Need to install bias adjustment

[janalex]

Hey guys,

Which pot would you use for a bias adjuster on a vintage plexi? Can you still find the straight pots out there where you physically move the wiper on the trace?

 

[janalex]

I see the piher trimmers at ValveStorm. Would horizontal or vertical be easier to install? Or is there a better option?

 

[neikeel]

 

[janalex]

Thanks

 

[janalex]

Followup question. The round RS vintage indicator light. Does it safely run at 220v?

 

[stickyfinger]

Under rated voltage is fine

 

[yafal]

Followup question. The round RS vintage indicator light. Does it safely run at 220v?

Was the Marshall originally 110 VAC? Did you modify it to 220VAC, setting the two primary parallel windings of the power transformer in series?

 

[LoudStroud]

Hey guys,

Which pot would you use for a bias adjuster on a vintage plexi? Can you still find the straight pots out there where you physically move the wiper on the trace?

Are you replacing the straight line trimmer or is there currently a set bias? In a 50 or 100 watt head there is typically a 25k trim pot, or straight line on ‘69 and earlier, in series with a 47k resistor, connected to the two 220k bias feed resistors.

Here is the 25k Piher trim pot available thru CEDist.com or you can find elsewhere.

https://www.cedist.com/products/trimmer-piher-single-turn-side-adjust-vertical-pcb-mount

 

[janalex]

My questions refer to two different amps in my collection.

My 67 jtm100 PA has no bias trimmer so I will be installing the one pictured

My 66,45/100 has a Cobbled together indicator light. It does not have a 110 V tap and only runs at 220 with a step up transformer. I have an RS round indicator light here and just want to be sure it is safe to install

 

[LoudStroud]

My questions refer to two different amps in my collection.

My 67 jtm100 PA has no bias trimmer so I will be installing the one pictured

My 66,45/100 has a Cobbled together indicator light. It does not have a 110 V tap and only runs at 220 with a step up transformer. I have an RS round indicator light here and just want to be sure it is safe to install

Re: the bias adjustment, I had to do the same with my ‘72 Park 75 that had a fixed bias.

As far as your indicator light question, somebody more knowledgeable than I will need to help you with that one. In later Marshall’s the indicator light is connected to the 6.3 V heater feed. Earlier years might’ve been to the actual 220 AC side.

 

[neikeel]

Round Amber (or replaced red from eg a Laney) is 220-240 fed from the mains switch.
Square reds 6.3v off heater taps.

 

[janalex]

Today I attempted to install the round jtm style indicator in my 66 plexi PA and noticed it wouldn’t fit in the original hole. Is it possible this is the original indicator ?

 

[LoudStroud]

Have you considered trying to replace just the bulb inside the original plastic dome gem? A friend of mine does that with the later Square lights, so not to have to splice or disconnect and pull the original leads from the wiring harness.

 

[janalex]

The bulb is working. The only reason I was going to replace was for the sake of restoration however seems the RS light we typically see in these was never installed in my amp as the hole diameter is too small.

 

[neikeel]

The correct lamp in 66 is the square red one, small hole with a notch.

 

[janalex]

The correct lamp in 66 is the square red one, small hole with a notch.

Thanks. The black flag PA came with a replacement indicator as well. The hole in the black flag is definitely larger than the 66 and could even probably fit the RS light. However I installed a red square. Is this much hole normal around the square?

 

[neikeel]

That hole is bigger than usual for a red lamp so it has been opened out for an 'in between' lamp.

 

[janalex]

Ok cool. I’ll stick with the red lamp though as it is period correct. Hard to tell unless up close