Headfirst MTL Lead gain and saturation potentiometers 220KLog

[hopkinWFG]

Dear all, know and seen few of the guys which did their DIY here... as i have difficulties looking for a 220K log potentiometer and was thinking if using a 250Klog be ok? Thanks Happy new year to all the good people here !!

 

[SpiderWars]

You could put a 2M2 resistor across it and that would make a 250k pot closer to 225k. And if you have several 250k pots measure them and see if one is around 244k-245k or as close as you can get. 245k in parallel with 2M2 = 220.45k

 

[hopkinWFG]

hi there thanks for ya reply... but i wonder if the resistors across is going to work? please forgive my ignorance here.. and is it really that crucial to getta right resistive values for potentiometers?

 

[SpiderWars]

To be honest I'd just use a 250k but I would measure a few and pick the lowest value. But putting a resistor across the outer terminals of a pot absolutely works if you really want to get as close to 220k as possible.

 

[hopkinWFG]

Thanks alot pal !! now i just searched thru amplified parts and i did find 220k Audio but it has a spilt shaft and its PCB mounted type not those with lugs... will you go ahead with that instead? funny how as i wonder how did the guys here came about sourcing their MTL clone pots? if your one of the builder will you mind sharing ? Thanks alot !!

 

[scottosan]

I s agree, just used a 250k pot. That’s not a critical area

 

[hopkinWFG]

I s agree, just used a 250k pot. That’s not a critical area

Thanks for helping Scott !! did you just build an MTL clone too? i am currently sourcing parts here... maybe if you were to frequent here i can check out some of the things am unsure off? and mind to show any pics off your build? i kinda ran into some heaters idea which read over the net about DC elevated heaters and AC heaters twisted pairs for high gain like the MTL with AC twisted pairs be just find? the pictures Burger sent on the heaters wires arent even twisted !! wonder what magic or does should i conform to ie wiring layout?

 

[SpiderWars]

I built a Superbass earlier this year (EDIT: lol, did it right out of the gate...I meant LAST year) and decided to try wiring the heaters similar to the way Soldano does the SLO (I also built a SLOclone about 15 years ago the same). Neither amp is noisy and I run the Superbass wide open into a PS-2 with a GEQ in front with slight boost. When I stop playing I hear some hiss but zero hum. Both amps have elevated heaters.

The heater wires are orange for preamp tubes and white/purple for power amp tubes. I like colors.

 

[scottosan]

Thanks for helping Scott !! did you just build an MTL clone too? i am currently sourcing parts here... maybe if you were to frequent here i can check out some of the things am unsure off? and mind to show any pics off your build? i kinda ran into some heaters idea which read over the net about DC elevated heaters and AC heaters twisted pairs for high gain like the MTL with AC twisted pairs be just find? the pictures Burger sent on the heaters wires arent even twisted !! wonder what magic or does should i conform to ie wiring layout?

I’m not doing this build but in a gain position you won’t find much audible difference if any. That’s a common substitution. There’s a difference between pure DC heaters and DC elevated. As for heaters the DC elevated is still AC heaters only with the center tap connected to an elevated DC voltage instead of 0V ground.

 

[hopkinWFG]

I built a Superbass earlier this year (EDIT: lol, did it right out of the gate...I meant LAST year) and decided to try wiring the heaters similar to the way Soldano does the SLO (I also built a SLOclone about 15 years ago the same). Neither amp is noisy and I run the Superbass wide open into a PS-2 with a GEQ in front with slight boost. When I stop playing I hear some hiss but zero hum. Both amps have elevated heaters.

The heater wires are orange for preamp tubes and white/purple for power amp tubes. I like colors.

Nice nice build !!

 

[hopkinWFG]

I’m not doing this build but in a gain position you won’t find much audible difference if any. That’s a common substitution. There’s a difference between pure DC heaters and DC elevated. As for heaters the DC elevated is still AC heaters only with the center tap connected to an elevated DC voltage instead of 0V ground.

i probably have to look into more of these readings and pictures !

 

[hopkinWFG]

@SpiderWars will you be cool to show more of how your elevated DC heaters is done in a more upclose picture? thanks really !!

anyway back to the topic which i may inturn getta 220k log but its PCB mounted type with spilt shaft... should i do it or just stick to 250K with normal solid shaft that look same thru out on my pot layout?

 

[SpiderWars]

As scottosan mentioned it's really only different from 'regular' heater wiring by connecting the heater supply center tap to a positive voltage.

The '68 SB with laydown PT only has one cap can on top, all other filter caps are inside the chassis. That one cap can is for the PI. I wanted 100uF for the PI so I used a 100uFx100uF cap can there. One 100uF for the PI, then I connected a 100k resistor from that PI lug to the other unused lug and then a 330k resistor from that unused lug to the ground lug. Then I connected the heater center tap to that new node. I think the voltage there was around 45vdc or a little higher. You'll have to experiment with those two resistors in that voltage divider (the 100k and 330k) depending on the voltage you tap off of. I thought I read that the Burger boards give you all the points on the board just for this but I'm not sure. I think you just need to choose those two resistors and if you use the same PT he might even tell you those values.

I had a Friedman-modded 70's 100W Marshall and what he did was use cap cans that were double the value he wanted for the screens (IIRC they were 100uFx100uF). But he only used half of those two cans (in series) for the screens. This left two unused sections but only one of them is grounded so he used that grounded, unused section for the heater CT and tapped off of the screen supply since it's right there. Very clean, clever way to do it imo.

I don't have a good picture of it but there's not much to see.

 

[hopkinWFG]

Thanks for explaining... i need to read somtime again to understand... well is late now for me .. will ask Jason on these!! Thx!

 

[SpiderWars]

Draw it out, that always helps for me.

 

[burger]

250k pot is fine, don't stress over finding a 220k

If using my chassis, you want Alpha 24mm M8 shaft.

 

[hopkinWFG]

250k pot is fine, don't stress over finding a 220k

If using my chassis, you want Alpha 24mm M8 shaft.

Thanks alot for the support Jason !!!

 

[burger]

No worries!

My boards support setting up a DC reference for AC heaters....twisted wires not required imho!

 

[hopkinWFG]

No worries!

My boards support setting up a DC reference for AC heaters....twisted wires not required imho!

just curious... and am a beginner here hope you can bear with me awhile.... the DC elevated heaters in ya design is tapped from the board name HTR near right hand side almost corner (1 green/white wire) that link to the centerpoint of the 6.3VAC heater supply from sec side of the power transformer?

and would also wanna ask if you can share the 5 pin DIN female connector that your using? any part number ? Thank you

 

[riq]

just curious... and am a beginner here hope you can bear with me awhile.... the DC elevated heaters in ya design is tapped from the board name HTR near right hand side almost corner (1 green/white wire) that link to the centerpoint of the 6.3VAC heater supply from sec side of the power transformer?

Yep - that's what I did on my 25 watt MCII build. I also went with parallel heater runs and I'm really happy with the noise level.