6v6 plexi-ish build questions

[donzoid]

So I just got done doing a 6v6 variant of Loy Dockery's version of an 18watt TMB and since I have not done one before with Cathode bias power I wanted to post this up for some feedback.

It's a "beaks and feet" build where I used EVERY recovered part I could. There are a few caps here and there and some new resistors but in general its CC resistors and even most of the caps are old (but tested good). The first PS cap is a radial I had lying about that's 22uF / 450v so I could put in a 5y3 later if I wanted.

I just did my initial voltage checks and am a bit concerned about a couple of items:
1. The heater volts are a strong 7ac and its no surprise this tranny was designed to heat about 20 tubes...what's the easiest way to bring that down a tad? add resistors?
2. The screens are measuring 5v higher than my plates...it isn't redplating, and I just played on it but is that normal on a cathode-biased set of 6v6?

Overall I think volts are too low so I may put in a lower value "throttle" after the rectifier. I kinda went crazy with that one 'cause its like 485 volts after the 5u4gb recto and right now the resistor is 800ohms...lol. Beefy, but I think too much.

Let me know what you think. Any advice is greatly appreciated!

[Ken Moon]

Wow, Ive never run 6V6s with over 420V.

I use Mills 5W resistors to drop the heater voltage - a 0.1R on each side should drop in to about 6.4V.

On this amp, you can see the two 0.1R resistors right there by the power switch and the lamp:

[img:768:576]http://i514.photobucket.com/albums/t346 ... red_10.jpg[/img]

Here is the Mills datasheet:

http://www.vishay.com/docs/31801/mra.pdf

[donzoid]

Thanks Ken! I'll look into those for dropping the heater volts.

The plates and screens are not even up to 300v, which is why I thought they were a bit low. I have read where older Fenders use some pretty insane volts (over 400). But I don't want to run them that hot either!

[drew]

As an alternative to the 0.1R resistors, you could use diodes, as Merlin describes here: http://www.valvewizard.co.uk/heater.html

As another alternative, you could put a CL-80 thermistor in each leg of the AC coming from the wall, which will bring the power down to about what the PT was spec'd for, and probably get your heaters down to what they're supposed to be.

[Ken Moon]

Cool.

I don't know how I missed that - my copies of Merlin's books look like textbooks from 20 years ago, with bent pages and yellow highlighter everywhere

[donzoid]

Thanks, Drew.

I've been reading Merlin's stuff, had not gotten that far yet.



It's a shame there isn't a LAN port where I can just plug in and download (into the 'ol noggin'). Nah, too creepy and probably will happen. I'll just keep reading.

[donzoid]

OK, I tried throwing in some .2 ohm / 1 watt resistors I had in the bins on the 6.3 volt lines, and they are introducing a "dynamo hummmm"...so I'm going to back those out and check the local parts place tomorrow for either a better grade of .1 or a pair of the CL-80's. I dropped the power resistor to 400 ohms too, now looks like I have about 320 B+ but its humming so bad I couldn't stand to leave it on and check the rest. Maybe with the thermistors, I won't need the resistor and that should help also.

[donzoid]

I've gone out and read Merlin's link that Drew included above. Does the insertion of diodes in one leg of the heater AC introduce any hum (as the resistors are), and based on his diagram just to make sure does this also work equally well on heater circuit with a center tap?


EDIT: Wish I had read that page before nailing down the flying leads on the main board....I left the original heater wires on there and now wish I hadn't as they wrap right around the sockets...hrmph.

[martin manning]

Did you put the resistors on only one side of the heater circuit or both? What kind of ground reference (center tap) do you have? I believe you will get hum if you don't balance the voltage drop about the CT.

[donzoid]

Hi Martin! It's a CT type so I put the resistors on each leg of the AC lines. The CT is grounded to chassis with the sec. CT. but I have been reading about elevating ground on cathode biased heaters, so it could be moved.

Does that answer the info you wanted? Just about to go to the local store to see if they have CL-80's and how much it would cost me...

[martin manning]

It doesn't have the hum when connected directly to the PT, just the higher-than-desired voltage?

If you placed a resistor on each side of the heater winding I don't know why that would cause a loud hum, and I don't think the specifics of the resistor brand would matter either. I guess what I'm saying is, well, hmm...

You can elevate the CT to the power tube cathode level to see if that helps. I'd add some more screen resistance in the power supply or at each tube to lower the screen voltage, or you could also try putting a resistor to ground from the screens to make a voltage divider like Andy Le Blanc has done. As I recall it was a pretty big value like 220k, but you could search TAG for it.

Link: https://tubeamparchive.com/viewtopic.ph ... highlight=

[donzoid]

OK well then the other variable I introduced may be the culprit...the resistor change from 800ohm to 400R. Before I take the resistors out of the AC heater lines I will try a different power resistor, as I was going to go lower in value than the 400 anyway (with that, I am at 320v on plates and wanted a bit more). Maybe that 400R is noisy.

[donzoid]

Yes, way less hum when connected direct...just that its abt 7v AC.

[martin manning]

Got schematic?

[donzoid]

perhaps I'd better re-check my solder joints on the resistors - if one isn't right that could certainly give me grief, and I'm not entirely thrilled with my own solder technique yet...