Davis 201A PA Conversion

[DRH1958]

Good catch on the B+ being shared by the first 3 stage. Thanks! I'll add the grid stoppers to the power tubes as well.

For the screen grid resistors, what would you suggest? Would 100ohm 1W do the job? (That's the lowest value I have in the 1/2W & 1W arena).

What voltages do you have for the plate and screens? The schemo doesn't specify, but it must be around 350-360v or so. They don't give the secondary PT main AC voltage or the DC output voltage of the 6CA4. I would be tempted to go to at least 200 ohms, depending on the difference in voltage the B+ rail provides for, although they thought screen resistors weren't necessary and many old PA and Hammond organ amps didn't have them. Figuring the data sheet max screen current of 22mA per pair, the 100 ohm will only drop 2.2v, so 200 ohms will double that. I personally like the screens to be 5-7 volts lower than the plates. Screen failure is the number one cause of power tube failure. Who cares if you lose a watt or two, you will never hear it and seems a logical trade-off for some added safety. Just a personal preference.

[Pdavis68]

Yeah, I've never really heard much good about headphones on guitar amps. Maybe I ought to just skip it. I've got modeling software on my computer and use that right now and it works well enough, I guess. Probably better than what I'd get with a phone jack. Just kinda curious about it and I kinda feel bad having amps that I don't get to use a lot. Need to join a band one of these days when I have more spare time so I can play at volume more.

[Pdavis68]

I'll measure the voltages tonight. Yea, I figure it'll be somewhere in the 350V arena. I think I have some 200 ohm 1W resistors. You know I'm not out for every last watt. That's why I got those EL844s. I prefer a quieter amp for playing at home...

[DRH1958]

I'm curious, the amp has 4, 8 and 16 ohm taps as well as 25V and 70V taps and a "Booster Output" which comes out after the tone stack. Would any of those be appropriate for headphones (I'm not expecting anything particularly close to what it sounds like going through a guitar speaker), just something to noodle around on while the wife and daughter are sleeping.

Not sure if this is what you might be interested in but here is an interesting link regarding your headphone question. This is for hooking up the headphones to the main speaker outputs: https://robrobinette.com/HeadphoneRe...Calculator.htm

[Pdavis68]

Thanks for the link on the headphones.

So I measured the voltages and they're not even in the ballpark of what's on the schematic for the two shown. I had my DMM grounded on the chassis and then I measured the 4 connectors on the can cap.

The schematic shows 40uF, 40uF, 8uF and 20uF, but the can has been replaced with a new can that's 40uF, 40uF, 20uF, 20uF.

So first of all, I did it with the "tube saver" off but going through the 200W bulb. From high to low for the 4 voltages I got:

269, 258, 249, and 240

Then I realized that going through the current limiter, it might be, well, limited... Plugged straight into the wall, I did it with and without the "tube saver" on. Without the "tube saver" I got:

291, 281, 271, and 260

With the "tube saver" on, I got:

272, 262, 253, 244

Given that the schematic shows 330V and 285V for the 3rd and 4th tap, I'm not sure what to make of this. Certainly not a 45V difference between those two, and nowhere near those voltages.

The AC voltage off the transformer is 250VAC. According to the 6CA4 datasheet, 250VAC RMS should yield 245V DC output voltage, but I'd expect well over 300V output. Something doesn't jibe.

[g1]

The AC voltage off the transformer is 250VAC. According to the 6CA4 datasheet, 250VAC RMS should yield 245V DC output voltage, but I'd expect well over 300V output.

Why are you expecting more than stated in the data sheet? Are you sure this is the stock PT?
Have you checked for AC voltage on the caps?

[Pdavis68]

Well, for one thing, 250VAC rectified should yield 250VAC * 1.4 = 350V DC and PSU Designer II says that I should have about 342V DC with full-wave 6CA4. I assumed I was misunderstanding the datasheet.

No, I don't know if it's the stock transformer. It's not the stock cap and the mic jacks have been replaced and resistors have been replaced. Really, I have no reason to think that anything is stock other than the case and chassis.

No, I haven't checked the AC on the caps. Can I do it with the DMM. Amp + computer + scope leads to tripping the switch in the switchbox for that corner of the house.

[Chuck H]

Shouldn't that be a 250-0-250 (so 500V) with a full wave rectifier, and unloaded, to get that 1.4x figure? Pretty sure.

[DRH1958]

Nope, the FW tube just takes the negative part of the sine wave and flips it up to positive. The peak voltage is the same. The voltage doesn't double but the pulsating DC frequency doubles form 60 positive pulses to 120 positive pulses. The voltages don't add, so pdavis has it right. But also, there will be voltage drop from the plate resistance of the 6CA4 as the load is placed on the amp. How fully loaded was the amp when you took the measurements? I also found that same 245 volt value from the data sheet with 250-0-250 as the output voltage. That value(245v) is for a 150mA draw, so if the current is lower, then the voltage will be higher because of less drop. Ohm's law.
I suppose since you know there has been work done to the amp, there could be any number of changes made, including how it is wired, so all bets are off.

[Pdavis68]

I didn't think about the voltage being for the current draw. Of course, to figure out the current draw, I need the voltages so I can calculate what the individual tubes are drawing. I'll try to work the math out tonight and see what the draw is. I'll also try throwing in the 6CA4 from my Hammond and see if that gives me a different value. I just found it weird that PSU Designer was giving roughly what I would have expected in voltages using a 250V secondary and 6CA4s. And I find it odd that the voltage drop from #3 to #4 different from what's on the schematic (~10V drop vs ~45V drop). I'll look over the resistors and see if they got changed. Yeah, there's no telling what's going on. The transformer, size-wise and color (black) match the original and whether or not it's an original transformer, it's definitely vintage. But it could be a new transformer with different resistors.

[Chuck H]

He mentions "250V rectified". Which it's not. It's 2x250V half wave rectified and joined. So 500V and not 250V. AFAICT nothing gets flipped. The two rectified DC pulses are joined.

When you plug numbers into Duncan PSU and select a FW rectifier the transformer voltage shown would be for one end of the HV secondary to the CT. Not end to end. A 250VAC secondary would be 125-0-125 for a CT wind or 250V end to end for a not CT wind. At least that's how "I've" always seen things spec'd.

[Pdavis68]

I think I'm getting mixed up in the descriptions here and for all I know we're all saying the same thing or we may be disagreeing. Not sure. Here's my understanding: The transformer, I believe, is 250V-0V-250V (so 500V CT). I apologize if I was unclear. I guess it wouldn't hurt to check the voltage unloaded. I'll pull the tubes and see what I get. Maybe that's what the voltages in the schematic are?

[DRH1958]

Here's what happens if I am interpreting it correctly. The positive half of the wave is passed as is and the negative half is now positive but the peak value is not changed so the rectified voltage is not increased. It stays the same as the zero to whatever voltage, not the voltage to voltage as in 0-250 not 250-250. So 250 X 1.4, not 500 X 1.4 is the output voltage. I think we are saying the same thing, only a different way or needed to clarify what the situation is.

[Pdavis68]

Here's what happens if I am interpreting it correctly. The positive half of the wave is passed as is and the negative half is now positive but the peak value is not changed so the rectified voltage is not increased. It stays the same as the zero to whatever voltage, not the voltage to voltage as in 0-250 not 250-250. So 250 X 1.4, not 500 X 1.4.

[ATTACH=CONFIG]34908[/ATTACH]

That is my understanding as well.

[eschertron]

Sorry if this has been done and I missed it, but have you measured the AC high voltage on the PT secondary? I'd think with the rectifier tube pulled you'd see if the PT was indeed 500vct or not. Then measure again with the full tube complement to get an idea of the actual load on the PT, and be able to predict the DC voltages more accurately.