I just lit up a new Princeton Reverb build and I've got some issues.
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My B+ is lower than expected. I've looked at GZ34 data sheets that suggest a ballpark GZ34 rectifier voltage increase stands around "1.3". With a 325 VAC HT secondary, that would estimate a 422.5 VDC rectified voltage, which is nowhere near what I've got.
I used a Hammond 291AEX PT that has 325-0-325 on the HT secondary (tests 322-324 with 121 wall AC) with a GZ34 rectifier. The B+ on the rectifier socket pin 8 tested incredibly low with a Mullard GZ34 (340 VDC vs. 390 target), so I put in a Gold Lion to try another (359 VDC). The previous PR I built tested 328 on the HT2, and I got about 390 at the first node with a JJ GZ34 which I'm fresh out of. Are GZ34 rectifier tubes just that variable? A few are on the way.
This is the first PT I've used with dual primary windings. I wired both windings in parallel to the mains.
Are these D.C. Voltages you report with the output tubes in the amp?
If so then what voltages do you read with them out of the amp, also if you set your meter for ac volts what ripple voltage do you read on the D.C. Voltage at the first filter node?
Please also report what - D.C. You have coming into pin 5 of each output tube.
When I die, I want to go like my Grandfather did, peacefully in his sleep.
Not screaming like the passengers in his car!
Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
I'll get you a full voltage report. Last night I played the amp and measured 308 VDC on each HT secondary at the rectifier socket. I just now disconnected the HT secondary and measured the voltages unloaded at 340 VDC per side. Is the circuit, then, drawing too much current for some reason and causing that large of a voltage drop? I'm going to move the HT center tap also, see if that will help.
I moved my HT center tap to an isolated ground, and that seems to have brought my HT secondary voltages to 325 VAC per side. I confirmed with Hammond Mfg. that both 120V windings should be wired in parallel to avoid burning up the winding. This was personally new and much-appreciated information.
I took voltages from the amp and found the following:
All preamp pin voltages are in the ballpark considering the lower B+, all scaled lower.
I can play this amp, it just doesn't sound anywhere near as good as the first Princeton Reverb I built. I'm playing this new chassis through the first PR's combo cab and speaker for reference.
Last edited by ViperDoc on Thu Nov 19, 2020 3:32 pm, edited 1 time in total.
You can't possibly have that high a negative voltage reading on pin 5 of the output tubes, you must be misreading a decimal point because they would then be so deep into cut off that they would not amp up anything!
A voltage of -30 is more like what you should have to work well with your plate and screen voltage .
Ps, looking close at the pictures of the amps output tube sockets I am seing the blue and brown wires from each end of the OT on pins 4, not on pins 3 where they should be, am I wrong about this, if not then that would explain a lot about what's going on here!
Pins 3 are the plate and pins 4 are the screen, the screens are fed off of the second power supply filter if the build is right.
When I die, I want to go like my Grandfather did, peacefully in his sleep.
Not screaming like the passengers in his car!
Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
Stevem wrote: ↑Thu Nov 19, 2020 11:49 am
The arcing is normal and not a cause for concern!
You can't possibly have that high a negative voltage reading on pin 5 of the output tubes, you must be misreading a decimal point because they would then be so deep into cut off that they would not amp up anything!
A voltage of -30 is more like what you should have to work well with your plate and screen voltage .
Ps, looking close at the pictures of the amps output tube sockets I am seing the blue and brown wires from each end of the OT on pins 4, not on pins 3 where they should be, am I wrong about this, if not then that would explain a lot about what's going on here!
Pins 3 are the plate and pins 4 are the screen, the screens are fed off of the second power supply filter if the build is right.
My apologies, I fat-fingered my pin “5”s. Those readings are off of pin 6.
Regarding the arcing from power rail to DMM probe, we’re talking like the anger of Zeus. Never seen that before!
Ok, but just so you know 6V6s tubes these days have no pin 6 or pin 1 so many times these lugs on the sockets are then used a terminal points to mount other things like the 470 ohm screen resistors on your sockets.
When I die, I want to go like my Grandfather did, peacefully in his sleep.
Not screaming like the passengers in his car!
Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
Stevem wrote: ↑Thu Nov 19, 2020 3:41 pm
Ok, but just so you know 6V6s tubes these days have no pin 6 or pin 1 so many times these lugs on the sockets are then used a terminal points to mount other things like the 470 ohm screen resistors on your sockets.
Correct. That’s all I’m using that for. Perhaps pin5 voltages would be more useful!
My guess is that (for unknown reasons) the bias voltage is insufficient (not even -12V), hence the power tubes are drawing extremely high idle current, thus pulling down the HT voltages.
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