Reducing piercing treble on this 1938 National Model B ?

[ginger]

The speaker is not the problem

[shooter]

same with both channels?
same when you have the guitar treble on 0?
same at all volumes?

[ginger]

Welll. no, guitar treble turned all the way down takes away the bite... but takes the tone away too

[ginger]

Thanks... i will play with it today , and get back to you .

[PRR]

Replace the R-C network across the OT primary. This has to be a HIGH-Voltage cap! I would even ues two 0.002uFd 600V caps in series. The 4.2K (4K, 5K) should probably be 10 Watt.

Then try the same but with 0.01uFd (two 0.02uFd 600V series) with resistor. That is what I *think* it wants to be, but try the plan values first.

But it could just be an over-bright (or jammed) speaker. Being voice-coil, authentic options are few.

[ginger]

Maybe an obvious question , but... is this amp parallel single ended ?!

[sluckey]

No, it's push/pull.

[ginger]

Well... that's what I thought , until I saw the 150 ohm cathode resistor.... which , depending on "conditions" , is usually around 400 ohms . Anyway , I'm going to pull the chassis today , take voltage readings , etc .

[sluckey]

The 150Ω resistor is lower than I usually see with 6L6s but who knows what the plate voltage is.    250Ω is more common with 6L6s. But anyhow, the cathode resistor has nothing to do with whether or not the amp is push/pull or single ended.
 

[ginger]

Thanks , yes , I know . Seeing that resistor value made me wonder... really low... i'm about to check plate voltage now

[ginger]

This amp has had what appears to be a tone control added... which seems to do nothing... any suggestions for a tone control that may be right for this National ?

[shooter]

can you sketch in where it's located, values used, might help

[terminalgs]

Maybe look at those three 12uf/25v bypass caps, especially the one under the 6L6's.   you could tack in a 25uf or 47uf to see if makes a difference...

[ginger]

Yes , I'm going to replace all 12@25V  with 25@50V.... just discovered the "mystery" tone pot has no ground connection through the pot body where the .005 cap is soldered , no internal tooth lock washer .... replacing that as I post this .

[ginger]

This is the "mystery " pot ,as I see it in the circuit... replaced the pot and cap... pot still does nothing but turn off all signal at one end of the sweep

[shooter]

I would remove it till the issue gets cleaned up.  I'm not quite good enough to tell if the 6N7s are fixed bias (real big grid R's).  If so, stitching in a TS might be more problematic.

I'd give PRR's suggestions a go, also re-cap ps, voltage checks.

does the piercing treble sound more like feedback squeal, or just too much treble.

[ginger]

Sounds like an amp with no negative feedback.... grating highs.. no squeel... no warmth

[sluckey]

Replace that .0047 tone cap with a .01 or .022 cap. Any better?

[ginger]

Replace that .0047 tone cap with a .01 or .022 cap. Any better?

I'll try that now

[sluckey]

I would remove it till the issue gets cleaned up.  I'm not quite good enough to tell if the 6N7s are fixed bias (real big grid R's).  If so, stitching in a TS might be more problematic.

That 6N7 is cathode biased. The grid resistor is only 250KΩ. Did you think it was 250Meg? Back during the time this amp was made, schematics used M to represent thousand (like the Roman numeral M). K (Kilo) did not show up on schematics until later. At that time M began to replace MEG to represent million. Lot of confusion during the transition years.

[ginger]

I just tried a .02 cap on the pot... signal doesn't cut out now , but tone is all or nothing at about 1 , on the 1 meg audio pot....no in between

[ginger]

I would remove it till the issue gets cleaned up.  I'm not quite good enough to tell if the 6N7s are fixed bias (real big grid R's).  If so, stitching in a TS might be more problematic.

That 6N7 is cathode biased. The grid resistor is only 250KΩ. Did you think it was 250Meg? Back during the time this amp was made, schematics used M to represent thousand (like the Roman numeral M). K (Kilo) did not show up on schematics until later. At that time M began to replace MEG to represent million. Lot of confusion during the transition years.

Yes , I saw the values on the schematic , and knew they had to be nonsense... A 500 meg pot ?!

[ginger]

I just wondered if this tone pot placement looked odd to your eyes

[shooter]

Did you think

that's always been my problem 

[sluckey]

I just wondered if this tone pot placement looked odd to your eyes

That's exactly where I was gonna suggest you put a tone control. Try a 500K or 250K pot.

[ginger]

ok... gonna try 250K

[ginger]

Replace the R-C network across the OT primary. This has to be a HIGH-Voltage cap! I would even ues two 0.002uFd 600V caps in series. The 4.2K (4K, 5K) should probably be 10 Watt.

Then try the same but with 0.01uFd (two 0.02uFd 600V series) with resistor. That is what I *think* it wants to be, but try the plan values first.

But it could just be an over-bright (or jammed) speaker. Being voice-coil, authentic options are few.

Just did this... no change

[ginger]

I just want to say... It's a REAL "pleasure"... following a 1938 rats' nest circuit .

[ginger]

I'm just gonna "alligator" resistors across the pot until it works

[ginger]

No resistor value , jumpered across the pot, changes the sweep... still all or nothing

[sluckey]

Put another pot in it.

[ginger]

Ok... second pot today... doing it now

[sluckey]

Ok... second pot today... doing it now

I thought all you did was put resistors across the original 1M pot?

[ginger]

No , today I replaced the 1 meg pot . But I just replaced THAT with a new Alpha 250K audio... I think it's working . HA... now all I have to do is solder the cap to the correctpot  lug , so it's not in reverse

[ginger]

OK... it works... thanks. I think I may go back to the 0047 cap .  Really , thanks

[ginger]

Although , I'm surprised that jumpering resisters across the new 1 meg pot didn't yield the same results

[shooter]

I'm surprised

doing service long enough you'll find there are 2 ways to deal with "that dilemma", theorizing to insanity, or go play 

[ginger]

I'm surprised

doing service long enough you'll find there are 2 ways to deal with "that dilemma", theorizing to insanity, or go play 

   Yeah , well , going to go buy batteries for my stupid camera now.... so everyone can see the insides of this thing ..... resistors from Tut's tomb

[ginger]

The 150Ω resistor is lower than I usually see with 6L6s but who knows what the plate voltage is.    250Ω is more common with 6L6s. But anyhow, the cathode resistor has nothing to do with whether or not the amp is push/pull or single ended.

PT is putting out 360V.... plate V is 405V

[PRR]

> The 150Ω resistor is lower than I usually see with 6L6s but who knows what the plate voltage is.

Note that G2 voltage is way-dropped by large dropper resistor.

This is how it used to be done. When steam engines and flatheads ruled the earth.