, or reduce the B+ voltage (typically to half, which actually gives one-quarter of the power, but it could be wrongly labeled). There's no reason you can't use more than one method at the same time.

FWIW, despite what your tech says, I don't think those mods have been done to the highest standards and IMO should either be removed or redone better. Using a mini-switch for triode/pentode selection is NOT a good idea - certainly don't operate it unless the amp is on standby. That type of switch is not rated high enough for safe operation at those voltages. I also don't like the haphazard hole positioning and labeling - although that isn't a problem as such, it does show less than 'perfectionist' workmanship, which is often a sign of other problems inside...

Do you have any pics of the inside?

 

Ed Roman

 

Unfortunatly I know Roy and this is typical response time for him due to his band's schedule.

I'd suggest you take it me at www.thermalrelief.com as we do quite a bit of the guitar amp repairs for all the Las Vegas stores. If fact when Roy's stuck he sends them to me also.

Give me a call and I'm sure I can straighten it out for you in a reasonable amout of time.

-Tony