Princeton Reverb Plate Voltages

[MSVguitar]

I completed a scratch build of a Princeton Reverb AA1164.  The amp sounds good and the reverb and tremolo work.  I checked all the plate voltages and found that they were 20 to 40 volts low compared to the schematic.  I changed the two 18K dropping resistors in the power supply to 15K and 16K and some of the plate voltages went up and others went down.  I posted the test voltages and a picture of the build.  I know that the voltages will drop once the power and preamp tubes are installed, the unloaded B+ is 421 VDC with a wall voltage of 120 VAC.  Are these voltages OK?

Also, the amp sounds good but I think it lacks some high end.  I installed a Weber 10F150T speaker.  Any thoughts on what to look for?

Thanks,
Mark

[bmccowan]

Nice build - looks like neat work. Your voltages look good in relation to each other. That's important. I also think those voltages will result in a fine sounding amp. Why is it less than spec? Hard to say. What PT did you use? Fender schematics are pretty unreliable in terms of total voltage.
Lacking high end? Or is it muddy bass? I find PRs tend to have flabby muddy bass, especially if the NFB is not connected or values changed. I ended up changing some of the tone cap values in mine. Good speaker choice, so not likely a problem. What OT is in there?

[MSVguitar]

The 'spec' are the voltages on the fender schematic.  I used a mojotone (heyboer) PT with a Mercury Magnetics Tone Clone OT.  The bass is not muddy, it actually is pretty tight.   It is lacking high end especially compared to my 65 Deluxe Reverb.  Probably not a good comparison because they are different amps. 

When I use my Telecaster on the bridge pickup (Lollar Vintage T), I need to set the treble at 7-8.  The NFB is connected with a 2.7K resistor.  Maybe my expectations just aren't in line with what a PR sounds like.

[AlNewman]

Which plate voltages are 40V out?  The power tubes?  I believe if you are going to change the 18k resistors between the 2 22u pre amp caps, they need to be balanced, as they are balance resistors between 2 parallel caps.  Somebody will probably chime in if I'm wrong.

[glass54]

Hi Mark
If you were to read "NOTICES" on Fender Schematic AA1164, Item (1) it indicates all voltages read with VTM with respect to Ground and tolerance of +/-20%
The Fender +HT (out output of Rectifier/1st PS cap also CT of output Tx) is +420V. Your Design HT is 420V, measured +368V, therefore approx -13%. That's well in Ballpark 
Just for interests sake, What is the actual AC rating of HT Tx winding?
By the way, VERY NICE BUILD 
Regards
Mirek 

[pdf64]

Was the 120V input mains voltage an actual measurement, or was it assumed?

What V AC do you measure across the 5V and 6.3V heater wires from the mains transformer?

The voltage of the HT winding of ghe mains transformer is almost 100V lower than we might expect, 584 rather than 680V. However, that voltage will depend on how much current is being drawn. What anode or cathode current are the output valves drawing at idle?

Are you sure you’ve got the correct mains transformer for a Princeton Reverb?

Regarding the treble control setting, the exact taper of the pot used will determine how much you get at a particular point on the track. To find that out disconnect the pot and set it to halfway, the mid point of its angular rotation. Then measure the resistances between the hot track end and wiper, and the cold track and and wiper.

[bmccowan]

Further - the Mojotone schematic for their PR kit publishes 386V on the power tube plates - you are about 5% below that. Close enough for rock and roll. PT choices are getting slimmer, with mfgs covering several amps with a single model, and Fender used different trannies without changing schematic #s. 6V6s will handle 410V but that doesn't mean they are happiest there.
Brightness - in general I think PRs are as bright as DRs, so maybe there is something to figure out there.

[HotBluePlates]

The 'spec' are the voltages on the fender schematic.  ...

Until recently I owned a number of blackface Fender amps from the mid-1960s.  None of their voltages exactly match the schematic, even when wall voltage is corrected.  So we can't be too concerned with matching a schematic.

...  It is lacking high end especially compared to my 65 Deluxe Reverb.  Probably not a good comparison because they are different amps.  ...

The Deluxe Reverb has a "bright cap you can't turn off."  To me it's annoyingly bright & crispy, to the point I always played my 1964 Deluxe Reverb with the Volume at/above "7" (then turned the guitar volume way low to get the loudness I wanted).

The speaker may also play a role here:  plug your DR's speaker into your PR.  You will likely need to place the amps back-to-back to get the speaker cord to reach.

[shooter]

"bright cap you can't turn off."

some snips, wire, solder, you can turn anything off, on is a little trickier 

[MSVguitar]

Which plate voltages are 40V out?  The power tubes?  I believe if you are going to change the 18k resistors between the 2 22u pre amp caps, they need to be balanced, as they are balance resistors between 2 parallel caps.  Somebody will probably chime in if I'm wrong.

V4 Pin 1 measured 223.8V
Based upon the response from everyone on the forum, I'm not concerned anymore about the voltages.  As for the 18K resistors, they are dropping resistors for the preamp filter caps and don't need to be balanced.  If they were in series like the reservoir caps on a Bassman, then they need to be the same.

[MSVguitar]

Just for interests sake, What is the actual AC rating of HT Tx winding?

Hi Mirek,
The AC rating of the HT for the transformer is 660v.

Thanks for your feedback!

Mark

[MSVguitar]

Was the 120V input mains voltage an actual measurement, or was it assumed?

What V AC do you measure across the 5V and 6.3V heater wires from the mains transformer?

The voltage of the HT winding of ghe mains transformer is almost 100V lower than we might expect, 584 rather than 680V. However, that voltage will depend on how much current is being drawn. What anode or cathode current are the output valves drawing at idle?

Are you sure you’ve got the correct mains transformer for a Princeton Reverb?

Regarding the treble control setting, the exact taper of the pot used will determine how much you get at a particular point on the track. To find that out disconnect the pot and set it to halfway, the mid point of its angular rotation. Then measure the resistances between the hot track end and wiper, and the cold track and and wiper.

The Mains voltage was set and measured on my variac.

The transformer secondary is rated for 660VAC.  As for the output tubes, they are drawing 18ma each at idle. 

The treble pot is a 250 K audio taper.  I did check the pot and it measured 238K total. The wiper to the cold track measured 19K at the mid point of it's rotation.  I didn't think of that and I think I will try a 250K pot with a 'J' taper.

Thanks for your feedback 

[AlNewman]

 As for the output tubes, they are drawing 18ma each at idle. 

 

And what's your B+ again?

[glass54]

mark
The Power transformer still puzzles me. Forget your "rated" values for now.
Standard Princeton replacement Hammond Transformer (Hammond 291AEX for example)will have secondaries of 650VCT @ 100mA and 550VCT @ 100mA 6.3VCT @ 2.25A 5V @ 3A. I suggest that if your transformer is "correct" you should see AC voltage to Rectifier somewhere in the 315 to 330 VAC range (with tubes in) as two 6V6's running @ 18mA DC and 4x 12A*7 should not be a huge load on HT secondary.
Please measure the AC HT no rectifier fitted, then Rectifier fitted to give us a better picture.
Silly question. Do you have a Manufacturers name and Model on the Transformer?
Regards
Mirek

[HotBluePlates]

"bright cap you can't turn off."

some snips, wire, solder, you can turn anything off, on is a little trickier 

Know your audience.

If the Listener did not know about the Deluxe Reverb Vibrato channel bright cap being the reason it's so "crispy" then they are also not (yet) the type to open the amp & cut out that bright cap.

[bmccowan]

Know your audience.

If the Listener did not know about the Deluxe Reverb Vibrato channel bright cap being the reason it's so "crispy" then they are also not (yet) the type to open the amp & cut out that bright cap.

Unless of course they are the type that just scratch built a nice PR - like MSVguitar did. I'm guessin' he could snip out a bright cap, or even plug into the normal channel.

[jordan86]

A bright cap is an easy experiment. I imagine it’s that treble pot though. My Princeton reverb build sounds best with the treble in the 7-8 range to my ear. Maybe something with that circuit.

[MSVguitar]

Please measure the AC HT no rectifier fitted, then Rectifier fitted to give us a better picture.
Silly question. Do you have a Manufacturers name and Model on the Transformer?

Mirek,

The AC HT without the rectifier is 310/310 vac
with the rectifier it is 309/309 vac.
The input to the primary is 120 vac that I set on my variac and measured on the chassis.

The transformer was purchased from mojotone and I believe is made by Heyboer.  I attached the transformer description and specs from the mojotone website.
The specs on the transformer shows 330/330 vac.  I am also helping a friend with the same build and using the same parts.  I checked his transformer and it measures the same as mine.
Looks like you helped me find the issue.

Regards,
Mark

[MSVguitar]

 As for the output tubes, they are drawing 18ma each at idle. 

 

And what's your B+ again?

366 vdc

[MSVguitar]

Know your audience.

If the Listener did not know about the Deluxe Reverb Vibrato channel bright cap being the reason it's so "crispy" then they are also not (yet) the type to open the amp & cut out that bright cap.

Unless of course they are the type that just scratch built a nice PR - like MSVguitar did. I'm guessin' he could snip out a bright cap, or even plug into the normal channel.

Thank you bmccowan!  I thought about clipping the bright cap, but I like how the amp sounds with my es175.

[glass54]

Now to really get boring, with 309VAC into rectifier I would expect around 420VDC (allowing for a drop of 15V DC across GZ34)
(Would that be reasonable ? PRR, Sluckey, pdf64, others please comment if you wish)
If you were using a 5U4 Rectifier, I would expect at least 385VDC.
If you were to try 2 Diodes (1N4007) I would expect around 435VDC.
The Mojo760 just doesn't add up for me 
Do you have another transformer with 300 to 330VAC available? (To "patch" in the AC HT)
(My experience only relates to Hammond or genuine Fender transformers)
....The plot thickens 
Regards
Mirek

[pdf64]

Now to really get boring, with 309VAC into rectifier I would expect around 420VDC (allowing for a drop of 15V DC across GZ34) …

See the OP

the unloaded B+ is 421 VDC with a wall voltage of 120 VAC

so I think the 309V AC is with just the rectifier valve fitted.

I share your concern with the mains transformer though, I think with just the rectifier the V AC should be 5 to 10% higher than the voltage at the rated load. So around 350V, rather than 309V.

[sluckey]

The Mojo760 just doesn't add up for me 

IMO the Mojo760 is too lightweight to supply the voltages seen on the Fender Princeton Reverb schematic. It's only rated 330-0-330 @ 75mA. The load current from the PR is loading the "no load B+" down considerably. That's why the math doesn't add up.

OTOH, the Hammond 290AX (replacement for 125P1B) is rated 325-0-325 @ 100mA. This is beefier than the mojo760, so the B+ will not be loaded down as much. I would expect over 400V with the Hammond 290AX.

[pdf64]

… the Mojo760 is too lightweight to supply the voltages seen on the Fender Princeton Reverb schematic. It's only rated 330-0-330 @ 75mA. The load current from the PR is loading the "no load B+" down considerably. That's why the math doesn't add up.
…

Perhaps so if the amp is trying to achieve high power output.

But it doesn’t seem to explain the totally unloaded, no valves other than rectifier, HT winding voltage being lower than 330V. When it should be higher.

[bmccowan]

Two things I note:
When searching around the various vendor sites, specs for "PR reverb appropriate" PTs are all over the map.
And, Mojo has a disclaimer that they are not responsible for the PT specs or wiring diagrams. That's comforting.

[MSVguitar]

… the Mojo760 is too lightweight to supply the voltages seen on the Fender Princeton Reverb schematic. It's only rated 330-0-330 @ 75mA. The load current from the PR is loading the "no load B+" down considerably. That's why the math doesn't add up.
…

Perhaps so if the amp is trying to achieve high power output.

But it doesn’t seem to explain the totally unloaded, no valves other than rectifier, HT winding voltage being lower than 330V. When it should be higher.

I sent mojotone tech support the information about the transformer no load B+ of 309 VAC last Friday night.  I have yet to get a response.  This issue is not a one of a kind incident, I bought 2 transformers from them, one for me and another for a friend who is building the same amplifier.  Both transformers have the same issue.
On Saturday I ordered 2 Hammond 290AX transformers and they will arrive Friday afternoon.  Once they are installed, I will make the voltage checks and post the results.

[glass54]

It will be VERY Interesting to see what you get with the Hammond 290AX, really good A-B comparison 
Design HT Winding "Secondary 1: 650/550Vct @ 100 mA"
....Continuing with my puzzlement, (is that a word?) the Mojotone 760 has very large heater ratings for a smallish transformer (4A each!)
Regards
Mirek

[MSVguitar]

It will be VERY Interesting to see what you get with the Hammond 290AX, really good A-B comparison 
Design HT Winding "Secondary 1: 650/550Vct @ 100 mA"
....Continuing with my puzzlement, (is that a word?) the Mojotone 760 has very large heater ratings for a smallish transformer (4A each!)
Regards
Mirek

The Hammond PT fixed the issue of low plate voltages.  See the attached pdf.
The Hammond PT unloaded 2ndary voltage is 351 VAC compared to 309 VAC for the mojotone.  With the rectifier installed the Hammond 2ndary voltage is 346 VAC with 482 VDC on the GZ34 pins 2&8.  With all the tubes installed the plate voltages are much closer to the voltages on the schematic.
The output tubes idle current is 23 ma with a plate voltage of 412 VDC.  I may go back and drop the idle current a little, but the amp sounds really good.

BTW, I checked the output power with both transformers and the mojotone PT measured 11 watts versus 14 watts for the 290AX.

Still no feedback from mojotone tech support.  I don't expect that I would be able to return the transformers so I will look for a project to use them on.  Maybe a Vibro Champ build.

Thank you to all for your feedback!

Best Regards,
Mark

[glass54]

 
...and thanks for sharing.
Regards
Mirek