OT question

[mort]

With a brand new Mercury OT for a Princeton Reverb, it's reading 219.5r from red to brown and 191.4r from red to blue.


It's a fresh build, just powered up today and has low output but sounds nice, and distorts unpleasantly at 8-10.

[Chuck H]

Well the Princeton Reverb isn't known for it's overdrive tone anyway. Aside from that, are there any differences from the Fender schematic? Whether there is or not an accurate schematic should be posted. A 28ohm difference in the windings doesn't seem out of the ordinary. I wouldn't be looking to the OT as a problem at this time.

Have you measured the output? Are the voltages consistent with what is typical for this design? etc.

[mort]

There are a few differences.


1 cap and 1 resistor in trem with values changed as per Robinette's mods.

switch added for choke vs 1k dropping resistor

3 position ohm selector added.




Otherwise it's stock.

Sorry for the wrinkled drawing. Voltages are in line with Rob's annotated schematic.


Untitled by Card Bucket, on Flickr

[Mick Bailey]

A difference in DCR on an output transformer primary is to be expected due to the method of construction. Remember, transformers operate on a turns ratio and the way they're wound is the first half of the primary is would over the core, the centre tap taken off, then the second half wound over the top. You can see the first half begins at a much smaller circumference than the second, so uses much less wire and consequently has a lower resistance. The only common OT fault that causes VERY low output and extreme unpleasant distortion is a shorted turn, which would be an exceptionally rare fault in a new OT. My though is your problem lies elsewhere but if you still have a reservation about your OT you could use R.G's 'neon trick' to test it.

[Chuck H]

AFAIK the Princeton Reverb has only about half the gain of the full AB763 type circuit. And the split load inverter doesn't contribute anything good to the overdrive charactersitics. So, missing from any clipped tone, compared to other Fender designs, would be the contribution from the long tailed pair inverter. Which is significant.

Bias could be critical in getting the best possible tone from the PR design. I would think a hotter bias would sound best. But of course you're limited by the plate voltage since these amps already run the tubes over the spec'd maximum.

I'm thinking you may have a cold bias condition right now. Bias should be checked the first time an amp is fired up. First with the amp in standby to see that there actually IS bias voltage so the tubes won't melt. And then again with the amp in play mode to measure current.

[mort]

AFAIK the Princeton Reverb has only about half the gain of the full AB763 type circuit. And the split load inverter doesn't contribute anything good to the overdrive characteristics. So, missing from any clipped tone, compared to other Fender designs, would be the contribution from the long tailed pair inverter. Which is significant.

Bias could be critical in getting the best possible tone from the PR design. I would think a hotter bias would sound best. But of course you're limited by the plate voltage since these amps already run the tubes over the spec'd maximum.

I'm thinking you may have a cold bias condition right now. Bias should be checked the first time an amp is fired up. First with the amp in standby to see that there actually IS bias voltage so the tubes won't melt. And then again with the amp in play mode to measure current.

It's funny you mention the possibility of no bias voltage. My only build error (known so far) was that I forgot to ground the negative bias supply and powered it up for less than a minute before seeing the unsoldered hole on the bias board and immediately shut it down. Already tried another pair of power tubes and still have the low output problem.

I usually reference Robinette's site but it won't load for some reason. Not sure if the site is down for some reason. I don't recall how to calculate bias from memory so I just tacked in a couple of 1r resistors on the cathodes and am reading 20mA on V5 and 25mA on V6.


The current voltages are present on both tubes, almost identically. (this is with used GT tubes, not the new JJ's)

396 plate
368 grid P4
-34.3 grid P5

To increase plate current I need a lower negative voltage, correct?


And to do that should the 100k bias resistor be raised to maybe a 110k or even a 150k?


Edit: I put the new JJ's back in and read across the 1r resistors again. .022vdc across each of them.

[Mick Bailey]

Your tubes are running a little cool (less than 10W dissipation). Increasing the 100K resistor is a good idea and you'll need to experiment. Double check you have the bias cap oriented correctly - positive to ground.

[Helmholtz]

Nothing wrong with your voltages and idle currents.
I don't think increasing idle currents would improve anything.

Do you have a scope?

[mort]

Your tubes are running a little cool (less than 10W dissipation). Increasing the 100K resistor is a good idea and you'll need to experiment. Double check you have the bias cap oriented correctly - positive to ground.

Yup. Double checked all the polarities including OT primaries, bias supply, all cathodes, etc. I'm guessing there's a simple mistake somewhere but haven't been able to spot it. Even unwired and double checked the volume pot and taper. Will try another volume pot just to rule it out. By the time it gets up to 8 it starts to sound kinda loud and full, but probably still lower output than should be. Haven't had a Princeton in a while so my frame of reference is fading but I do know it should start filling up a bedroom or living room at 5.

Nothing wrong with your voltages and idle currents.
I don't think increasing idle currents would improve anything.

Do you have a scope?

I do not. Maybe I should order one of those small cheap ones to have something to work with in that department, but never have used one.

[Helmholtz]

Your tubes are running a little cool (less than 10W dissipation).

10W would mean 83% plate dissipation. I'd consider that very hot for a fixed bias amp.
Even using the absolute max rating of 14W would result in over 70% dissipation.

[Mick Bailey]

JJ 6V6 (they call it a 6V6S) can be run much hotter and with higher plate voltages than a traditional 6V6. They're more inclined towards a 6L6 construction. The caveat though when setting an amp up with these tubes is that a subsequent owner may fit regular tubes which could give problems. My own preference is to bias an amp on the cool side, but that's from a repairers perspective. Apart from some tonal differences arising from distortion characteristics, when I've scoped amps under different bias conditions there's very little power output variation available.

Just a thought, make sure your impedance switch is wired up correctly and you don't have a mismatch between speaker and OT.

[mort]

It's in the 16 ohm position with a 1x12 16ohm cab.

[Helmholtz]

Did you check cathode and plate voltages of the small tubes?

[mort]

Did you check cathode and plate voltages of the small tubes?

Yup.

V1 has 152/1.179 & 152.2/1.161

V2 367/6.84 & 367/6.82

V3 155/1.101 & 152.3/1.188

V4 1st half oscillating and didn't have a shorted jack on hand. 2nd hald was 186 on the plate, 23.4 on the grid and 42.5 on the cathode.



I noticed that the negative bias voltage is the same or very close on either side of the 220k resistors. Is this normal because no/very little current is drawn across those resistors?

[Mick Bailey]

I noticed that the negative bias voltage is the same or very close on either side of the 220k resistors. Is this normal because no/very little current is drawn across those resistors?

Yes, that's normal - practically no current draw so no voltage drop.

Edit, just thinking about the low output and unpleasant distortion - this can sometimes be caused by a large asymmetry in the output stage. Check the AC signal to the 6V6 grids by running a fixed-frequency signal into the amp's input and gradually increasing the volume. Both grids should see approximately equal signals.