Heavily modded Marshall JMP - PT heater rating

[Rockwell666]

Hi guys, A bud brought me a 74' superlead JMP and he wants an all tube effects loop and an extra gain stage added. not to mention some relays to bypass loop, dual masters and a mute footswitch.
Don't worry the amp is in rough shape with holes drilled everywhere from the previous owner. it had been returned to stock, albeit poorly.
So I installed everything, ordered a Hoffman relay power supply (5v regulator from heaters) and 3 relays boards, 5 pin din footswitch and rewired the preamp for a freidman HBE style gain.

I am experiencing two issues that I want to address before installing tubes and testing.
1)- with no tubes installed, amp on but in standby, I am getting only 6.0v of heater voltage. once those tubes are installed (4xel34 5x12ax7), i am certain it will drop too low.
2)- relay power supply - 4.91v on output. im happy with that. the relays appear to be working (leds on/off with footswitch) i haven't had time to test to see if the contacts are moving BUT on 2 of the 3 relays when i measure across the diode and press the footswitch the voltage sways and returns as normal. The first relay that sees the 5v supply, the voltage drops right off when i engage the footswitch but the others down the line appear fine. I looked for a bad common connection and couldn't find it. I haven't dug much deeper but i cant help but feel that all this stuff is working the PT heater winding too hard and potentially the one relay coil is fried eating up the voltage. (i will double check footswitch connections)

Does anyone know the heater winding rating for those old marshalls? it is a laydown style PT with no voltage selector switch.

I am thinking i will disconnect the relay power supply and see if that helps bring the voltage back up. if so a sep 6.3v trans will be required. mind you the 3 relays shouldn't be drawing much more then 550ma (+2x300ma for 12ax7's). so that's an extra 1.2A over stock. i don't see 0.5A being a problem but 1.2A is enough to cause sag.

suggestions? experience?

[Stevem]

You lost me with this, but in a 100 watt super lead the 3 preamp tubes and 4 output tubes need 6.900 amps for the filaments and if your looking to tack another 1.5 amps on to the original PT that is not gonna happen, on the other hand if your 3 added relays are pulling down that 6.900 amp heater feed without any tubes in then your PT is bad.

[Rockwell666]

That's what I was thinking!! I was like how the hell is the heater voltage so low with NO TUBES, just the relay power supply. I will double check (maybe my DMM batteries are tired).

[Stevem]

What does the meter read when probing a wall outlet?

[tictac]

a schematic would be helpful to see if your transformer and DC PSU connections are correct...

TT

[Rockwell666]

Wall Voltage measured today @ 119.2VAC with all the tubes installed I am seeing 5.9vac on the heaters. I will get a schematic drawn up this weekend. Its quite the doozy. all AC heaters. You can hear the laminations in the power trans vibe alittle upon flipping the power switch. ive seen this in ALOT of vintage units.
My bud wants to run 6L6's for a more modern sound so if its drawing too much current that would help....will it sound as good...prob not haha

[Rockwell666]

as im drawing out the relay supply and footswitch at work from memory I realize I might have some of these wires wired wrong. I bought a commercial 3 button footswitch and changed the LEDS. see attached. I will have to recheck my wiring.

[Rockwell666]

You lost me with this, but in a 100 watt super lead the 3 preamp tubes and 4 output tubes need 6.900 amps for the filaments and if your looking to tack another 1.5 amps on to the original PT that is not gonna happen, on the other hand if your 3 added relays are pulling down that 6.900 amp heater feed without any tubes in then your PT is bad.

UPDATE: the footswitch wiring is correct. I double checked it.
With everything installed I get 5.6v on the heaters. I removed the relay power supply from the heater circuit...5.7v...removed one tube at a time until there was nothing drawing from the heater supply....6.0v unloaded... you are right Stevem the PT must have a small short in the heater winding.

[bal704]

That voltage seems low. My heaters usually run between 6.5V and 7.0V unloaded.

[Rockwell666]

4x1.5A for el34's = 6A
5x300ma for 12ax7's = 1.5A
7.5A total + 3 relays..... all the laydown Power transformers I see are rated at 7A for the heater winding. Heyboers MPT100 is rated for 7.5A but its an upright. you think that would be ok? I emailed them asking if they could do an MPT100 but laydown.
Anybody have any other suggestions?

[pdf64]

...the PT must have a small short in the heater winding.

That doesn't seem feasible; a shorted turn should draw fault current, causing the mains fuse to blow.

[Rockwell666]

...the PT must have a small short in the heater winding.

That doesn't seem feasible; a shorted turn should draw fault current, causing the mains fuse to blow.

I cannot figure out why the heater voltage is so low..is the Pt just getting weak?. the PT does have another Primary Tab that looks to be unused maybe 120 vs 110v. the Caps were redone in 1998. The Dagnall tran is marked T3490.

The heaters voltage is a reflection of the wall voltage and im assuming the gauge of wiring used.... regardless the amp seems to function fine I am just alittle concerned for the life of the EL34 tubes.

[pompeiisneaks]

That's definitely a possibility, if you've got it on the 110 winding, and you're at 120, that may be why you're getting lower voltages. I'm trying to do that math in my head. If I do the winding ration of 120 to 6.3 I get 19.05 and if you apply that to 110 you get 5.77, but not sure if that is 'right' either heh, It seems logically to me that if you put 120v into a 110v you'd RAISE voltages, not lower them, otoh if you're in the 120v and your house tends to be closer to 110 it 'would' lower the other side voltage.

~Phil

[Rockwell666]

i have never seen a vintage amp with low voltages that wasn't fixed by swapping primary taps...it has me scratching my head. i read somewhere on the internets (ha) that some early 70's JMPs had lower then normal voltages...
5.7v would be bang on for what im seeing. its not like the trans is being overloaded, in standby the heaters are low, its not like its running near max volume and then sagging....

[Rockwell666]

I tried the other winding, its a not go. PT must be tired. time to find a suitable replacement.