New Princeton Build - Very low volume

[daveneary]

Just finished my first build of a  Fender Princeton Reverb  following the Hoffman design for a 1164 circuit. 
The good news is that it fired up first time and the Reverb and Tremolo effects both work and there  are no buzzing or hisses.
The main issue is that the volume is very low.
 If I turn the volume up beyond 7 its got a not unpleasant distortion.

If I play a guitar straight into the return from the reverb I get the same overdriven sound and volume.  Can anyone help me track this down. I've taken plently of voltage readings and think I have set the 6V6 to 20ma Bias with a plate voltage of 440 ish.



Regards.
Dave

[Ambugaton]

Switch the leads on the OT primaries

[Ambugaton]

Is it a high pitched squeal?

[daveneary]

The amp sounds Ok  - no oscillations etc. just quiet - like you would expect at  2 to 3 on the volume dial. When I have it at 10 but at this level it distorted like you would expect if it was driving the preamp tubes. 

[daveneary]

Switch the leads on the OT primaries

Sorry for the ignorance but cant you confirm the lead to swap (primaries) are the side attached to the pin 3 of the  6V6s and the B  point on the board between the  first and second power supply caps ? and the Secondaries to the Jack and earth on the OP transformer ?

Regards.
Dave

[Fresh_Start]

20ma of plate current alone seems like cold bias to me.  Even without screen grid current included, I'd still be inclined to bias it somewhat hotter.  However, I can't find a tube data sheet with plate voltage anywhere near 440 VDC.  Is your bias voltage in the neighborhood of -34 volts DC?

Where are your tone controls set? If you put Treble & Bass on 10, does it get any better?

Tubes - have you tried "known good" tubes in every slot, especially the 6V6s?

Speaker OT impedance mismatch possible?  IOW are you trying to push an 8 ohm speaker off a 4 ohm tap or vice versa?

The next step would be checking every resistor in the circuit to make certain the value is correct, at least in terms of labeling on the resistor.  Sometimes you can't measure the actual value of a resistor in the circuit because there's another resistor in parallel.  I'd start with my pots - make sure they're the right values.  Then the 6.8K mid resistor in the tone stack.  Then get out your highlighter and go to work.

Sorry for not being able to give a more focused response.

Chip

[daveneary]

20ma of plate current alone seems like cold bias to me.  Even without screen grid current included, I'd still be inclined to bias it somewhat hotter.  However, I can't find a tube data sheet with plate voltage anywhere near 440 VDC.  Is your bias voltage in the neighborhood of -34 volts DC?

Where are your tone controls set? If you put Treble & Bass on 10, does it get any better?

Tubes - have you tried "known good" tubes in every slot, especially the 6V6s?

Speaker OT impedance mismatch possible?  IOW are you trying to push an 8 ohm speaker off a 4 ohm tap or vice versa?

The next step would be checking every resistor in the circuit to make certain the value is correct, at least in terms of labeling on the resistor.  Sometimes you can't measure the actual value of a resistor in the circuit because there's another resistor in parallel.  I'd start with my pots - make sure they're the right values.  Then the 6.8K mid resistor in the tone stack.  Then get out your highlighter and go to work.

Sorry for not being able to give a more focused response.

Chip

Thanks for the response. I have checked all the resistors and they are all what they should be.

I have set the Bias previously at 30 ma with no real difference. The Bias voltage available was -40 I will check this again tomorrow.

The tubes are new - and I have no reason to suspect them at this point...but will get a couple of spares.

THe speaker is a SC112 cab rated at 8 ohm.

The OT is a  Deluxe reverb type.

Dave.

[Fresh_Start]

I have checked all the resistors and they are all what they should be.

ARE what they are supposed to be or are labelled what they should be? 

I've found pots that were pretty far off stated value.

Also, virtually every one of us has put a 6.8 ohm resistor where a 6.8K resistor is supposed to be or vice versa.

Can you swap one of the other 12AX7s for the one in the V4 spot?  I ask because it sounds like the problem comes after the reverb circuit return.  What tubes are you using?

Respectfully,
Chip

[super&plexi]

If you're still having problems, maybe try checking the input jacks and speaker jacks and do you have good unpainted star washers on them,  they have to ground to the chassis and in some cases not ground, and things not being tight can give you low volume. Also wrong value pots. If it were me I would  probably probe it, see where the volume dropped out, push on it with chopsticks see if you get a volume rise. Heck,  smack  it with a hammer.  Used to work for Boogie. I don't know if they still do it, but its worked  for me ( procede at your own level of caution on that one)  I'm sure someone will chime in pretty quick with some more helpful information, and congratulations on getting this far on your first build. Also,
   at the bottom  of Slukey's page push the word '' this", it links to a new thing they've got, might help now but should surely help in the future. Cheers! 

[Willabe]

at the bottom  of Slukey's page push the word '' this", it links to a new thing they've got,

Huh? Where? I don't see it. 

Ok, found it.
 

[super&plexi]

Oh yeah, thumbs up to all other replies and as regards FS's input, I respectfully ask if you actually checked the resistors with an ohmmeter, or just by the color bands. A couple times I've got one listed as 1.5 K and its been over 1 meg, or 2,2k such thing like 16K. Especially nos carbon comp. And one each of the new flame proof guys... one completely open, one completely closed- and yeah,  goes without saying gotta have spare tubes. Probably the first thing to check in a situation like this. And I just suffered a completely bunk DR transformer on my TB/mstr build, that's why I ended up going with that crate OT. It Ohm'd out fine in regards to primary and secondary, but must have had a wire touching its own chassis or something. Never figured it out just worked wit
h another output transformer and wouldn't work with that one. You know I never thought about 1 of it's wires touching it's own  chassis so thanks for getting me to think about that. Oh yeah do your speaker jacks/ wires have to be isolated from the chassis? And isn't that the tone stack that you can turn off the bass and treble and crank the mids and you got the Vox thing?. Have you tried it with TMB all @ 5, because I think with manipulation of those tone controls you can actually make the volume disappear

[Ambugaton]

I have had problems with low volume due to a wrong connection to either pin 6 or pin 7 on the preamp sockets... This may be far out there and I have not looked at a princeton schematic. Who knows... maybe it would help. My personal experience was with a JTM45P and I had switched the connections with pin 6 and 7 which resulted in a functioning amp with low output. Trace all connections with the schematic and mark them off as you go.

As far as the OT primaries go they are the ones attached to pin 3 of each power tube socket. They are typically blue/brown and a crap shoot to connect them right the first time.

[daveneary]

I have checked all the resistors and they are all what they should be.

ARE what they are supposed to be or are labelled what they should be? 

I've found pots that were pretty far off stated value.

Also, virtually every one of us has put a 6.8 ohm resistor where a 6.8K resistor is supposed to be or vice versa.

Can you swap one of the other 12AX7s for the one in the V4 spot?  I ask because it sounds like the problem comes after the reverb circuit return.  What tubes are you using?

Respectfully,
Chip

i have checked the resistors with a meter.  i checked the range for the pots and used a meter during the construction also. The pots were within 10 percent of range.

Valves are all JJ.

i will swap the 4th valve and check the connections.

thanks for your help.

Dave

[EL34]

Have you done this yet?
http://el34world.com/Forum/index.php?topic=17701.0


This is important for me and other here to know if this has been completed as described on the page above

[daveneary]

i will try that this evening. i did something similar last night scribling out each connection as i checked. is the chop stick so you can do it with power on ?
Dave

[EL34]

No, power off and unplugged


Use a wooden chopstick if you have to

You are double checking every jumper wire on the board, every wire leaving the board and every part value

[daveneary]

If you're still having problems, maybe try checking the input jacks and speaker jacks and do you have good unpainted star washers on them,  they have to ground to the chassis and in some cases not ground, and things not being tight can give you low volume. Also wrong value pots. If it were me I would  probably probe it, see where the volume dropped out, push on it with chopsticks see if you get a volume rise. Heck,  smack  it with a hammer.  Used to work for Boogie. I don't know if they still do it, but its worked  for me ( procede at your own level of caution on that one)  I'm sure someone will chime in pretty quick with some more helpful information, and congratulations on getting this far on your first build. Also,
   at the bottom  of Slukey's page push the word '' this", it links to a new thing they've got, might help now but should surely help in the future. Cheers!

Spot on that man. I resoldered the speaker jack removing the feed back and ext speaker connections. very loud now.

thanks for your help. adding the feedback connection sends the amp oscilating...but at least I now have a usable amp.

regards.
Dave.

[sluckey]

adding the feedback connection sends the amp oscilating...but at least I now have a usable amp.

Swap (reverse) the OT primary leads on pin 3 of the output tubes to fix the squeal.

And if your ext spkr jack has the switch lug connected to the ground lug just cut it.

[Willabe]

I resoldered the speaker jack removing the feed back and ext speaker connections. very loud now.

Were both speaker jacks 'switching' jacks with the switch wired to ground when no jack is inserted? With 2 speaker jacks only 1 jack should be a switching jack (or you can leave the 2nd jacks switch not connected), that's the jack you plug into when only using 1 jack. If you have a switching type jack that's switch is wired to ground, with nothing inserted the switch is closed and will short the output to ground = no volume.

adding the feedback connection sends the amp oscilating...but at least I now have a usable amp.

Try flip/flopping the 2 OT primary leads that go to the power tubes plate.

Edit; Sluckey beat me to it, again. 

[daveneary]

Yes both switching. I will not use an ext speaker and will just cut the switch as described. is the switching of the ot wires about changing the phase to get _ive feedback rather than +ive ?

Regards.
Dave

[sluckey]

Yes both switching. I will not use an ext speaker and will just cut the switch as described. is the switching of the ot wires about changing the phase to get _ive feedback rather than +ive ?

Regards.
Dave

yes

[daveneary]

That's sorted the feedback. Thanks for the tip. 

I think somewhere along the way I've knackered the reverb. When I first got the +ive feedback ..I pulled the speaker cable in a panic (I  know this could blow the OT). i only realised when I heard a crackle. The OT is fine but could it have blown the reverb transformer? If I tap the reverb tank I get the rattle in the speaker so I think its on the reverb send side of things.
Any thoughts ?.

regards.
Dave.

[sluckey]

If I tap the reverb tank I get the rattle in the speaker so I think its on the reverb send side of things.

I agree.

[daveneary]

there was a wire touching the chassis on the reverb drive side. So all working now thanks for the help.

 I have made the published mods to slow and deepen the tremolo and also connected the phase inverter to the next highest point on the voltage divide.   These both improved the sound and usability of amp to my ears.

i will do the feedback mod to vary he resistance and use a humdinger mod to try to reduce the hum.

i have not managed to get the amp to distort much other than at the 9/10 level ..is this normal. ?
Dave

[HotBluePlates]

More or less. Princeton Reverbs are not buzz-bombs.