Music Man EL34 Tube Phase Splitter Conversion Nightmare

[CJ]

here we go with a real nightmare,

hard to read schemo, weird circuit,

trying to change a solid state phase inverter to 12AX7 inverter,

first off, look at the weird screens, instead of all 4 screens tied to the same HV node via 470 1w resistors like Fender, we have one tube from each side of the P-P pwr amp tied thru the dropping resistor,

next up- ultra weird grounding scheme, the whole circuit floats off the chassis, even the speaker jacks are isolated with fish paper, but there is a small cap to ground, to bypass AC,

bad idea to mix transistors and tubes running at 750 plate voltage, (screen volts run about half of that so the tubes don;t pop)

see here>

 

[CJ]

we discovered the weird grounding scheme after a new tube socket was installed and all the tube grounds made to the chassis, 

what else can we cry about?  howabout two different heater schemes depending on which amp you have, the solid state inverter or the tube inverter (two different models so at least we have a schematic to work with while converting to 12AX7,)

one side of the heaters connected to chassis or ground wtf, over?

also, we have two different pwr transformers depending on which chassis we have,

and check the bottom schemo, the hi lo switch changes the pri tap, so the HV goes up or down, but what about the heater circuit? that has to move around too, unless we are missing something,

 

[CJ]

what else can they make weird?  howabout the input jack grounds,

two for the price of one, 1 chassis, 1 audio gnd,

must have been some ham radio guy who designed this, >

 

[CJ]

anything else weird?

of course!  check the supressor grids on the pwr tubes, they go to the chassis, not the audio gnd like the cathodes,

then we have a cap from the opamp board gnd to chassis gnd,

so the whole circuit floats except the input jacks and the supressor grids,

check the output transformer protection diodes, they go to the chassis,

added bonus, two 50 pf screens caps, never seen that before either,

 

[CJ]

so what started out as a simple conversion turned into a full on rework and schemo analysis,

instead of lifting all the tube grounds (beefy soldered grounds to chassis) we just left them and tried the amp anyway, it works, but there is hum when the master vol is turned down all the way, and some osc when it is cranked past 4,

so we tried grounding the speaker jacks and the hum went away, the amp sounds great but we still have to fix the osc. problem, use the scope and isolate-then add cap somewhere,

note that master vol needs to be changed from 10K to 1 meg, if going from solid state to tube inverter,

so next time we will just fix the stock solid state circuit even though it is not as dependable,

we also had to add two caps to the cap pan and a choke coil, so 3 day project instead of 1/2 day,  ;D

here is the schemo that uses the stock tube inverter, notice that the supressors and protection diodes go to audio gnd in this version>

 

[CJ]

final thing to watch out for if doing this conversion, the power transformers are different, the bias supply will be a problem, luckily we are running cathode bias (self bias) so we do not need the neg supply voltage, just a pair of 65 ohm 25 watt resistors in series, for a bias of about 25 volts,

so the solid state circuit has a lower voltage winding for the preamp board, the tube version has the minus 46, then knocks down to +/- 16 with resistors and zeners, in fact, there is no bias supply for the solid version, looks like a current source type deal, so if you do the solid to tube conversion and want fixed bias, you will also need to come up with -46 volts somehow,  ???

also weird, the bridge circuit shown for the low voltage supply has two diodes on the pwr board,  and two on the preamp board, never seen a bridge circuit split up like this, so as you look at the pwr board, you think you have the same half wave circuit as the other version, but no! the diodes circled in red are on the pwr board, green = preamp board,

ok rant off, we are all boned up on the pitfalls of the Music Man conversion,

here are the two pwr supplies>

 

[CJ]

one last thing, there is a tube socket cutout that is covered with a plate and has a heater terminal board bolted to it, we did not see this in time and thus punched a needless hole in the chassis,
if you use the socket cutout, you will need to move the heater terminals board,

 

[Gus]

Why change it?
I like the output stage You still have the tube, transformer and speaker interactions
Cascode, NPN with tubes
so you can use a more "perfect" IC based PI  circuit to the NPN bases
A friend has told me that the different IC part numbers can make a difference in the amps sound.

One thing on the list of things to do is to adjust change some parts(EQ, gain) in a MM amp leaving the stock circuits as an experiment.

 

[CJ]

funny deal, right after we convert to a tube splitter, another MM comes in with a tube splitter,

cap can before and after, caps and choke needed for 12AT7 power supply,

note tube socket covered and where we put the new one,

 

[CJ]

shot of where the splitter board goes,

 

[CJ]

close up

see the left side of that 100K?  it is just touching the solder grommet, so when the amp was played, it vibrated away, causing a drop in power, been like that all these years,
found it by accident while taking voltages, good for a free pizza,  ;D
would look alright to the QC guy, eh?
so we stuck in a 100K 1 watt and soldered it good,

 

[CJ]

ok, camera cleaned out now, 

HV non oscillator circuits on the pwr tubes,

half of that stuff you could rip out, look at the old Fender amps, non of that stuff,