DIY Audio Projects Forum

FourPlay!

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-= Gregg =-
* Ratings are for transistors - tubes have guidelines*
Home: GeeK ZonE
Work: Classic Valve Design

Charles your wiring is easy to see where a wire goes and it will be easy to work on and modify.
One suggestion you mentioned that the anodizing is not very conductive and you are right. In regards to the "blue velvet" however it is preferable to connect a small wire to one of the <two> small screws on the back of the pot. It will allow you to gnd the pot threw it's own wire back to the star gnd and not to chassis gnd. OK another suggestion is, using larger AWG for the heater wiring. Small wire, big currents = more voltage drop across the wire itself and therefore larger/stronger magnetic fields around the wire just waiting to inject into some little signal line. Also using separate gnd return lines for the heaters allows one to make a twisted pair feeding the heaters therefore taking full advantage of any cancellation of common mode signals. With these smaller tubes they draw around 300mA each so not as much but something to think on when looking at larger builds.
In the end though my gut is with your conclusion. The hum injection is coming from the proximity of the power and output trans. If you could shield the OP's i think you would get the quiet you are looking for. Very nice looking amp though well done. I built this amp and did a lot of experiments with it before placing into it's cabinets. I used one cab for pwr and other for amp but i could get injection from pwr trans into out trans when they were 10" apart no matter what the orientation between them was. Not a fault of either they do a good job but good shielding is a must especially since all these trans are open frame types.

Charles; What is the frequency of the hum? f-mains or 2xf-mains?

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Matt
It's all about the Glass!
http://www.CascadeTubes.com
Cascade Tubes Blog

Hello Matt, good question I didn't think about. It's 50Hz, not 100. The level on the scope is burried in the other noise signals, so I can't really measure it. I found out by comparing it from the headphone to my smartphone signal generator app. Clearly 50 and not 100.

Interestingly, I shorted the inputs, put the volume pot at 0, and I still see about 2mV noise on the output (no load for this check). So Laurie-54 is correct, these open-frame Edcor OPTs are picking up everything flying around my lab desk...

Hi Laurie, thanks for all the hints. I will study the possibility to implement them. Shielding the OPT's... smart idea. I was going to shield the power-tranny, but after all, better shield the receiver and not a single noise emitter.

Due to the large number of turns in the primary they pick the smallest magnetic field out and do their thing. It's almost like they were made for induction.

I know that the selected output trafos impedance matches up with particular headphones with the same or similar impedance. What is the byproduct of using headphones with lower impedance? For example 66Ω with the 300 tap on the 600/300 OPTs. Would I get more bass? less bass? Less efficiency? Just trying to understand the effects of not matching all of my phones with this amp. BTW I find the amp to work fine with my 66Ω AKG 7XXs at around 3 o'clock on the dial.

Hello All!!

I found that some Tube amplifiers has an standby switch, that cuts B+ to the tubes, why this design (that's obviously works) hasn't it?

Also, May I mount the 7806 regulator to the metallic chassis instead of on a heat sink? with the needed electric isolation... there is a lot of metal to dissipate heat!!!

Thank you!!

Hello Willy,

I asked myself the same B+ delaying question. I even bought a small pcb kit to delay the B+ (I use solid-state rectifiers). However, I have read in this long thread that the designer Bruce founds it unneeded, so I skipped it. I might add one in another build I'm planning for this amp. But first I have to solve my hum issue. Sailing season is beginning here so my attention is shifting to it until Fall (amps are a winter hobby for me).

The PCB I'm using is quite compact and doesn't allow large heat sinks. Consequently, mine gets pretty hot, but not too much. If your physical design allows to mount it on the chassis (properly isolated), I wouldn't see an issue and you would certainly get better cooling and less heat inside the amp.

Regards,
Charles

Thanks for your response Charles!!!

I forgot that the B+ delay was mentioned in the thread... there are so many pages!!!
I'll get rid off the heat sink, I tend to complicate simple things...

Sailing!! Nice!! I agree, audio its a winter hobby, I passed all the summer outdoors even in my backyard!!

Good winds!!!