• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

Triode connected SV6550C with 500V HT

Keep in mind that the voltage drops significantly under load- is that 500V with a simulated load (with resistors)?

500V might be too high for the screen grid (G2) IIRC.

You could possibly use a choke-input power supply to drop the voltage to a bit lower, or some sort of regulated circuit.
 
SHiFTY said:
Keep in mind that the voltage drops significantly under load- is that 500V with a simulated load (with resistors)?

500V might be too high for the screen grid (G2) IIRC.

You could possibly use a choke-input power supply to drop the voltage to a bit lower, or some sort of regulated circuit.


I didn't try with load. Just with the rectifier, and the capacitors. The peak voltage was 505V without any load. Due the amplifier has the output power about 25W, and the transformer is 230VA, there will be not too big drop. (I use separated transformer for the heaters).

As I know, the high voltage results runaway with the valves. But the voltage goes low, when significant current flows on the second grid, due the triode connection. So the dissipation will be low.

Choke input rectifier can be good solution, I will try it. The regulator out of question, because I want to keep the PSU simple.

sajti
 

dhaen

diyAudio Moderator Emeritus
2002-06-10 12:01 pm
U.K.
www.keystrobe.co.uk
Your choke might buzz in choke-input mode if it's not designed for that. Sometimes a small cap ~0.1uF as an input cap quietens it.

Alternately, why not add a small 25-0-25v transformer. Wire the secondary is series with your HT winding in anti-phase to reduce the AC by 50V.
10-20VA is probably big enough, and it'd fit under the chassis.
 

PRR

Member
Paid Member
2003-06-12 7:04 pm
Maine USA
www.diyaudio.com
> I know that 6550s are rated to 450V

FWIW, GE 6550A is rated 500V in Triode mode.

Numbers on 6550 vary between makers, and some fudge the numbers more than others. While SY is right to stay inside book-spec, especially on the Screen where abuse is hard to notice until the screen melts, I am a wildman and would not be TOO concerned running 11% over in home HiFi. 6550 is an industrial tube, and the ratings assume hard work with economic life in industrial service. They were run harder in some non-industrial amps, no real problem. And 6550 is again in multi-factory production... if you melt a couple, it isn't cheap but it isn't a Big Problem like it was around 1988 when we had to grub the trash to find un-dead 6550s.

> if I use less than 100k for the 1st grid

Won't do much to reduce screen-grid abuse. However, notice that 6550 grid resistance is 50K in fixed-bias, 250K in self-bias.

Look. If you wanted POWER, you'd run them 6550s in pentode mode. Or at least UL mode. Using triode mode means you want a smaller nicer amplifier than an Ampeg SVT. And that usually points to self-bias. And that means "wasting" some of your precious B+ voltage. In this case you have B+ to spare and you want to waste some.

Your underloaded transformer will end up around 500V DC. Pencil 450V across the tubes. 6550 plate rating is 35W or 42W depending who you believe. 75mA gives 34W, a nice safe first-guess. Squinting at the average Triode curves, this needs -50V grid bias. So drop 50V in a cathode resistor, 450 in the tubes, everything is groovy. Your transformer is in fact "exactly right" (except for being heavier and more expensive than it had to be). Figuring 150mA for two tubes, the cathode resistor is 333 ohms 7.5 watts, which could conveniently be three 1K 5W or 10W resistors in parallel. Or twelve 33 ohm 2W resistors (cheaper by the dozen) in series, and you short-out 1, 2. 0r 3 resistors to trim the bias for the specific tube you use (remember tubes vary +/-20% in grid bias for the same current).
 
Hi PRR,

many thanks for all the comments.

I have Svetlana 6550Cs. They are rated to 450V in triode mode. I think that 10% over the specification is not too much, but I don't want to kill my tubes if not necessary:D
I saw specification, due the 6550 has lot of variations. I know that the first grid resistor of 6550A is specified to 50k with fixed bias AND 500V. But most other variation rated to 100k, AND 450V. This is why I asked if others can handle up to 500V with 50k:scratch:
Svetlana rated to 100k as well. And most of the manufacturers use 100k (ARC, c-j...)
I have some JJ KT88s, they have no problem up to 550V, but I prefer the sound of 6550s...
I don't need more power. My output transformers rated to 30W, and 4kohms plate to plate.
Maybe self bias can be good idea, but I don't like the large cathode capacitors.
Yes this transformers are large. So one of them can handle up to 3power amplifiers. And I have separated 220VA transformers for heating, with 4x6.3V secondary. Should be good for multiamplifying, or tube based 5.1 for SACD or DVD Audio :D

sajti