Tubelab Simple SE: 6K7VG and XPWR033 Power Trans
I've got one of George's amps on my workbench for testing, and I have the luxury of having both power transformers. I'm using the standard parts list with 5AR4 rectifier, Jan Sylvania 12AT7WA driver, and Shuguang EL34-B output tubes in triode mode for testing. OPTs are Edcor CXSE25-8-5K. Connected speakers are nominal 8-ohm. I get 404 VAC to the board from the XPWR033 with B+ of 474 VDC. The glow on the plates of the EL34-Bs concerns me with this combination; although, they play music just fine. I've only left this configuration powered up long enough to take a few very basic measurements. The Allied 6K7VG measures 388 VAC at the board with B+ of 450 VDC. With this combination there is a very slight glow on the plates that's much less concern. BTW, mains input is 118 VAC, and measurements are at idle. I like the build quality of the Edcor, but I'm afraid I'll burn up some tubes. Any advice from experienced users would be appreciated.
I've been told that any visible glow is not good. It means the plates are hot enough to emit visible light. If they are that hot, they are also starting to outgas whatever is dissolved in the metal. This will erode the getters and contaminate the vacuum.
You should run the tubes at a lower overall dissipation. This requires either reducing the B+ voltage or reducing the tube's current draw. You already know one way of lowering the B+ voltage - use a different transformer. You can lower the voltage by reducing the line voltage on the primary side (variac, buck autoformer, IRCL device, etc). You can add more DC resistance to the rectified side of the power supply (bigger R in the CRC filters) but I believe too much R will negatively affect bass performance of the amp.
Reducing the idle current in the tubes is generally accomplished by increasing the value of the cathode resistor (in self biased amps, such as the Simple SE) or adjust the bias voltage (fixed bias amps).
Look at the "transformers and tubes" (I think that is what it is called) on Tubelab site and pick a cathode resistor that is going to give the appropriate dissipation. Keep the Edcor if you like the build quality better...
You can also decrease the value of the first capacitor tied directly to the rectifier, which should get you closer to choke input and decrease your B+, with the added bonus of stiffening your supply. Power Supply Designer II, a freeware program by Duncan Amplification is your friend.
I would also like to lower my B+ by just a few volts. My simple se sounds great, but 6l6gc, and KT77 tube plates glow a bit. My line voltage usually measures 123 v., and my B+ is ~460v.
Already have a CL90 in the primary of the Edcor (370-0-370). Can I put several of these in series or in parallel to drop the line voltage(?)
Tried the Duncan amp softwaare, didn't really know what I was looking at.
Also I see Variacs for only ~100 bucks.
Are these used for routine listining too (as oposed to starting up a vintage amp for the first time).
If so, do Variacs usually have the kind of control necessary to drop voltage by just a very small amount?
If you decide to try to lower the primary voltage, I wouldn't add more IRCL devices. Instead, consider building a bucking autoformer. It'll be a lot cheaper than buying a variac. I'm not sure I'd want to leave my tube amplifier connected to the variac all the time anyway - what happens if some curious visitor decides to give the big knob a twist? Here's a good thread on building a bucking autoformer. You can most likely find the part you need at the local Radio Shack.
I mentioned the RS catalog number in post #14. It's 273-1511. I'd suggest wiring the 120 volt side in series and in phase with the 12 volt side. Take your bucked voltage from the end of the 120 volt winding and the junction between the two windings. Here's a schematic diagram:
Use the schematic at the top. The lower schematic will work too, but is not the preferred configuration. Remember that an autoformer provides no isolation between input and output, so consider safety when mounting the thing.
I got tired of playing with passive devices to 'adjust' the B+ voltage.
Now I just swap in a different rectifier tube.
Highest B+ voltage
5AR4 / GZ34
Lowest B+ voltage
'Pristine tested matched Cryo-treated Black-Plate NOS Brown-Base Perfect-Logo from Blackburn/RCA/TungSol on a Thursday in August of 19XX' 5AR4 tubes are expensive and seem to be hoarded by the 'magic speaker wire' crowd. I'm sure they're wonderful and selectively remove any sonic deficiencies from the rest of the amplifier.
Instead, I pick up 'Used, tested good, faded printing, a bit of a scratch on the base, from a boring manufacturer' of the other three rectifiers. Mid-60's RCA/GE/Sylvania are fine. With a mild amount of Ebay patience, you can get 3 of them for $20 shipped. If you can find JAN (Joint Army Navy == mil-spec) versions for the same price (and it's not hard), grab them.
Then I smile when I turn on the amplifier and realize that the warm-up profile isn't quite right (the 5AR4 does have a delightfully-slow voltage ramp) causing some sort of mystical damage to the output tubes for several seconds. Or when I look at the 1930's era datasheet for a metal-case 5Z4 and realize that I'm in voodoo-scary-no-man's-land (80 Volts and 35 mA beyond the recommended operation). The ~50uF input capacitor on the Simple SE is way too big for most of these tubes. I ignore ALL OF THAT.
The old tubes just work. No Chinese 5AR4-style flashes. No JJ 5AR4 failed-after-three-power-cycles frustration.
They might be complaining about the abuse, but if they don't like the job, I have a box full of scratch-and-dent tubes sitting nearby, feeling lonely and confused, who would be DELIGHTED to take their place. "Gee, it's good to be warm again. I thought nobody wanted me. Please don't put me back in that little box, mister. Look closely, I have a warm glow and a cool glass envelope."
My biggest fear is that my tube collection will form a union and start making demands. But until that day, I'm the evil overlord.
My power transformer Edcor XPWR035) is rated for 200 mA on the rectifier winding. I think that all but the 5AR4 require more than this. I guess I [U]cannot[U] run these other rectifiers?!?
It is 200 mA on the high voltage winding. The 5V winding is rated for 2A. But, as you have correctly noted, some of the alternate rectifiers need more than 2A. Check the data sheets before you try.
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