5F10 Schematic question

[Danelectron]

Yes its normal for the B+ side voltages in 'unloaded' circuits to be higher. Pins 3 (plate) and 4 (screen) will read right up to the B+ level without a tube installed. The tubes 'pull down' the 'pin' voltages because when the tubes are installed, current is flowing through the tube between the plate and the cathode, and to a lesser extent between the screen and the cathode. The more current that flows, the lower the plate (and screen) voltages get.

Tubeswell, This makes sense. I removed the bias components and they checked good. I started going through the entire circuit and so far all is O.K. I'm going to pull the coupling caps and check them going into the power section. I'm suspecting I will find an error or bad component when I get to the power section. I have a 50 k pot coming and will set up the adjustable bias. I'll post any further discovories as I go through this. Thanks for your suggestions. Dan

[tubeswell]

Hi Dan

FWIW the best way to check coupling caps for leakage that I know is to unsolder the 'tail' end only (the end that is hooked up to the next stage's grid), and clip your high VDC meter onto that end and then turn the amp on (with the other end of the meter hooked up the the chassis/ground return path). If the cap leaks, you will see DC on your meter. See fig 4-4 on p57 of attachment

[Danelectron]

Tubeswell, That's a good troubleshooting guide. I did have a bad cap coming from the cathode of the 12AX7. I replaced that and the voltage went to about -37 volts. I set up the adjustable bias with a 39K resistor and a 50k pot, but I can't set the bias any higher(more positive) than -37 vdc. Strange. I'm going to pull the pot and bias resistor out and double check the resistance. I did solder the wiper to one of the legs on the pot and the other leg to the 39k resistor. I have to be missing something simple. I did get through the wiring lay out and didn't see any errors there. Thanks, Dan

[Danelectron]

50VAC should work with 6k8 and 56k in the bias supply voltage divider, but its better if you make the bias supply adjustable. Replace the 56k with a 50k pot wired as a variable resistor in series with a 39k resistor on the pot ground leg. Wire the pot wiper to one of the other pot's lugs - to ensure that if the wiper contact fails you still have a bias supply voltage.

Tubeswell, The pot I'm using has three legs, the center one being the wiper. It's a trimmer pot and I'm not sure of the power rating. How large does this need to be? It is plastic and doesn't have a ground leg or any real metal on the housing to ground to anything. It's physically about the size of a dime and about 3/16ths" thick. Is there a more proper type I should be using? Thanks, Dan

[tubeswell]

Tubeswell, The pot I'm using has three legs, the center one being the wiper. It's a trimmer pot and I'm not sure of the power rating. How large does this need to be? It is plastic and doesn't have a ground leg or any real metal on the housing to ground to anything. It's physically about the size of a dime and about 3/16ths" thick. Is there a more proper type I should be using? Thanks, Dan

Those little 1/2W trimpots are fine - I've used them in a few builds. The bias supply only sees a handful of mA. Remember to connect/short the wiper to one of the 'side' lugs (it doesn't matter which one) - it will still function as a variable resistor either way, but if the wiper contact fails, you will still have a bias circuit (albeit a coolish one)

[FiddlinJim]

Dan, did you ever get your bias circuit figured out? I've just Harvardized my 5F2-A and I'm having some trouble, too. I'm using the same PT from Allen. In the attached image, I replaced the 56K resistor "B" with a 49K + 10K bias pot and I'm getting somewhere around -48 as the bias voltage (220K junction "C"). I measured the bias tap as 51VAC at "A". Have you settled on a voltage divider scheme that gets your bias voltage in useful range?

[RWood]

What I would do, Jim, is temporarily replace the 49k resistor with a 100K pot wired as a variable resistor. Clip your voltmeter at C (or better yet, use a bias probe) and then dial in the 100K for the best range of values. Run your bias adjust pot up and down to check your range, then when you are satisfied unclip the 100K and measure the resistance you ended up with. Then hard wire a fixed resistor there in its place.

On some amps I've calculated the maximum plate dissipation that I want from a particular tube, and I'll set the range so that even when max'd out, it cannot be biased too hot. You can do that fairly easily when you sub in a pot for that fixed resistor.

RWood

[FiddlinJim]

Thanks RWood. I'm a little surprised it is so far off using the 49K - 59K range. Do you expect that I'll be increasing or decreasing the resistance from 56K to get the desired -27 (or appropriate current via the bias probe)? I'm all twisted up and down with the negative voltage thing...

[Danelectron]

FiddlinJim, I haven't gotten back to this amp yet. Busy at work this summer. I agree with RWood. I set mine up with a 39K and 50K pot, but can't set the bias higher than -37 so I need to adjust the 39k to some lower value. Or it's possible that the 25/25 cap may be leaky. I will post when I find the solution. It's been a rediculously busy summer.

[FiddlinJim]

An engineer here at work just 'splained some things to me and we've come to the conclusion by doin' a little math that the appropriate value for the bias adjustment might be closer to 4.3K instead of 56K, so I'm going to put a fixed resistor in at < 4.3K and the bias pot (although I'm going to try RWood's method as well). My friend explained that if the original bias tap value was more like 35VAC instead of the 50VAC I have in there, then the 56K value might be right to put the bias voltage closer to -27 VDC. So, I'm wondering if the original PT's bias tap was like 35V...

[FiddlinJim]

So, I clipped in a 50K pot and dialed'er in to get the current about 22ma for the 6v6s and found that it liked about 30K, so I soldered in a 3.9K resistor (for a fixed non-zero base load) and a 50K bias pot and she's runnin' like a dream. A little less power but cleaner than a 5E3; a little more everything than a 5F2-A. Noice! I did have the high-freq oscillation problem, but immediately switched the OT primaries between the 6V6s and it sounds great now. I haven't done any chasing of noise, but there seems to be very little hum or preamp noise, so I'm a'likin' what I hear. I need to do some 6AT6 / 6AV6 taste tests. The bias voltage ended up being more like -35 to get 22ma of current on the power tubes.

[Edgar Sterling]

Did you ever figure this out? I remember I had this same exact problem when I built my Harvard last year. I had the diode turned the wrong way. When I reversed the polarity everything worked great.