My 5F6A build vs. others

[MWJB]

It's not quite as simple as that, just adding a 4.7K resistor. The original 5F6A schem has a 5K presence pot with a .1uf cap from wiper to ground. This arrangement was noisy, so Fender built later bassmans with the same presence control as found on the 6G12A Concert and that which is seen on the 5F6A reissue.

These use a 4.7K load resistor. This resistor is in parallel to ground with a 25K pot & a .1uf cap. The pot & cap are in series with each other, the pot is wired as variable resistor. This is much less prone to noise and has a gentler sweep to my ear. It is the same arrangement that Kendrick & Victoria use.

[tweed5259]

"V1: plate 225v / 218v cathode:1.6v screen:321v" I assume you mean grid instead of screen, you shouldn't have hundreds of volts dc here.

"V2: plate 81v / 93v cathode: .5v screen:150v" V2 plate voltage at pin 6 should be mid-300s to 400vdc, cathode voltage (pin 8) should be more than twice what you have. Voltage on the grid (pin 7) in this case should be about the same as the cathode at pin 8. Only the cathode at pin 3 should have lo dc. Something is wired wrong

"V3: plate 281v/ 255v cathode:43.5v screen:323v" You should be seeing a few 10's of volts at the grids, not hundreds. Seems a big difference between plate voltages, I'd expect 10 or 12v difference at most. Check resistor values.

-46v at the 6L6 grids is not a measurement that means anything, it doesn't tell us how much current the tubes are drawing as different brands have massively different requirements. Install 1ohm 2W resistors between pin 8 of each power tube and ground, and read the voltage at pin 8 in mV DC.

mY METER READS 23.1 on the 1ohm resistor.

I'm gonna check that v2 right now, here's the voltages on the preamp tubes by pin.

V2
Pin 1: 99v
Pin 3: .5v
Pin 6: 161v
Pin 7: 98v
Pin 8: 98v

[MWJB]

You should have 321 at pin 6.

Pin 3 should be more than twice what you have.

How does that 100K resistor at pin 8 check out in ohms?

23.1 mA is low, wind it up to 30-35mV.

[Bruce / Mission Amps]

mY METER READS 23.1 on the 1ohm resistor.

I'm gonna check that v2 right now, here's the voltages on the preamp tubes by pin.

V2
Pin 1: 99v
Pin 3: .5v
Pin 6: 161v
Pin 7: 98v
Pin 8: 98v

OK, among other possible issues, you have a problem right here at this tube...
The supply voltage should be in the low to mid 300v range and half of that should be dropped across the tube from lug 6 to 8.

Your first preamp tube is fed from the same node on the B+ rail as V2.
V1 seems to have good voltages on it and you say you have tried other preamp tubes... so, I think V2 is miswired or you have used misread resistors that are not what you think they are.
It is not unusual to see a novice builder miswire this socket or put the 100K resistor on wrong or feed B+ from the wrong side... etc

[tweed5259]

You should have 321 at pin 6.

Pin 3 should be more than twice what you have.

How does that 100K resistor at pin 8 check out in ohms?

23.1 mA is low, wind it up to 30-35mV.

Ok, so I did wire V2 all wrong... now I have on V2

Pin 1: 221v
Pin 3: 1.3v
Pin 6: 366v
Pin 7: 220v
Pin 8: 221v

and I'm getting 30mV at the 1ohm resistor tied to ground from pin 8 of the power tube.

now I'm gonna go see how it sounds.

[tweed5259]

AMAZING! So, I've got v2 wired up correctly now and I biased the amp properly to 35mV and it sounds awesome! Much like the Victorias and even a lot like the real 5F6As, I also wired the presence pot with a 25K linear pot with the .1 cap and the 4.7k resistor and that sounds much better. So i learned how to properly bias, how to check voltages, pretty good day, thank you so much everyone, this has been a great great help to me!

it still sounds a bit "new" to my ears, but it is new amp, so after a few hours it should settle in.

thank you all very much~!

[Bruce / Mission Amps]

AMAZING! So, I've got v2 wired up correctly now and I biased the amp properly to 35mV and it sounds awesome! Much like the Victorias and even a lot like the real 5F6As, I also wired the presence pot with a 25K linear pot with the .1 cap and the 4.7k resistor and that sounds much better. So i learned how to properly bias, how to check voltages, pretty good day, thank you so much everyone, this has been a great great help to me!

it still sounds a bit "new" to my ears, but it is new amp, so after a few hours it should settle in.

thank you all very much~!

Excellent.... sometimes just listening to another's point of view is all you need to fix something like this.

[scole]

would the changes to the presence circuit work for a 5f4?

[MWJB]

Not really a cut & paste job, as the 5F4 has a cathodyne PI with the first stage triode's cathode as the NFB load (pretty well what the 4.7K does in the later 5F6A). 5F6A/6G12A etc all have long tail PIs and NFB is tapped at the LTPI tail.

[scole]

i couldnt find the other post about this but how do you get the presense on the 5f4 to have more effect, with the stock setup it doesnt seem to make much of a difference.

[sloblues]

voltages on your 5F6A build

where are you guys getting your voltages?
The orginal Fender schematic shows this as the voltages on a 5F6A:
V1: 150vdc/150vdc 2.5vdc cathode
V2: 180vdc/325vdc and 1.2vdc first cathode,180vdc on the second cathode
V3: 225vdc/230vdc 33.5vdc on the PI 'Tail'
5881 Power tubes: 432vdc plate/430vdc screens

Are some of you saying Victoria 'clones' run much much higher voltages? I use a variac to put 110VAC into my 62 Pro and it sounds incredibly better now that the voltages are in the range it was designed for.

[tweed5259]

THE OT'S though...

I have a MM Tone Clones series Tweed Bassman OT in there now, but I've come to find out that MM uses paper bobbins... I know they say they don't sound different than paper, but then why would anyone say they do sound different...

Is this a worthy cause to purchase a new OT with a paper bobbin?
Or will it really not make a difference?


thanks.

[MWJB]

The difference won't be night & day, yes there may be some difference, whether it's distinct enough for you, specifically, to hear...after an hour spent swapping parts is another matter.

I'd hold fire until you are sure you have explored other, more practical options to tailor tha sound to your ear.

[tweed5259]

The difference won't be night & day, yes there may be some difference, whether it's distinct enough for you, specifically, to hear...after an hour spent swapping parts is another matter.

I'd hold fire until you are sure you have explored other, more practical options to tailor tha sound to your ear.

Ok sounds good, but, and again forgive my inexperience, can you list me a few other, more practical options?

[MWJB]

Well there are plenty of resistor & cap values that will have just as great, if not a greater impact on the tone & voicing of your amp. Without knowing which 5F6A circuit you have built the amp to and what specifically you think it's lacking/got too much of, I can't really make any recommendation.

Basically once you're happy that it's functioning as it should, live with it a while and have a clear idea of what you think needs changing.

To give you an idea though, 5F6A's may have been fitted with 0.1uf or 0.022uf bass caps, they may have had 100K or 56K treble slope resistors, etc. A lot of builders would skip the 8uf preamp filter and go straight in with 20uf (reissue uses 40uf). There's a bit of room to move whilst staying true to the 5F6A mold.

Then you've got the various brands & specs of tubes to try...when you've tried these things and still feel that there is a tangible difference between your amp and one fitted with a paper bobbin OT (A/B tested, after eliminating whatever different factors you can) then that's the time to try one.

The critical ear testing you do along the way will be good experience.