New Amp Day - StewMac Mojotone '65 P-Reverb Princeton Reverb Assembly & Review

[clintj]

Swap the reverb send and return cables and repeat the bump test to see if you get a crash. It's not unheard of for a tank to get damaged in shipping. Make sure the foam shipping blocks aren't still packed in the tank as well.

[jonfields45]

Originally Posted by clintj

Swap the reverb send and return cables and repeat the bump test to see if you get a crash. It's not unheard of for a tank to get damaged in shipping. Make sure the foam shipping blocks aren't still packed in the tank as well.

Already tried the cable swap. But, those foam blocks are sitting there... I just removed them and it did not fix the problem. I'm getting a spare 12AX7 tomorrow too and am going to try replacing the tubes on either side of the tank. BTW, the tank input is roughly 1+ ohms and the output side ~200 ohms. I think it is OK. I'll try swapping tubes next. The volume of the tank "crash" is also way off from my expectations (too quiet). The reverb has a lot of hum too as I turn it up. I wonder if the triode on the reverb return side has a short between its cathode and its filament.

I guess having only one output tube was enough to get sound yesterday, but two is way better :~).

[jonfields45]

The problem was loose RCA plugs on the reverb tank cables.

My only gripe is if I turn the reverb up beyond where I would like to use it (higher than 3) there is an increasing hum as I go further. No problem at reasonable reverb settings. Grounding the return triode's grid does not eliminate the noise (so it's not the spring picking up the noise). Grounding that triode's cathode does silence it (saturates the tube and probably zeroes the AC gain…). I'm going to think about this a while before putting the amp fully together. It would seem there is leakage from the AC filament to the cathode, but the tube tests good, and a replacement did the same thing. Checking around the cathode bias network everything looks fine with my meter (resistance to ground is correct and the ground is solid).

[clintj]

Lay a cookie sheet or similar across the open chassis and see if it helps. Fender closed the shielding around the circuit with aluminum window screen type material in the top of the cabinet. With it open like that you're subject to any environmental noise.

[jonfields45]

Originally Posted by clintj

Lay a cookie sheet or similar across the open chassis and see if it helps. Fender closed the shielding around the circuit with aluminum window screen type material in the top of the cabinet. With it open like that you're subject to any environmental noise.

The cookie sheet did not help. I verified that the ground is quiet where the reverb recovery triode's cathode bias is connected (screwdriver from that eyelet to the chassis). Since the reverb is crazy over the top compared to a 65 Princeton a friend owns as I turn it up, and the tube tested as on the very edge of high in-spec gain, I might reduce the gain of that stage after verifying that the current resistors are in-spec. It could be as simple as a bad solder joint on the plate resistor (driving up the resistance and gain).

[jonfields45]

No more gilding the lily. It sounds great!

[Bluside]

Well done.

[GoPappy]

Congratulations. Well done!

[jonfields45]

I went to Guitar Center and tried out the PCB based Princeton Reverb on the floor. Fender changed the reverb return circuit quite a bit in the PCB versions (the foot switch grounds the reverb after the triode instead of before, and the peak amount of reverb is dramatically less). It was not quite worst case Guitar Center noisy, but for similar amounts of reverb I heard similar amounts of 60 Hz hum.

Case closed.

[rockabilly69]

Originally Posted by jonfields45

I went to Guitar Center and tried out the PCB based Princeton Reverb on the floor. Fender changed the reverb return circuit quite a bit in the PCB versions (the foot switch grounds the reverb after the triode instead of before, and the peak amount of reverb is dramatically less). It was not quite worst case Guitar Center noisy, but for similar amounts of reverb I heard similar amounts of 60 Hz hum.

Case closed.

My close friend built the same as you with the same results for the reverb. It comes on way quicker than the PCB version. So congrats, you've now got a great amp that you built yourself!!!

[jonfields45]

Originally Posted by rockabilly69

My close friend built the same as you with the same results for the reverb. It comes on way quicker than the PCB version. So congrats, you've now got a great amp that you built yourself!!!

The hum too? Thanks!

[rockabilly69]

Originally Posted by jonfields45

The hum too? Thanks!

I'm not sure about the hum, but definitely had way more reverb than he needed past about 3 on the knob. I just talked to him on the phone and he told me that although he got a majority of the parts for his Princeton from Mojotone he wound up building his PR from the layout on Rob Robinette's page...

https://robrobinette.com/AA1164_Princeton_Reverb.htm

He also said he didn't get hum as much as white noise at higher reverb levels (more of a hiss than a hum)

[TiffanyGuitar]

I built the champ kit in January and it sounds great. I did have to get an amp tech to go through it for me because it was failing the first tests - even though the light came on. He found a bad solder joint. Then after I brought it home I found an issue with the 12ax7 and thought it was a bad tube socket. So I replaced that socket - and sure enough that solved my issues. The thing sounds great. I have thought about trying to one day do the Princeton kit but worry it might be too complex.

[jonfields45]

Originally Posted by TiffanyGuitar

I built the champ kit in January and it sounds great. I did have to get an amp tech to go through it for me because it was failing the first tests - even though the light came on. He found a bad solder joint. Then after I brought it home I found an issue with the 12ax7 and thought it was a bad tube socket. So I replaced that socket - and sure enough that solved my issues. The thing sounds great. I have thought about trying to one day do the Princeton kit but worry it might be too complex.

The possible tube socket failure that would lead to reverb hum (that is only silenced by grounding the V3a cathode) would be leakage between the filament and the V3a cathode. The filament supply has two 100 ohm resistors to ground (a DC measurement filament to ground would be 50 ohms). Testing the V3a cathode resistance to ground, if there was a short, would measure a little less than 50 ohms, but it measures just right at 1.5K... It could be that both tubes I tried were marginal in a way missed by the tube tester. I suppose more subtle leakage in the tube socket between the filament and V3a cathode is possible.

I've stumped a fellow EE with tube experience. He agrees if V3a hums, V1a should similarly and it doesn't. This is more about OCD than circuits :~).

If I ever get an excuse to open it up, I'll knock down the max reverb and max hum by 6 dB.

If you build another kit, I could talk you through the debug. I had a intermittent cable killing the reverb and miss wired the filament supply to the V5 6L6. This gave me a poor sounding class A amp with no reverb for a very short while.

[Bob Womack]

Congrats on your build, on completion, and on a great amp!

Bob