Frondelli Master Volume for Bassman Micro EF80

GlideOn-Designs

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That's cool. I was just hunting for 1.8M on my go-to amp-supply sites -- not so much. So 2M would be handy. EDIT -- but now I see my friendly small supply sites (AES, tubedepot) just carry 1.5M and 2.2M. No, wait, AES has carbon film 1/2W at 2M. Whew. I forget how the math works here... is it just two resistors in series? :confused:



That's very helpful; just getting a more linear response would be elegant, and what I've seen in the past suggested Lar-Mar may be the most popular and widely used post-PIMV on several forums I was reading.

The other supplier hassle is Alpha's dual gang pots seem to come most often with their nasty little 0.31 bushing diameter, but it looks like AES has a version with the nicer 3/8 bushing.

Yeah, there's a lot riding on the pot to be of good taper and reasonably accurate value to behave as desired. Alphas are hit or miss, so are these newer yet unknown Chinese manufacturers making 2w Carbon pots - I've used sucessfully though.

Should you have the room, it is totally worth getting a PEC/RV4 spec pot ($40!), but if you do it right, you do it once! The accuracy, taper and robust feel is second to none.
 

King Fan

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Yeah, there's a lot riding on the pot to be of good taper and reasonably accurate value to behave as desired. Alphas are hit or miss, so are these newer yet unknown Chinese manufacturers making 2w Carbon pots - I've used sucessfully though.

Should you have the room, it is totally worth getting a PEC/RV4 spec pot ($40!), but if you do it right, you do it once! The accuracy, taper and robust feel is second to none.

Heh, I'm not up to PEC $$ yet -- plus I've got Alphas all across right now (chosen for reasons (not $) I now regret). Not my favorite but they're getting the job done.
 

Hyakuya

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The Alpha I'm going to use was literally the only option I can find in the UK for a 250KA dual pot that isn't audiophile price. Tocos do one but the guys who sell them here say the channels are so badly matched that they sent most of them back and won't get any more.

Fingers crossed the Alpha does the job.
 

King Fan

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Can anyone explain which formula (series or parallel) we're using for that 1.8M resistor from wiper to 'ground lug'? Here's a schematic from Rob's 'real 5F6a' version, and though the grid stop architecture is different, and the grounded lug here runs off through to bias circuit, I'm seeing it as *in series* with the pot's resistance when the pot is turned up to max, ie, no signal to ground. But I also keep feeling like that's wrong!!!

1658619816927.png
 

andrewRneumann

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Can anyone explain which formula (series or parallel) we're using for that 1.8M resistor from wiper to 'ground lug'? Here's a schematic from Rob's 'real 5F6a' version, and though the grid stop architecture is different, and the grounded lug here runs off through to bias circuit, I'm seeing it as *in series* with the pot's resistance when the pot is turned up to max, ie, no signal to ground. But I also keep feeling like that's wrong!!!

View attachment 1008357

It's in parallel. Some truths are self-evident. :p

(A signal appearing on the top of the pot has 2 choices to get to ground (when volume is max): 250K or 1.8M. How about both for 220K?)
 

King Fan

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It's in parallel. Some truths are self-evident. :p

(A signal appearing on the top of the pot has 2 choices to get to ground (when volume is max): 250K or 1.8M. How about both for 220K?)

Thanks, o wise one. I sensed I was seeing it wrong. You've also just given me an example of how well-chosen words can be better than complex pictures.

Heh, the one clue to me was the || calculator giving the 'right' answer. As you say, never ignore the obvious. :)
 
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Hyakuya

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So would it be an option to use a different pot so long as the 1M8 resistor is adjusted accordingly to achieve the 220K.

i.e. a 500K pot and a 400K resistor
 

King Fan

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So would it be an option to use a different pot so long as the 1M8 resistor is adjusted accordingly to achieve the 220K.

i.e. a 500K pot and a 400K resistor

Interesting. Reading the 'secret life of pots' thing, would it alter the taper? Would it alter the actual R load seen upstream and the actual source resistance seen downstream, and would that matter?
 

andrewRneumann

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So would it be an option to use a different pot so long as the 1M8 resistor is adjusted accordingly to achieve the 220K.

i.e. a 500K pot and a 400K resistor
Great question!

It messes more with the taper. You might still be happy with it.
(I forgot to label the axes: X is percent pot rotation, Y is total grid leak resistance.)

250K pot + 1.8M safety resistor
1658679071135.png


500K pot + 400K safety resistor
1658679151768.png


Keep in mind that this "safety resistor" is only there to keep the valve from immediately red-plating and melting down (thermal runaway) if the pot wiper were to lose contact. If the amp is continually operated on the safety resistor, which far exceeds the maximum allowable grid leak resistance, thermal runaway may still occur. The safety resistor gives you a little more time to figure out something is wrong and shut the amp down before damage occurs.
 

King Fan

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Keep in mind that this "safety resistor" is only there to keep the valve from immediately red-plating and melting down (thermal runaway) if the pot wiper were to lose contact. If the amp is continually operated on the safety resistor, which far exceeds the maximum allowable grid leak resistance, thermal runaway may still occur. The safety resistor gives you a little more time to figure out something is wrong and shut the amp down before damage occurs.

I wasn't smart enough to see that this is a safety resistor in the cathode-bias Bassman Micro; Rob emphasizes that aspect on the 'real' Bassman, where the grid leaks handle the fixed bias current as well. But I guess losing the wiper here would still remove the grid leaks -- and as you say, running on the safety resistor alone would be bad. Hmm, would one know the wiper had failed, or would runaway be the first sign? OTOH, would the 400K resistor be a lot less evil than the 1.8 - 2M resistor in case of wiper failure?

And (dream on, KF) is there a way to wire the pot so wiper failure reverts to 220K? "Secret Life of Pots" indeed... :)
 

andrewRneumann

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I wasn't smart enough to see that this is a safety resistor in the cathode-bias Bassman Micro; Rob emphasizes that aspect on the 'real' Bassman, where the grid leaks handle the fixed bias current as well. But I guess losing the wiper here would still remove the grid leaks -- and as you say, running on the safety resistor alone would be bad. Hmm, would one know the wiper had failed, or would runaway be the first sign? OTOH, would the 400K resistor be a lot less evil than the 1.8 - 2M resistor in case of wiper failure?

And (dream on, KF) is there a way to wire the pot so wiper failure reverts to 220K? "Secret Life of Pots" indeed... :)

Good point--there is a little more protection for cathode bias, but the way I understand it you can still runaway a cathode biased tube. All the grid needs to do is keep rising in voltage as the cathode rises in voltage... with enough grid-leak resistance, I think this is a possibility for all tubes, regardless of bias type.

If you lose signal on one side of the power amp, you are going to lose a ton of clean power. Basically you are now limited to class A operation on the working tube and that will have a very small amount of headroom available to it. As I've watched and learned on this forum, I've come to realize it is never wise to keep operating an amp that is not working the way it should. Shut it off and investigate!

400K would be a lot less evil in the event of wiper failure. One would have to experiment and determine if the change in taper was annoying or maybe not even noticeable.

And for your idea...

1658682673105.png
 

King Fan

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LOL, no, I can dream it, but somebody like you would hafta tell me how to do it... :):):)

Oh, and Andy, does this tell us anything about the safe max grid leak resistance?

1658684353984.jpeg


Remembering that in the Micro, the grid leaks are part of a voltage divider/attenuator with the 470K grid stops...

1658684300736.jpeg
 
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andrewRneumann

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Oh, and Andy, does this tell us anything about the safe max grid leak resistance?

Yes, Rg1 max is listed at 1MΩ. This is technically the resistance between g1 (control grid) and cathode (k), but I don't think anyone worries about the relatively small value cathode resistor. The question is: is that value for fixed-bias or cathode-bias?

Another datasheet for the EF80
1658685539730.png

Je ne parle pas francais, but I'm pretty sure it's 500K for fixed bias, and 1M for cathode bias.

The 470K attenuators do not come into play and can be ignored.
 

Hyakuya

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Now it's done....

IMG_20220728_122731.jpg

only initial impressions but it seems to work very well.

I've not used this amp for a while now but I'm sure the presence pot now has more impact than is used to.....

Anyway... thanks to all for the help.

Back to building the cab for my Champ Micro now. @King Fan - I recall a post you made about finishing a cab but I can't find it. I'm thinking of Danish Oil but can you remind me what you did to yours cos it looked bling...
 

King Fan

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… can you remind me what you did to yours…

Sure. I did 3 coats of wax free shellac as a sealer and about 8-10 coats of rub-on satin poly, with wet finish sanding at 1500 before the last couple coats (the rub-on dries nice and quickly). Lots of web info about rub-on varnish — IME the easiest way, tho Danish oil may be a similar process. Oh and the shellac is optional — poly makes a decent sealer.

Thanks for taking the lead on the Lar-Mar (and thanks to Rob) — I’ll be following in your footsteps. Share added impressions as you get them!
 

jjlemon

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The amp I've just finished building has a post phase inverter master volume, and I used a safety resistor from the wiper to ground. I had intended to use a 1Meg resistor but for reasons which now elude me, I ended up using 560K resistors with the dual 500Ka pot. Simulations showed this wouldn't be too bad in affecting the taper, and in practice the master volume has a pretty good sweep and usable range, as can be seen in Andrew's simulation above. I doubt I would notice much difference in taper if the safety resistors were removed.
 

robrob

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Can anyone explain which formula (series or parallel) we're using for that 1.8M resistor from wiper to 'ground lug'? Here's a schematic from Rob's 'real 5F6a' version, and though the grid stop architecture is different, and the grounded lug here runs off through to bias circuit, I'm seeing it as *in series* with the pot's resistance when the pot is turned up to max, ie, no signal to ground. But I also keep feeling like that's wrong!!!
At max volume you have 250k in parallel with the 1.8M resistors.
Parallel resistance equation: 1 / (1/1,800,000 + 1/250,000) = 220,000 and 220k is a typical 6V6 and 6L6 grid leak value. A 2M resistor would increase total resistance slightly.
 




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