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Hey guys, I scratch built a 5e3 from parts scrounged from other builds,to Trinity specs. The amp works great ,sounds great and all, but for some reason the voltage at pin 8 of V2 is only 46vdc,when it is supposed to be 70 .I have tried a bunch of different 12ax7s in there, checked the values of the resistors there and still no joy.... All the other voltages are really close or dead on to the voltages spec'd in the docs except this one spot. Whats up?

I feel your pain G-log..1rst build of mine was the same thing and we tried everything, thinking I had done something wrong, and it's running like that today unfortunately....2nd 5E3 build from Stephen, tested perfect with 70VDC at pin 8 V2....I just finished last night the 3rd and final tweed and same as the first...45VDC at pin 8 of V2 and 18VDC at pin 7..I lifted the 1M resistor at the 56K eyelet and jumped it to grd and only got 3VDC there so I put it back as per schematic..but I notice Leo's design never had that 56K in circuit but i'll let you know if I find a solution..

for the record, all the high voltage issues I had with the first two builds with high B+ voltages running the Supplied Sovtek 5Y3 rectifier tube, this last kit mysteriously came with a JJ and wouldn't you know B+ voltages tests at 382VDC and thankfully I had a Sovtek to try in it and readings of 420VDC...so don't run the Sovtek unless you want them running hotter....all the emails, phone calls, ordering parts, burning up gas and money running across town to get Zeners and switches as per recommendations and it was nothing more than a rectifier tube..unfortunate someone couldn't have shared that with me first..

....and the Sound of Silence......(cricket chirp, cricket chirp,...and so on).

Could be an error in the documentation.
Have you compared your voltage to other Tweed voltages?


If the voltage is low on the cathode there I'd say something is pulling it down (few more uA of plate current flowing then normal) but yet you say all other voltages are fine. I'd still check all the resistors and caps again (make sure no caps are leaking a bit of DC).

If it sounds good though I'd leave it!

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http://pdfelectronics.com

Thanks guys.Famly health issues kept me away from this for a few days... I think the amp sounds great as it is,but I was reading Ernie's threads about voltage issues and thought I might compare my voltages with some other finished amps.I haven't compared it to other 5e3 amps( although a friend of mine has a real one that I am going to try out as a comparison).As I stated before all my other voltages are super close to Stephens documented numbers,but my amp is a bit different as well,it has an early 60's Bell& Howell Filmosound PT and a Mojo 4/8/16 output trans(I wanted to use a Celestion Blue and already owned a 16 ohm version)NOS 5y3 and a matched pair of RCA black plates I have been hoarding for many years. I have a couple of zeners on the center tap to drop the voltage into correct specs as well as a home made VRM,so lots of other variables in there to make things interesting(if not confusing!).I did read one thread where Stephen said not to worry about that particular voltage reading being off,but my curiousity got me and want to know all I can about these amp thingys
Here are my voltages
117vac wall
B+360v at first filter cap
pin8 rectifier 390v
V4 6v6 pin 3 354
pin 4 326
V3 6v6 pin3 352
pin 8 at cathode resistor 20.7v
V2 pin8 46v
pin6 194v
pin3 1.2v
pin1 155v
V1 pin8 1.92v
pin6 123.8v
pin1 116v

....Anyhoo...Here's a couple of pix of the cabinet I made for it.

Beautiful job on the cabinet G-log..very nice !! although the amps sound great, I feel the same as you in that whats causing that strange low voltage on pin8 and on pin 7 where there shouldn't be voltage, it exists. I'm determined to figure it out, as the 2nd build was perfect, yet the 1rst and 3rd tweed kits are out of spec...Please let us know as I will too if you find a solution..

Thanks Ernie! I will post more info when I get it.I won't give up on this....Thinking about building another anyway for awesome stereo tone......

PM sent G-log

Let me try to explain. The Deluxe uses a cathodyne type phase inverter. It drives the output from the Plate (pin 6 ) and Cathode (pin 8 ). It DOES NOT MATTER what the voltage is on Cathode, pin 8 - 50-70 . What matters is a balanced voltage swing. So if the plate swings 10 V positive, the cathode swings 10V negative. i.e. Phase inversion.
The difference in the voltages measured will be a result of the variances in the resistors in the path to ground - R13 vs R11 + R10 and the B+ . It all figures in. Tweeds are not precision audio instruments with matched resistors. But again, the amp will work fine with either value on pin 8. Your ears have already told you!
The voltages on the Tweed documents and manual were based on a documented build example. Maybe a good idea to add a range to this document.

HTH

_________________
Stephen
Web: www.trinityamps.com. Facebook: facebook.com/trinityamps. Twitter: @trinityamps

g-log - nicely done cabinet!!

_________________
Stephen
Web: www.trinityamps.com. Facebook: facebook.com/trinityamps. Twitter: @trinityamps

We sold Tweed kits with Sovtek rectifiers for years and only recently decided to try the JJ. Nothing mysterious about it actually. Typically we have run our Tweeds at 400V with the Sovtek. If you are more comfortable, or like the JJ better, that's great. If you have a VRM, it doesnt even matter. Set it and enjoy it!

A zener was a last resort because it seemed there was nothing we could do to bring that voltage into the typical 400V range once the 126V Mains house supply was rectified. The Zener is a great thing to know how to do. You will find that a very useful tool.

_________________
Stephen
Web: www.trinityamps.com. Facebook: facebook.com/trinityamps. Twitter: @trinityamps

Thanks Stephen! That answers a lot of questions, I suspected that particular voltage wasn't super critical. I really did learn a lot putting this one together,with all the bits & pieces from other projects and various sources. It is a killer sounding amp for sure