Mojo Princeton Reverb: output section turned frequency doubler

[fidot]

Howdy folks!

First post here :). Nice to meet ya all!

A just built (off of Mojo kit) Princeton Reverb is driving me nuts. Just started testing it, it sounded horrible, and, chasing the problem, narrowed it down to the output section.

It seems that I built a... frequency doubler!

Allow me to point you to the scope pictures at the bottom of this post. First one; "grid inputs"; is taken off of the coupling capacitors coming off of the PI towards power tubes (V5,V6). You don't see bias 'cause probes are AC coupled; bias is applied correctly (tested and scoped).

Second one; "One Input And Output"; has yellow probe intact; and green was moved over to the output jack (running into an 8 ohm load resistor box for test purposes).

When I saw that first; I so did not believe my lying eyes, I had to confirm it with a frequency meter: yep; it's doubling the frequency!

What's even more fun is - and I noticed this accidentally and recorded it - during startup; as the amp comes alive and currents start to flow; it seems that it "wants" to do the right thing - but then, negative half cycles of the output flip to be positive. See for yourselves:

I am driving it with 1KHz, 250mV peak to peak sine thru input 1. Volume pot is at about 2.5 (out of 10).

All voltages (checked with no input signal) are within the spec of Mojo's schematic (https://cdn.shopify.com/s/files/1/0604/9615/0624/files/Princeton_Reverb_SCH_2.pdf?v=1750277593) with the exception of the  Phase Inverter's cathode voltage. They show 15V; mine's in the neighborhood of 50. I thought that was a problem at first; but then realized that it probably is a typo: Fender's schematic is 50V, and my PI output looks good (at the very least; I do see two perfectly legitimate and phase inverted signals past PI's coupling caps). Just to be sure; I checked and re-checked all resistors in that circuit's node as well.

I have attached the schematic with my measured voltages, currents, and notes in the relevant section. Measurements were taken with input signal turned off.

Power transformer's primary sections' DC resistances measure as:

- V5 <-> center tap: 161R
- V6 <-> center tap: 144R
- Total (end to end): 305R. Isn't math a magical thing confirmable by multimeters? ;)

I did incorporate the following mods (all on the schematic):

- power valve's screen and grid stopper resistors
- power valve's adjustable bias (mine is currently set at the schematic's -40V, in my test I did for this post I recorded -41V); I have tried bringing the bias up to increase idle current up to ~30ma (don't want to go higher; afraid to redplate) with no effect on my problem.
- power valve current test resistors (1R, off of cathodes to ground)

For the sake of completeness; other mods also include:

- rectifier's backup diodes
- adjustable bias Rob Robinette style (pot + resistor divider)
- selectable NFB ( "standard" / "low" (5.6K feedback resistor) / "none", in this test; NFB was turned off)

I have tried a few things beyond checking and rechecking voltages; resistance values; currents; and wires going to the right pins:

- swapping power valves to no effect
- swapping a pair of power valves with another pair of 6V6s to no effect; both pairs work fine in a Fender 5E3 Tweed I have around.

I am frankly at a loss. I do not understand what / why / how this can be happening guys... any help would be greatly appreciated.

PS: If you wonder how does this sound.. the best I can describe it is imagine I hid myself in one of those green steel dumpsters, about 1/8-1/4 full with used diapers (for just the correct amount of dampening of course - just like they stuff foam into hifi speakers!), closed the lid, and was banging on a sheet of aluminum? Kinda like that... Another metaphor: "alien TV".

PPS: I might've inadvertently discovered a new guitar effect! Ha. But chords are not playable, you have to play one note at a time and have a long pause in between them... hmmm... I might be onto something ;)

[fidot]

[deleted]

[stratomaster]

10mA at 390V is incredibly cold. You may just have severe crossover distortion. What happens if you warm up the bias?

[fidot]

> What happens if you warm up the bias?

Nothing really. I wouldn't want to go beyond ~30ma (11.4 watts) not to fry the tubes.
The output wave changes it shape and amplitude a bit - as well as odd peaks/values relashionship to even peaks/values -  but it doesn't drastically change at all as I go from 10ma to 30ma.

Now; I just stumbled upon something I am not sure I can interpret properly.

Playing with variac - I drop B+ to about 200 volts; bias to -8.75; plate current ~20mA and I get the proper wave!

"Cooling" the bias (== lower bias V) shows - eventually - the same frequency doubling I get at regular B+ voltage.
"Hotting" the bias fixes it; and then just distorts the wave (but frequency looks good).

But!

If I go back to stock variac setting of 125 input volts; ~390 plate; and I try "heating" the bias up to ~30mA plate current - nope; still the same "double" wave. It changes shape a bit, but does not fix itself.

[pdf64]

My first thought was that the OT primary was wiring incorrectly, so kudos for eliminating that.
And you eliminated the possibility that the negative feedback was was actually positive (by disconnecting the feedback).
All I've got left is that there may be a bad decoupling at one or more HT / bias supply nodes, eg a failed cap or connection.

[tubeswell]

Besides what the others said, double check the load resistors for the cathodyne. Make sure they are both 56k and that one of them isn’t accidentally some lesser value (like 5k6 or something)


PS 1k5 grid stoppers on the 6V6s is pretty minimal. You can easily go to 47k on a tetrode or pentode without it affecting the tone. You can also try a 1M grid stopper on the cathodyne.

[fidot]

Thank you guys for the replies so far :) - I respond to some points further down in this message; but first...

I have a suspicion based on some more twiddling and - shameful to admit - chatting with AI chatbots.

Could it be the output transformer? Some strange, weird, non-detectable by DC examination failure mode? Here's why I think this.

I mentioned yesterday that I accidentally stumbled upon this: if I lower B+ to ~200V and set the bias to -8.75, my output looks okay. It even sounds okay-ish (but of course weak and .. 'farty'?).

The reason why I started lowering B+ voltage to begin with was to examine plates with my scope, which can't go past 300. I figured; if I can manage the thing to run at 200 - 250 (without the rectifier giving up), I can actually look.

Well, I did - deliberately and systematically - today.

What I *expect* to see is the push pull - ie, one is running, the other is cut off, and then they "swap".

Instead, I see what I show in "Both Plates" attachment. Notice they're in phase, and very jumpy (my camera captured several refreshes on the scope's screen)?

This to me looks like a smoking gun. I am in some weird single ended mode there - even though this signal is good frequency wise. Because everything else component wise is good, by elimination, it *must* be the OT...

Another reason I am suspecting OT is that at normal B+ voltages (390 B+; ~20mA idle current bias) there is no effect if I swap the OT's plate leads

Some weird gremlin aliens non-detectable with DC analysis of OT? That's my logic.

Whatdd'ya think?

--

Replying.

> Besides what the others said, double check the load resistors for the cathodyne. Make sure they are both 56k

I did; a number of times :( See attached PI.Resistors.Board.And.Schematic.jpg. What makes me happy with my PI is that the signals I see on the input grids of my power tubes are exactly right; perfectly out of phase between the pair; and offset exactly right; with correct bias applied...

> My first thought was that the OT primary was wiring incorrectly, so kudos for eliminating that.

I actually didn't mention that prior (I mentioned swapping tubes); but I did indeed try swapping OT primary plate leads (brown and blue) with no effect. From what I understand about push pull OT's, there must be significant effect with orientation change - which makes me blame the OT

> All I've got left is that there may be a bad decoupling at one or more HT / bias supply nodes, eg a failed cap or connection.

Checked for ripple.

B+: 390ish volts DC with 3.4V / 120Hz AC ripple
Bias supply: ~35-40 volts DC (depending on my setting) with 0.036V / 60Hz AC ripple.

All measurements referenced to amp's ground; same physical spot where everything on the power amp side is connected (couple terminal lugs on one of the power transformer's screws).

Both ripple frequencies make sense to me.... B+ is doubled due to the tube diode and bias is line ripple due to a single 1N4007 doing the rectifying.

[fidot]

... this might be my smoking gun.

Putting OT's primary (blue/brown) on the LCR meter (unsoldered from tubes' pins of course) gives me 9.5 millihenries.

My tranny is Mojo - they don't publish the spec sheet for it; but! Hammond does. Spec sheet for 1750E which is a drop-in for Fender Princeton Reverb OT shows 24H at 1KHz. (for their 1750E). I am 3 orders of magnitude off!

I have another OT - for a SE amp, 5k impedance / 25watt. Just for **its and giggles, hook that up to my LCR. That one is 15 henries.

So I think I am onto something here.

[fidot]

All is well in the world.

Replacing OT fixed this mess :) Aliens evicted; and guitar noises abound :)

[Merlin]

Putting OT's primary (blue/brown) on the LCR meter (unsoldered from tubes' pins of course) gives me 9.5 millihenries.

Sounds like it had a shorted turn then