Princeton reverb build in a trainwreck chassis

[martin manning]

So it’s still a cathodyne, but now it’s a fixed bias cathodyne. That’s also called the “Ampeg” mod.

[telentubes]

Thanks for that clarification. I'm going to go back and change the other post.

[norburybrook]

Right then..A good day of soldering and I'm done. I just need my ceramic caps and a fuse holder which should be here tomorrow and I can fire up.

It's a pretty untidy build I'm afraid but squeezing everything in to a trainwreck chassis has proved tricky and not ideal, working in reverse gave me some head scratching moments Anyway I've got it all in and wired, my only concern is the distance of the bias tremolo controls from the actual bias board/bias pot. I'm hoping the long cable run wont cause issues. Also I've added a bias pot how should I set this and where do I measure things?

I've used the basic mods on Rob Robbinettes page, grid stoppers, the bias pot, upgraded B+ resistors and Bias cap. I used my Dumble bus bar grounding method with this as it's served me well. everything on the board/pots tied in to a single ground at the input. The main filter cap and everything else grounded together at an 'output' ground.

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onward and upwards.....


M

[martin manning]

You could use the OT primary resistance method (slow), or the OT primary shunt method (scary), or maybe you can find a way to add some 1Ω current sense resistors somewhere?

[telentubes]

The PR can be a rats nest by nature. Yours looks pretty well organized.

[norburybrook]

You could use the OT primary resistance method (slow), or the OT primary shunt method (scary), or maybe you can find a way to add some 1Ω current sense resistors somewhere?

thanks Martin, I've removed the Cathode grounds and inserted 1R resistors. I'm just waiting for the postman with my SM/Ceramic caps.



M

[norburybrook]

Ok finished wiring. Put valves in and started up on the light bulb limiter, no issues.

Fired up normally and the 6V6's are red plating regardless where I set the bias pot. I have a 25k lin pot with a 10k resistor.

I have no sound, there is a hum.

Can i leave the rectifier valve in and remove the others to take voltage readings?

M

[martin manning]

Yes. To be very thorough, you could measure AC voltages with the rectifier removed, and note that you can measure the bias voltage in that configuration.

[sluckey]

I think I see pin 5 of the 6V6s have a grid stopper connected to pin 1. Pin 1 is just a tie point and should be connected to the on board coupling caps. But it looks like you just connected pin 1 to pin 8 to ground. If so... That's wrong and is causing the red plating because there is no negative bias on pin 5.

[norburybrook]

I think I see pin 5 of the 6V6s have a grid stopper connected to pin 1. Pin 1 is just a tie point and should be connected to the on board coupling caps. But it looks like you just connected pin 1 to pin 8 to ground. If so... That's wrong and is causing the red plating because there is no negative bias on pin 5.

Steve, i don't have pins 1 and 8 connected. You're correct that there's a grid stopper between 5 and 1. I have pin 8 going to ground now via 1R resistors.


M

[martin manning]

I’d see if there is negative voltage on pin 5. Then with the rectifier in check the other power tube pins.

[norburybrook]

I’d see if there is negative voltage on pin 5. Then with the rectifier in check the other power tube pins.

I've spoted the problem I haven't connected pins 1 from the 6V6's to the grid leak resistors on the board!!!! Doh!



M

[norburybrook]

LOL, well that was an oversight

Connected them up and everything is working. I have an Ay7 in the reverb and it's a bit weak so I need to change that, I've no idea what the pan is as it come out of an old Peavey but it's working and sounding nice just a bit quiet. The footswitch is working in reverse fro the reverb and trem but if I switch the cable connections at the amp end it makes a horrible noise and stops working.



The bias trem is very subtle but I've a feeling once the bias is set right that may change...so...

I have a B+ at the 6V6 grid of 398vDC what should I bias at in mV for JJ 6V6's ?


edit: it seems with around 400V DC plate voltage 19mA is the maximum safe bias voltage. I will set accordingly tomorrow and report back.

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M

[martin manning]

JJ 6V6S are a beefier tube than a 6V6GT. 14W. So you could set them at say no more than 0.7*14/398 = 25 mA. Recheck the plate voltage and see where you end up. You could go a a couple of mA cooler and see what it sounds like.

[sluckey]

The footswitch is working in reverse fro the reverb and trem but if I switch the cable connections at the amp end it makes a horrible noise and stops working.

The connection to the reverb FS must be shielded but the connection to the trem FS does not have to be shielded. So, if you have a typical Fender two button FS, you need to swap connections at the FS end. May be easier to just swap the switches around.

Or, you could swap the connections INSIDE the amp if your layout permits.