What are these components on the Super Lead ST1 board?

[Dave L]

Hi guys, I´m looking through a ´75 Super Lead I got a while back, I´m not really tech savvy but just reading off some color codes and matching it to schematics. Trying to learn something, I suppose.

It seems more or less stock and/or what you might expect to find, but what are these two supposed to be? I can´t really match them up to any typical values, and it does look like there´s been maybe a PPIMV mod here judging from the soldering and the extra hole in the front panel. Are these the 220K coming off the PI I see on the schems switched to 82K, or am I just way out of my depth here?

EDIT - after a bit of extra googling it seems like 82K also is a stock value, but mostly used for the 6550 versions. This does have EL34s now, but these 82K ones probably stayed on in the conversion. Any thoughts on the benefits of going to 220?

 

[griff10672]

Hi guys, I´m looking through a ´75 Super Lead I got a while back, I´m not really tech savvy but just reading off some color codes and matching it to schematics. Trying to learn something, I suppose.

It seems more or less stock and/or what you might expect to find, but what are these two supposed to be? I can´t really match them up to any typical values, and it does look like there´s been maybe a PPIMV mod here judging from the soldering and the extra hole in the front panel. Are these the 220K coming off the PI I see on the schems switched to 82K, or am I just way out of my depth here?

EDIT - after a bit of extra googling it seems like 82K also is a stock value, but mostly used for the 6550 versions. This does have EL34s now, but these 82K ones probably stayed on in the conversion. Any thoughts on the benefits of going to 220?

it looks like someone had put a resistor over at least one of those .... most likely to bring the voltage where they could bias EL34's or vice a versa .... if it was originally running EL34's .... they did it to bias the 6550's

 

[Dave L]

Thanks Griff!

 

[Dave L]

I checked the bias with my old Eurotubes probe now, and they were in the 26,5-27 range. Not entirely out of spec for the EL34s and some dudes do like them a bit below 30, so going off of that I can´t tell if it is set up for biasing 6550s or not. It´s going to the shop to swap a broken pilot light, so I´ll have the tech look if there´s range to go up a bit on the bias while he´s at it.

 

[rstites]

I think it was the importer who went with 6550's, so it is possible that these changes were made before it was even sold by the importer - or at least to their specifications. I believe it was only the US who got the 6550's.

 

[Dave L]

This was sold in Sweden originally, so it´s from those scandinavian export lines that also differed a bit sometimes. I don´t know if we ever got the 6550s, though.

 

[Longo]

I have one just like that. Same year, 1975, with the original EL34.
Also the same resistors.
How does yours sound?

 

[shar-vell Dan]

I checked the bias with my old Eurotubes probe now, and they were in the 26,5-27 range. Not entirely out of spec for the EL34s and some dudes do like them a bit below 30, so going off of that I can´t tell if it is set up for biasing 6550s or not. It´s going to the shop to swap a broken pilot light, so I´ll have the tech look if there´s range to go up a bit on the bias while he´s at it.

My 100 watt EL-34 Marshalls are around 38-ish.
6550's would be mid 50's.

 

[griff10672]

your Bias depends on your plate voltage ..... and then where do you want to run them .....

I always go with 70 % ....

can't go wrong with this simple vid from Eurotubes

 

[shar-vell Dan]

Yes in relation to PV but a "bias range resistor" has a "range" based on its value.
If the OP has no experience biasing a 48 yr old 100 watt Marshall a "forum" is not a place to start.
For optimum performance take your amp to a qualified technician.

 

[psychodave]

Let your ears determine the bias. Make sure to play it for a while then adjust. After you like how it sounds, check to make sure you're not too hot. I tend to like around 60% on most amps.

 

[Racerxrated]

With any Marshall, the OT is designed around EL34s. While 6550s typically will bias in the 50-60ma range, I'd be careful biasing 6550s like that in a Marshall that expects EL34 current draw.
It's safer to bias a 6550s like an EL34 in a Marshall..somewhere between 30-40ma will work well. Long ago I biased 6550s higher and then compared the tone with a lower number at 35ma....didn't notice much difference. Lower=safer so that's how I biased going forward.
Food for thought.

 

[Dave L]

It does sound good as is, and like I mentioned I will not be doing work on it myself. Just trying to orientate myself on the state of it before the tech takes a look, never know what you´ll find in a fifty-year old amp. But the bias is one thing we will be talking about, for sure.

 

[Dave L]

I have one just like that. Same year, 1975, with the original EL34.
Also the same resistors.
How does yours sound?

Interesting, I suppose some of them ended up with those resistors and EL34s even if mostly was a 6550 thing. I guess any combination might have happened on any given day at the Marshall plant, as always

 

[FourT6and2]

Those resistors are the PI output grid leaks. 220K for EL34. 150K for 6550. Sometimes lower, including 82K. Together with the grid stoppers on the power tubes, they add up to a total resistance that shouldn't be exceeded for the tube type. As long as you stay under the spec on the data sheet, you're fine and you can use whatever value you want for the sound you like.