Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Jennings on December 18, 2025, 05:42:59 am
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I’m refurbing a little three tube ‘50s cheapy combo. Twin triode series preamp (a 12AX7), via a coupling cap and 500k volume pot into an EL84. The third tube is an EZ90 rectifier. Originally it has a switched (for mains on/off) tone pot and .02uF cap to ground directly on the input jack. Basically acting like another guitar tone knob…presumably for 50s guitars which didn’t have one fitted.
Question is, do I swap it for something like a switched 500k (that’s what I have to hand) and make a tone control in or after the preamp stages, or can I utilise the original 100k effectively for some kind of tone shaping or gain adjust somehow? I feel 100k is perhaps a bit low for most of the tone circuits I’ve seen.
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The Two Rock 35 has an input like this
(https://i.imgur.com/htNRxB2.jpeg)
if you want to give a try and see if you like it, use the 500K pot you have and put a 6.8K between the pot (pin #3) and ground, pin #1 connected to the grid stopper of V1, wiper (pin #2) connected to pin #1 (use the pot like a rehostat)
do a search to understand wich is the use of that rehostat
(the schematic show a rotary switch but some models of Two Rock use a pot in that position)
https://www.two-rock.com/wp-content/uploads/2023/03/Studio-Pro-35-rev3.pdf (https://www.two-rock.com/wp-content/uploads/2023/03/Studio-Pro-35-rev3.pdf)
On the manual read about the Pickup Loading feature
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Here you can find suggestions on how to use the 100K pot as a Single Pot Tone Control
http://www.muzique.com/lab/swtc.htm (http://www.muzique.com/lab/swtc.htm)
and here there is a 500K Single Pot Tone Control
(https://el34world.com/Forum/index.php?action=dlattach;topic=23314.0;attach=71129;image)
Here other Tone Controls and Single Pot Tone Controls documentation
https://monster.partyhat.co/article/amplifier-tone-stacks/#Overview (https://monster.partyhat.co/article/amplifier-tone-stacks/#Overview)
Franco
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Thanks so much Franco...that's a great set of suggestions and food for thought. For some reason the first image (Two Rock input) just displays as a blue blank with "content not viewable in your region" for me, but the links and second picture all work / display just fine. I hadn't thought of things like variable pickup loading.
I'm tempted to go for an option which gives me some apparent treble boost rather than just degrees of treble cut. Reason being on simple amps I rarely use the single tone control as I find I leave it at the maximum treble setting. I think my 100k existing pot would load down the valve stages too much...but another rummage in my parts stash reveals I have a 1M pot with push/pull switching. So I'm tempted to swap out the original rotating switched 100K pot for that...I'll add a switched mains plug on the lead for mains switching duties (which'll also help keep some hum source distance from the amp circuit area), and then use the 1M pot as the basis for my single tone control, and the push/pull switch to maybe take it in/out of circuit. On that basis I might go for a combined semi active control...introducing partial (for treble boost) cathode cap bypassing in one direction, and treble cut (cap to ground from audio path) on the other.
I'll draw an as-found and modded circuit diagrams too when I get a moment. The amp in question is a little Besson combo dating to circa 1955 (from the caps).
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Now do you see it ?
The previous image is hosted at IMGUR and if you are in UK you can't see it because of your govern choiches
Franco
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Ah yes…I always find politics and economics come in the way of people and life, sadly. Thanks for the image :thumbsup: To start with I’ve gone for the Princeton style 1M pot giving a .0046uF to ground treble cut in one side and 470pF volume pot treble bypass on the other…initial power up gives good results. I need to properly adjust the preamp bias still (the original was biased for lower gain than I’d like with 4k7 resistors only on the cathodes). Diagram pending.
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Quick update to say thanks again for the suggestions, and to say that the amp's all finished and sounding great. I never did end up using the original 100k switched pot though, and simply changed it out. Lovely range of clean tones, and great raunchy cranked drive too. With my bias changes in the preamp I had to move the volume control or the dual triode were overdriven from the word go. Much better gradual response with the volume between stages. Schematics attached.