'BF Princeton' weird voltage- help!

[The Captain]

Hi. I've scratch built a BF Princeton which is up and running and general Voltages are in correct ballparks: BAR only 165V at pin 1 of V2 (should be ~ 260v iirc). I've checked as advised there is 'no continuity between hot ends of 100k pair at V1 and 56k at V2 pin 6' and all wiring correct/ continuity etc..

any ideas anyone? or any help much appreciated, thanks- Captain.

[Chuck H]

Maybe: (in order of possibility)

bad tube
wrong value cathode resistor
bad cathode bypass cap
wiring error (cathode bypass cap polarity, etc.)

After trying another tube check the cathode resistor and bypass cap with a componant tester. You will need remove them from the circuit. Triple check your wiring. (in this order)

HTH

Chuck

[The Captain]

Thanks Chuck, I have ruled each of these out- what im getting is 300v from the second filter cap (point A) which goes to a 220k on the board, on to the anode of V2 (here measuring 165V). Someone has said that's 'an expected drop in V, and not to worry about it'; Id like to ask others opinions on this answer if possible; maybe Im just being too picky?

Secondly, my 1M Vol pot is losing all volume at last 1/4 turn. Ive swapped pot out for a new one- the same. Its measuring, in situ in circuit, up to ~300k then drops off to ~0 at max. Can anyone think of ideas on this? ALL circuit is 3x checked ie wiring/ values/ etc etc.

Much appreciate any help, captain.

[Chuck H]

O.K. Well, IIRC a BF Princeton should have a 100k plate load resistor. But even with a 220k I think your voltage drop is excessive. Your plate load resistor is currently dissapating over a half watt of current. This would be unusual for that amp. Also, if your not using 1 watt plate load resistors, you should expect a failure. Typically you should see less than half that amount of current. So there is a problem. You should recheck the things I posted.

As for the volume control. Well...If you changed it out once and it's doing the same thing, it's not the pot. I can't be sure how your measuring the resistance either. It should be removed from the circuit. You either have a mistake in your wiring or a parasitic oscillation that is shutting the amp down at higher volumes.

Chuck

[Chuck H]

Ooops...Pin 1 V2 is the trem circuit. My bad. Just looked at my schem and 220k is correct. As for the voltage, there should be 415v behind the plate load resistor. It is tapped from the screen supply node. What is your screen voltage???

Chuck

[The Captain]

Yes ts the trem part, perhaps why Bluebell Audio's quick advice was 'just go with it'. If it doesn't affect the general tone of the amp with or without trem on, then Im not worried: why I ask here for another opinion..

I use a torres 508v PT, so I realise I will be shy of schematic/ guide figures (I have 300v from point A/ junction of 1st and 2nd main filter caps.. screen voltage is it?). The big jump down to 165v across the 220k from 300v is my query as its 100v shy, whereas Im only say 30v shy with the plate resistors at V1 (160v as opposed to "190v" on the schematic).

The vol pot is totally baffling- perhaps its realted to lower general voltages? anyway appreciate you having a peep at the schematic there Chuck, cheers Captain.

[MWJB]

Hi Captain,

If you had a 100K plate resistor at V2, pin 1 I'd only expect to see 200v-ish on the plate (a 12AX7 plate in a Fender with 100K resistor usually shows around 68% of the voltage applied to the plate resistor). So, 165v with a 220K plate resistor does not seem out of line.

As Chuck inferred, the difference is largely down to the lower voltage at the screen node.

I seem to remember that heater voltages on that PT were a little lower than what you'd, normally expect from a Fender style PT. I connected up the 230V tap, rather than 240v, to get them up over 6.1VAC, if I remember right? If you switch to the solid state recto, you could try connecting the 220v primary of the PT then check heater voltage with tubes installed. If you get under 6.9VAC then that's OK and your B+ will be nudged up slightly. THIS IS NOT A RECOMENDATION FOR ANYONE ELSE, NOR IS IT STANDARD/RECOMMENDED PRACTICE - just relates specifically to the Captain's PT, with which I am already familiar.

Also watch power tube bias, lots of current through the 6V6s will pull down your B+, see how the amp sounds with 25-30mA per tube?

Remove your volume pot from the amp (or check the old pot) and check resistance between the tab that should be grounded (LH) and the wiper (middle tab) with the pot turned fully up. Even measuring the pot in situ, you should be getting 300K+ with the volume turned fully up. This will help you establish whether the pot is at fault, or whether you have a parasitic like Chuck suggests (not uncommon in scratch built amps).

Regards, Mark.

[The Captain]

Hi Mark, thanks for thoughts there. As my ltd sense says I wouldn't neccessarily be affecting overall tone even if V2 pin 1 was higher, I will stick as is: the thought had occurred though.. what if I changed the 220k for a ~100k or so to nudge the V2 P2 voltage up? theoretically could this be applied to the 100k plate r's on V1 too? (~68k's to get me up from 160v to nearer the "190v" guide/ schematic. Im not neccessarily planning changes to V1 P2 its rather the idea Im interested in).

PT wiring- I'll go through the above ideas again in due course. At the mo though the amp sounds good: dead quiet, a proper useable tone (not cello bassy like the ***ing 5e3 ). Bias tested at 28.3 mA for both 6v6s. I'll try a 120pf treble cap for the 250pf for more highs, as the Twin style bright switch idea didn't work on this circuit. The PT is running faultlessly though and a good even temp- its a quality component for sure.

Vol pot may well have me howling at the moon soon (took out, tested fine, back in, tested daft) the term 'parasitic oscillations' makes my teeth gnash- it must be this is all I can think. Cheers Capt.

[MWJB]

If your tremolo is working as intended I wouldn't worry about the voltage at V2, pin2. If, not, maybe try 100K instead.

I'd stick to 100K at V1 personally, 68K or 56K may make the amp overly bright/scratchy sounding with a 12AX7.

Triple check tone stack wiring.

PO - Did you copy the Fender layout exactly? Use shielded cable for grid wire at V1, 2 and from the vol pot to V1, pin 7. Shorten or shield wires to 6V6 grids.

[The Captain]

If your tremolo is working as intended I wouldn't worry about the voltage at V2, pin2. If, not, maybe try 100K instead.

I'd stick to 100K at V1 personally, 68K or 56K may make the amp overly bright/scratchy sounding with a 12AX7.

Triple check tone stack wiring.

PO - Did you copy the Fender layout exactly? Use shielded cable for grid wire at V1, 2 and from the vol pot to V1, pin 7. Shorten or shield wires to 6V6 grids.

Bright and scratchy?.. sounds ideal! I think it'll stay as is though. So just the vol pot- it is bugging me as Ive checked wiring for the nth time. Im 120% certain all is spot-on. I copied the fender field guide layout/ schematic, using shielded for V1, 2. I'll try it for V1, 7 and poss on 6v6 grids but cant see it solving the last 1/4 turn drop-off.. cheers Capt

[MWJB]

"Bright and scratchy?.. sounds ideal!" If you want to push up the preamp plate voltages (which is what a smaller plate resistor would do), improving hi end & fidelity, reduce the value of the power supply dropping resistors, whilst keeping the 100Ks on pins 1 & 6, first. To get into BF territory you want around 200v here.

A good place to use a 56K resistor if you want to brighten the amp/increase mid character is the treble slope resistor, that's the 100K that connects the 250pf treble cap to the mid & bass caps.

That last 1/4 turn 'drop off' is not right, it shouldn't happen. Something is shorting out the wiper at full rotation. With all tone controls & volume flat out, you should read approx 355K between the wiper (middle tab) and vol ground reference (with stock pot values).

[The Captain]

Excellent- as I used your method of whacking tone caps & the 100k around the pots to save space, changing that 100k for a 56k is a doddle so I'll do that asap. im a bit confused as to the treble cap value- is I were to lower it say (from 250pf as is to ~120pf) would this brighten or darken in theory?

Maybe as I always use bass pot at min and treble pot at max this is why the vol's dropping off last 1/4 turn? I'll check this over wknd. Shouldn't do though should it.. tyhasnks mark, capt.

[AaronShafer]

EQ grounding weirdness?

Hey, that volume pot is beginning to get to me as well, so a couple of thoughts:

I've had volume pots do the same thing except in reverse: if you turn all the way down, the amp spikes in volume. I finally found that by adjusting my grounding scheme, I could remove unwanted ground loops and get the EQ working properly.

I prefer to ground the volume pot and input jacks at the first cathode ground (usually) and then ground the rest of the EQ somewhere else in the circuit, e.g. at the power tube cathode ground (in a cathode-biased amp).

Also, if it is an oscillation issue (which I've never run into, as you describe it) you may check that your shielded wire is only grounded on one side; if you ground the shielding at both ends, you definitely create a ground loop. You may have already checked that, but I thought I'd throw that in since I didn't see it in an earlier post.

Cheers,
Aaron

[The Captain]

Thanks Aaron I'll go over these thoughts in more detail- at the mo its still there, and as I rarely dont have amp up loud Im living with it as is last 1/4 turn vol drop off.. its definitely bugging me though! I have grounded things as you say in fact, separating preamps/ jacks from cts/ main filter caps etc, & also using shielded wire on important bits- so there's barely any discernable boxes to tick as ikt were to find this prob.. but hey-ho I guess thats the nature of amp building.