New Princeton Reverb build--Low B+ post -GZ34

[ViperDoc]

OK, good to know. I put the cathode current at 18 mA, that's at the far end of the pot. Can't go lower.

Now the plate voltage is at 435 VDC and there's about 420 VDC on the screens.

The amp plays fine for about 30 seconds and then cuts out again.

[pdf64]

So in the cut out condition, get solid readings on each 6V6’s plate, g1, g2 and cathode.

[ViperDoc]

So in the cut out condition, get solid readings on each 6V6’s plate, g1, g2 and cathode.

Yes, although one side seems to meander with a fizz sound before landing on a stable reading, usually on the plate. This happened with both sets of tubes.

[ViperDoc]

Cyber Monday gave me all the motivation I needed to buy an oscilloscope. Hopefully it will help figure this baby out. All advice welcome.

[ViperDoc]

I decided to pick up a replacement OT in case I need it. I originally placed an order for another Hammond 1760E. Somehow that got fat-fingered in the warehouse and I was sent a 1650E:



The 1650E has some leads I'm not familiar with, namely what appear to be a fourth and fifth OT primary pair intended for the screen grids. Is this intended for Hi Fi or some other application? Otherwise the spec on this tranny is the same as the 1760E, except it's about *4 times as large/heavy*. This is one big piggy bank of an OT, like for a 100 watt Marshall!

I don't think I'll be using it, but since this baby is about 4x the cost... maybe I can find its rightful owner on eBay.

EDIT: Just a bad joke there. I'll certainly have to return this unit; this thing is so big, my PR chassis might have more room for Lou Ferrigno!

[pompeiisneaks]

Those are what's called 'ultra linear taps' for ultra linear tube performance, not just for hifi but more common there. They're used to help the tubes perform more linearly.

Here's at least one article that talks about it: http://www.r-type.org/articles/art-115.htm

~Phil

[ViperDoc]

Those are what's called 'ultra linear taps' for ultra linear tube performance, not just for hifi but more common there. They're used to help the tubes perform more linearly.

Here's at least one article that talks about it: http://www.r-type.org/articles/art-115.htm

~Phil

Thanks, Phil!

[ViperDoc]

While waiting for some parts, I placed 1R PT cathode resistors on my PR 1.0 build and found 38 mA on those cathodes and -31 VDC on the grids. I will retrofit it with the manning bias and return and report. I heard a previous target of 18 mA for cathode current on a Deluxe. Any other recommendations?

My backup 2.0 OT should arrive next week as well as a signal generator. Not sure where to start with all of that, but I'll see what I learn soon enough.

[martin manning]

The new bias circuit will be fail safe, and get the bias established more quickly. For the bias setting, most people use a percentage of max plate dissipation on the output tubes. The current is easy to calculate: (Pdmax * %Pdmax)/Vplate. For 6V6, Pdmax is 12W and for a percent Pdmax use something like 60 to 70%. Accordingly, if your plate voltage is 430, and you want to target 65% Pdmax, that would be (12 * 0.65)/430 = 18 mA, and you would dial up 0.018 V on pin 8 with a 1Ω to ground. The plate voltage will change with plate current, so do it one more time and call it good.

[ViperDoc]

Thanks Martin. My startup plate voltage was 400 VDC.

[ViperDoc]

Well I just replaced my "Zeus bias" circuit which previously offered a minimum of 42 mA of power tube cathode current in my first Princeton Reverb build (the gods must be pleased) with the Martin Manning bias circuit so kindly provided earlier in this thread. It works just great. I couldn't find a 100V-rated 10 uF cap so I went with a Sprague Atom 10uF 150V. It fit on the board just fine. I immediately noted two things:

1) Now my 6V6 tubes don't appear to be on the verge of ascension to Mt. Olympus!
2) Now the onboard tremolo sounds very useable! Before, it sounded quite lifeless and unnoticeable unless the intensity was cranked, and even then it was almost asking to be removed. Not anymore. Sounds just dandy. My only wish was that it was slower, which I'm tackling on my PR 2.0 with upped tremolo cap values and LED cathode bypass.

Thank you, Martin!

My 6V6 plate-to-cathode current measured at 398 VDC, and punching that into the Robinette tube bias calculator set the JJ6V6S 70% cathode current at just over 24 mA. I noted a slightly-less-than 2 mA difference between each 6V6 cathode, so I left the high one at 21 mA which gave me 21 mA and 19 mA, give or take. Sounds pretty good! I'll play with it tomorrow and see what I think. I might heat that up a bit.

I will also proceed with demoting Zeus on my second PR build. I still need to diagnose its cutout issues.

[ViperDoc]

All right. I removed and installed the updated bias circuit on my PR 2.0 build and was able to obtain 18 mA and 19 mA of current on the power tube cathodes. There were a couple of "whoooooOOOOOSH-pop!" sounds I'm not familiar with while probing the power tube sockets, and still no sound comes out of the amp. There is the expected clicking/popping from the speaker as I'm probing. This is the amp I previously could play and sounded good once I got the "Zeus bias" diagnosed and cured, but now it won't play at all. Any thoughts on what to do before I replace the OT? I just picked up a scope and function generator, but, having no experience with either, I'm willing to wander unless any of you have some pointers. Thanks.

[martin manning]

Ok, catching up here... This amp was was working fine after the updated bias circuit was installed, but now it produces no sound? IOW, the cutting out problem is now just out?

[ViperDoc]

The amps worked "fine" in the very beginning with the lightning bolt bias. Then it started to cut out and now cuts out every time I turn it on. I just today replaced the board components to the updated Manning bias. I kept the pre-existing pot and resistor; even though the range is unnecessary, I can hit 18 mA easy on the adjustment.

I heard the whoosh sounds before I had a guitar plugged in. The speaker connection is definitely there, but when the amp cuts out, no guitar to be had.

I have floating input jacks on this build, but the jacks are grounded to the preamp bus and I get 33K on the input in between the jack and the V1A grid pin, that all looks fine.

[ViperDoc]

Well now I'm playing the amp, right now it sounds awesome! testing...

So far its sounding better than it ever has. I'm thinking either lead dress or debris somewhere...