help with bias circuit mod

[mikepukmel]

So i got a kit for a JTM45 type amp. The amp has a variable 22k trim pot to adjust the bias. So far so good. But the trim pot included in the kit is too flimsy looking for how important its function is. I scoured the web looking for a solid, chassis mount pot like the Fender 10k bias pot and could not find what Im looking for. in fact the Fender 10k pot is the only one I could find like that, good solid metal chassis mount pot.

So I thought maybe I could modify the circuit and use the Fender 10k pot instead of what was there.



Since the amp isn't built yet, I have no idea what actual voltages and currents will be there.

The kit value voltage divider goes between:

56/(15+56) and (56+22)/(15+56+22) if my grade school algebra is correct or
0.79(Vin) to 0.60(Vin)

Some guesswork, and replacing the 15k with a 22k, and the 22kpot with a 10k pot, gives
0.72(vin) to 0.64(Vin).

What I have no idea is if its better to have the swing roughly centered, or should the swing be on the negative side or the other way around, less negative side.


If the final voltage is out and I can't get the tubes biased, can I just futz with the value of the resistor that is now 15K to bring the swing around the value need?

I'm guessing yes, but do I run the risk of killing the tubes if its too far out to start with? Ive read some quick remarks by people who have done some modifications, but they didn't say how they went about it.

The kit has a nice table with target voltages for the assembled amp. They have -42v at pin 5 on the KT66's.

But this Im assuming is a loaded amp, all tubes in place, standby off. So, you pretty much need to have the amp done before you can get to that point.

And this amp taps the bias off the HV secondary which seems like it should 'move' more than if there was a separate bias tap.

Thanks,
Mike

[Enzo]

Yes, the bias will move a little with the HV sag...so what? These are guitar amps, not rocket surgery.

How sturdy does a trim pot have to be? There is little current through it, it is inside the amp where fingers won't be mauling it. A zillion amps use small trimmers.

But I have a hard time imagining it difficult to find a 22k-25k or thereabouts pot with metal hardware for panel mount.

I think these are real cool, but they are $15.

https://www.mouser.com/ProductDetail...DcTVYCNZP1I%3d

Here is a plain old 24mm pot, in stock, three bucks:

https://www.mouser.com/ProductDetail...6K6lZnnZUPw%3d

[g1]

I don't think you need to worry about killing the power tubes that fast when you first power up.
Just set the pot at max. resistance to start and monitor the tube current as it starts up. If it's too far out, tweak that 15K like you mentioned.

But wait a sec., something is wrong with your calculations for your new values. Changing the pot from 22K to 10K will make the bias voltage less negative. Increasing the 15K will further compound the issue, rather than compensate for it.
(56+22)/(15+56+22) is not .6
Max. negative grid voltage is with pot at full resistance, not at zero ohms.

[dstrat]

Verify you have proper bias voltage range before you plug tubes in.

[mikepukmel]

Yes, the bias will move a little with the HV sag...so what? These are guitar amps, not rocket surgery.

How sturdy does a trim pot have to be? There is little current through it, it is inside the amp where fingers won't be mauling it. A zillion amps use small trimmers.

But I have a hard time imagining it difficult to find a 22k-25k or thereabouts pot with metal hardware for panel mount.

I think these are real cool, but they are $15.

https://www.mouser.com/ProductDetail...DcTVYCNZP1I%3d

Here is a plain old 24mm pot, in stock, three bucks:

https://www.mouser.com/ProductDetail...6K6lZnnZUPw%3d

Thanks Enzo, looking now...

[mikepukmel]

I don't think you need to worry about killing the power tubes that fast when you first power up.
Just set the pot at max. resistance to start and monitor the tube current as it starts up. If it's too far out, tweak that 15K like you mentioned.

But wait a sec., something is wrong with your calculations for your new values. Changing the pot from 22K to 10K will make the bias voltage less negative. Increasing the 15K will further compound the issue, rather than compensate for it.
(56+22)/(15+56+22) is not .6
Max. negative grid voltage is with pot at full resistance, not at zero ohms.

DOH!
Burned by excel again! (Of course, it s not my skills working with it, heh) I certainly did think I put the right formula in.
Arghh. Back to the drawing board. Thanks for the catch.

(And I, uhh, studied, uhh, math. At one point. Or so they tell me. )

[mikepukmel]

Verify you have proper bias voltage range before you plug tubes in.

Thanks Dstrat. Wasn't sure with this amp, if it would drift enough to be a problem once the tubes were in. But as Enzo has reminded me, several times, this isn't a cylcotron.

[mikepukmel]

Trial 2 (no beer this time)

Vout = 0.789(Vin) pot at min
Vout = 0.839(Vin) pot at max


with values in the schematic.

Put a 10k in, and get

Vout = 0.789(Vin) pot at min
Vout = 0.815(Vin) pot at max

If the voltage is around -45v with the pot at mid, is it really that small a swing, i.e. around 3v?
I can't quite remember, but thought that the full swing on my fender type thing was a lot larger than that (pot min to pot max).

[66 Kicks]

The voltage at the diode varies as the total load presented by the bias circuit varies, so simple algebra won't get it. I know what you mean about the 10K bias pot -- it seems to be the only screwdriver-actuated chassis mount bias pot that is readily available.

I would change the 180K to 100K, leave the 15K, change the 56K to a lower value, and change the 22K pot to the 10K pot. What the 56K get changed to depends on the AC voltage of the PT secondary and the desired bias range. If they gave you the rms voltage of the PT, I can give you a ballpark value for the 56K replacement.

[mikepukmel]

The voltage at the diode varies as the total load presented by the bias circuit varies, so simple algebra won't get it. I know what you mean about the 10K bias pot -- it seems to be the only screwdriver-actuated chassis mount bias pot that is readily available.

I would change the 180K to 100K, leave the 15K, change the 56K to a lower value, and change the 22K pot to the 10K pot. What the 56K get changed to depends on the AC voltage of the PT secondary and the desired bias range. If they gave you the rms voltage of the PT, I can give you a ballpark value for the 56K replacement.

Thanks 66! The power transformer sheet says its 323 - 0 - 323 v ac rms, unloaded. Ive been looking around on the web for an article since this amp has been clone-built to death, but haven't found anything that shows the desired swing.

[mikepukmel]

Yes, the bias will move a little with the HV sag...so what? These are guitar amps, not rocket surgery.

How sturdy does a trim pot have to be? There is little current through it, it is inside the amp where fingers won't be mauling it. A zillion amps use small trimmers.

But I have a hard time imagining it difficult to find a 22k-25k or thereabouts pot with metal hardware for panel mount.

I think these are real cool, but they are $15.

https://www.mouser.com/ProductDetail...DcTVYCNZP1I%3d

Here is a plain old 24mm pot, in stock, three bucks:

https://www.mouser.com/ProductDetail...6K6lZnnZUPw%3d

Hi Enzo, yeah, no one will be fiddling with it, but, eek, this particular one is REALLY flimsy. I have had other equipment with trimmer pots, that looked a lot more sturdy. This has to be the lowest quality, cheapest thing made by humans today.

[mikepukmel]

Here I go again with the "what if's". Is it worth it, for longevity, and fun mod's, to just go and put a 50v transformer into this amp to feed the bias circuit?

[Dave H]

The power transformer sheet says its 323 - 0 - 323 v ac rms, unloaded. Ive been looking around on the web for an article since this amp has been clone-built to death, but haven't found anything that shows the desired swing.

To use a 10k pot in place of the 22k for the circuit in the OP why not just use resistors of half the value?
i.e. R29 91k, R30 7k5, R31 28k and VRx 10k
That should give the same range as the original circuit (about -38 to -50V)

[dstrat]

Hi Enzo, yeah, no one will be fiddling with it, but, eek, this particular one is REALLY flimsy. I have had other equipment with trimmer pots, that looked a lot more sturdy. This has to be the lowest quality, cheapest thing made by humans today.

last amp I built had one of those flimsy trimmers but once soldered in and set I have not touched it since over 2 years ago.. but I understand your concern.
Hay its your amp build it the way YOU want!!

[66 Kicks]

Thanks 66! The power transformer sheet says its 323 - 0 - 323 v ac rms, unloaded.

32K would be a good starting point as a replacement for R31/56K.