Seemingly high voltages on a Mojotone Deluxe reverb kit build

[ARhodes]

I just finished and tested my deluxe Reverb kit build.  The kit is from Mojotone.  I am measuring voltages that exceed the nominal specs on the mojotone schematic.

The AC supply measured 120VAC.

The transformer HV output measures 355-0-355 off standby, at idle, with volumes set to zero.

Terminal A voltage is 460 with the nominal specified as 427
Terminal B voltage is 457 with the nominal specified as 425
Terminal C voltage is 363 with the nominal not specified
Terminal D voltage is 303 with the nominal  not specified

Some Plate voltages:
V8 is 460 (nominal 427)
V7 is 459 (nominal 427)

V1 id 209/204 (nominal 196)
V2 is 194/194 (nominal 195)
V3 is 450/459 (nominal 400)
V4 is 204/207 (nominal 190)
I did not measure V5, V6 at this time

The rectifier filament is 5.13
The other tube filaments are 6.31

So, some of these plate voltages are over the maximum specified for he respective tubes.  Is this going to be a problem?

The amp plays reasonably well.  The reverb and tremolo seem OK, though I would like to slow down the tremolo a bit.

I have not played a DR in the past so I cannot comment on the quality of the sound compared to "normal" yet.  Because the plate voltages are "too high", I have minimized the time the amp is powered up.

So, are these measured voltages OK?  They definitely do not match the numbers indicated by Mojotone for this kit.

-Tony

[tubeswell]

JJ6V6S can cope with 460 on the plate

[dwinstonwood]

What's interesting is that the Fender Deluxe Reverb schematic shows 330-0-330:
https://el34world.com/charts/Schematics/files/Fender/Fender_deluxe_reverb_ab763_schematic.pdf

Dividing 330 / 355 = 0.93
460 x .93 = 427ish
457 x .93 = 425ish

But, like tubeswell posted, the JJ 6V6S's have a limiting value of 500V:
https://www.jj-electronic.com/en/6v6s

[ARhodes]

Ok, thanks for that info.  But I think V3 is also running over its specified voltage too.   

Are there other ways to get the voltage to a safer level.

Is the plate voltage a problem by itself, or is it just that plate POWER is the limiting factor.  So, if the plate current is adjusted accordingly can the high
Plate voltage be acceptable?

[sluckey]

How is the bias set? What is the voltage on pin 5 of the 6V6s? What is the plate current?

I ask because if your amp is biased cold then the plate voltage will be high. Biasing hotter will cause the plate voltages to drop.

So, give us more info.

[ARhodes]

Well, despite having measured the plate voltage SEVERAL times, this time it measures lower.  Both V7 and V8 have 448 volts on the plates!  I cannot explain it because I checked it several times before.

The bias voltage on Pins 1&5 is -45.4 on V7, and -46.2 on V8 (presumably because the 5% 220K resistors are not very closely matched?

The cathode current (measured using 1R resistors to ground) are 18.2mA and 17.0mA on V7 and 8 respectively.

The bias adjuster pot is annoyingly critical.  The bias voltage change causes the plate current to jump from about 18.2 to 19.4mA with essentially no rotational motion.

I will attach the schematic, layout, and the photos.

[ARhodes]

more pix

[ARhodes]

more

[ARhodes]

more

[ARhodes]

again

[ARhodes]

more

[ARhodes]

hopefully it will allow me to post the schematic and layout in one post....

[ARhodes]

The plate voltage of V3 is definitely still just about 450.  I will get back to you on an estimate of the cathode current.

While the amp seems to sound good, at idle, volume set to zero, there is a faint whine: sounds like a tea kettle whistle.  Faint.  If there is any significant background noise (like my Tektronix 2430 oscilloscope fan), then I have to get very close to the speaker to hear it.  At first I thought the whine was from the kitchen, and I was faintly hearing it in the workshop.

The whine does not seem to change with the volume knobs.  The hiss and crackle (also pretty faint) does change with volume.  I will be changing the preamp anode 100K resistors to metal film types instead of the carbon composite types.

One of the nice things is that there is NO (noticeable) hum.

I can supply more pix as needed.  I can show some oscilloscope captures

[sluckey]

The bias voltage on Pins 1&5 is -45.4 on V7, and -46.2 on V8

The cathode current (measured using 1R resistors to ground) are 18.2mA and 17.0mA on V7 and 8 respectively.

That's biased a bit cold. Crank the bias pot until you have about 22mA to 24mA. Plate voltage should drop. Pin 5 will probably be about -36V. The amp may even sound better.

[dwinstonwood]

...Both V7 and V8 have 448 volts on the plates...

...The cathode current (measured using 1R resistors to ground) are 18.2mA and 17.0mA on V7 and 8 respectively.

That's probably less that 60% plate dissipation. That's a bit on the cool side, and as sluckey said, you can bias the tubes hotter - say, 22mA, or so for around 70% dissipation - to bring the plate to cathode voltage down even more.

[dwinstonwood]

Sorry, we posted at the same time!

[ARhodes]

OK, I can bias it hotter. 

But V3 is still running over the rated voltage.  I have not checked V5 and V6 yet.

And there is a faint "squeal".  The oscilloscope pix are below.  It appears to be a 24KHZ signal, a ~3KHz signal, and around a 111Hz signal in there.

What should I do to narrow down the source?

[sluckey]

OK, I can bias it hotter. 

But V3 is still running over the rated voltage.

What do you call the rated voltage?

When you bias hotter, all B+ voltages throughout the amp will drop due to the heavier current load on the PT.

[ARhodes]

Well, the 12AT7 specs say 300V is the max for the plate.  The Fender schematic calls out 410V, as does Mojotone.  This tube is fed from B thorugh the reverb transformer.  My B is measured at 457 with the nominal voltage speced at 425. 

I am reading 450 at the plates of V3 on my amp.

I realize that Fender intentionally exceeded the specs of the 12AT7 in this location, and seems to have had adequate longevity.  However, at an extra 40 volts (110% of Fender-specified and 150% of the tube ratings), I am a little concerned. 

So, if I am thinking that if I can knock off 30 volts from the AC output of the transformer (and not affect the filament outputs) then that is good.

Would a 5U4 rectifier drop the voltage in a useful way?

[mresistor]

Would a 5U4 rectifier drop the voltage in a useful way?

Sluckey has already advised decreasing the bias voltage to increase plate current which lowers overall amplifier high voltage due to loading the ps.  Try that first.  Bias should be set to 70% max plate dissipation in a DR and then adjusted according to taste.  Merlin has even stated that fixed bias P/P amps can be safely run at 80% max dissipation. Not that I would do that for heavy duty use but it should allay your fears if any.


Yes a 5U4 would decrease voltages in the amplifier but you have to make sure your PT 5v winding can handle the increased amperage.  The 5U4s draw 3 amps.


I see the specified mojo DR power transformer has a 5v winding rated at 3 amps.  MOJO761EX

[jeff]

I just finished and tested my deluxe Reverb kit build.  The kit is from Mojotone.  I am measuring voltages that exceed the nominal specs on the mojotone schematic.

The AC supply measured 120VAC.

The transformer HV output measures 355-0-355 off standby, at idle, with volumes set to zero.

Terminal A voltage is 460 with the nominal specified as 427
Terminal B voltage is 457 with the nominal specified as 425
Terminal C voltage is 363 with the nominal not specified
Terminal D voltage is 303 with the nominal  not specified

Some Plate voltages:
V8 is 460 (nominal 427)
V7 is 459 (nominal 427)

V1 id 209/204 (nominal 196)
V2 is 194/194 (nominal 195)
V3 is 450/459 (nominal 400)
V4 is 204/207 (nominal 190)
I did not measure V5, V6 at this time

The rectifier filament is 5.13
The other tube filaments are 6.31

So, some of these plate voltages are over the maximum specified for he respective tubes.  Is this going to be a problem?

The amp plays reasonably well.  The reverb and tremolo seem OK, though I would like to slow down the tremolo a bit.

I have not played a DR in the past so I cannot comment on the quality of the sound compared to "normal" yet.  Because the plate voltages are "too high", I have minimized the time the amp is powered up.

So, are these measured voltages OK?  They definitely do not match the numbers indicated by Mojotone for this kit.

-Tony

Rect tube?
Had a real prob with 5Y3's being 'too strong'
(maybe sold a "close enough" rect tube rebranded as a 5Y3)
I was getting 420V where I shoulda been getting 360
Dropped in different 5Y3 everything was A O.K.


Trans voltage seems to match Mojo's schemos so maybe try different rect tube. Only thing I could guess

[tubeswell]

Did you re-bias yet?

[HotBluePlates]

What's interesting is that the Fender Deluxe Reverb schematic shows 330-0-330:
https://el34world.com/charts/Schematics/files/Fender/Fender_deluxe_reverb_ab763_schematic.pdf

... V3 is still running over the rated voltage. ...

The schematic voltages weren't 100% correct when they were first drawn, and Fender raised voltages over time by tinkering their power transformers.

...  I am measuring voltages that exceed the nominal specs on the mojotone schematic.

The AC supply measured 120VAC.

The transformer HV output measures 355-0-355 off standby, at idle, with volumes set to zero.
...
Some Plate voltages:
V8 is 460 (nominal 427)
V7 is 459 (nominal 427)
...
I have not played a DR in the past so I cannot comment on the quality of the sound compared to "normal" yet.  Because the plate voltages are "too high", I have minimized the time the amp is powered up.
...

Try this:
Turn the amp off and measure the resistance between the 2 power transformer Red wires that connect to the rectifier tube socket.  What do you get?

I used to own a 1964 Deluxe Reverb.
   Although the tube chart says "117v" I measured 6.3v on the heater wiring when applying 120vac wall voltage.
   The power transformer was a 125P23B, the first of several transformer models used in the DR.
   I measured 220.4Ω across the entire high voltage winding (Red wires), and got 334-0-334v output with that 120vac input.
   I got 394v dc at the 6V6 plates when drawing 24.9mA of plate current, using a British GZ34 rectifier.

I compared measurements with someone owning a 1965 Deluxe Reverb with the 125P23C power transformer:
   They measured 122.3Ω across their high voltage winding (Red wires).
   Their PT output was 348-0-348v.
   They had 442vdc at the output of the rectifier (slightly less at the output tube plates).


If your PT winding resistance is very low, that's great for keeping it cool but likely contributes to higher voltage output.  You could add some 50Ω 3w "build-out resistors" between the Red wires and Pins 4 & 6 of the rectifier socket.  That will tend to lessen B+ voltage and increase sag/softness.

[ARhodes]

Thanks for the info.  I will check the resistance.

-Tony

[ARhodes]

Well, I tried the simple expedient of removing the GZ34 (5AR4 equivalent?) and installed a 5U4 rectifier.  That dropped the voltages by a reasonable amount.  6V6 Plate voltage is 439 at about 19.5mA.  I calculate this as a dissipation of 8.56 watts, or about 61% of 14watts.  I suppose I can go up to 70%, and I will try it and see if I can tell whether the sound is any "better".

It seems as if this may be a decent work-around.  Not really a complete solution, but maybe the best alternative for this Mojotone transformer.

I still have yet to measure the transformer resistance. 

-Tony

[HotBluePlates]

Well, the 12AT7 specs say 300V is the max for the plate.  The Fender schematic calls out 410V, as does Mojotone.  This tube is fed from B thorugh the reverb transformer.  My B is measured at 457 with the nominal voltage speced at 425. ...

The rating doesn't matter so much (which is not something I'd usually say).  There were commercial hi-fi products from the 1950s that placed almost 500v across a 12AX7, which is also rated for only 300v.

I think Fender tried to maximize the power output of the 12AT7 driving the reverb transformer, and really cranked up the voltage for that stage.  You could feed the 12AT7 from the next down-stream filter cap, to apply less voltage to the transformer & tube.

And if you are a "touch of reverb" guy, you could replace the 100kΩ linear Reverb pot with a 100kΩ audio taper.  The amount of reverb you get at "3" now will shift to be at "7" on the audio taper pot.

[mresistor]

Hey Tony   I'm running a Single Channel AB763 in Deluxe Reverb trim using a 5U4GB recto.  The later Fender amps used them alot.  Not a big deal if your PT has 5V 3A winding. The yield a little more sag and are cheaper... or were when I picked up a bunch.