Question about valve data results - mono block conversion project

[casssax]

Hi,

I've been working for some time now converting an old 1959 Realistic AF-12 to a guitar amp.

It is a 12 watt push-pull 6V6. The schematic is very similar to a Fender Princeton Reverb (except it has no reverb or vibrato).

What I've done to it is to add hi and low input jacks that go to V1.
The output of V1 goes to the tone stack (just treble and bass)
The output of the tone stack goes to a pre-volume control.
The output of the pre-volume control then goes the the second half of V1.
The output of the second half of V1 goes to a main volume pot
The output of the main volume pot goes to the first half of V2.
The second half of V2 is the phase inverter.



I took voltage readings and input them to the Valve data page http://el34world.com/charts/valve/ValveData.php?e=view&f=18515

I'm hoping I took the readings the right way (black multi-meter to ground and red on the pin)

I don't really understand the results. The -195 on pin2 of the first stage seems strange. 

I was hoping someone with more experience than I might know if this looks normal or if it look like there is a problem.

The amp is working and it sounds pretty good. I'm running it through a 12" 8ohm speaker from another amp I have.

There are some intermittent buzzy sounds and the amp seems very sensitive to outside interference (like pops when you engage a foot switch or if light plugged into the same wall outlet is turned on or off)
 

Code: [Select]

User Name Amp Type File Date Notes File Name
casssax realistic af-12 07-25-2015 No 18515

- Valve Pin 1 Pin 2 Pin 3 Pin 4 Pin 5 Pin 6 Pin 7 Pin 8 Pin 9
V1 12AX7 75 -195 0 * * 63. .002 0 *
V2 12AX7 250 .002 2.4 * * 169 12.9 40.6 *
V3 6V6 * * 327 308 .15 * * 18.8 *
V4 6V6 * * 327 308 .048 * * 18.9 *
V5 5Y3 * 337 * 340 * 340 * 337 *

bias resistor = 252 ohms
voltage drop = 18.75V
V4 = 304V
V5 = 306V
average voltage = 305V

V/R = 18.75/252 = .074A/2 = 37.2mA average plate current

305V * .0372A = 11.346 watts

Any input would be appreciated,

Thanks.

[eleventeen]

For sure, the voltages on your V1 and V2 look strange. Your output tubes (6V6) look completely nominal; assuming your 37.2 ma is for two tubes. That's right on the money.


You say you have -195 volts on V1 grid (pin 2) Where does this amp with a completely normal 5Y3 rectifier produce -195 volts? (ans: it doesn't) There's no place you can produce -195 volts. Furthermore, if there WAS -195 v on that grid it could conceivably damage that tube. At minimum, the tube would be shut off like a big dog. Evidence of being shut off is the zero volts on the cathode pin 3. Most typically, we see very small volts on the cathodes of preamp (not talking about PI) tubes. Like 1.5-2.5 volts, as you have on V2 section 1-2-3. Yet you say you have sound going through it.  (Incidentally, nobody can really say whether a 12AX7 section 1-2-3 is the "first" half and 6-7-8 is the "second" half.)

[HotBluePlates]

Adding the original AF-12 schematic, so we have a reference point.

Is "-195v" really missing a decimal point? Perhaps it's actually "-1.95v" or "-0.195v"? I ask because the original stage was grid-leak biased, so the cathode is connected directly to ground.

If you leave the grid-leak biasing, yank out C4 & R5, leave R6 at 3MΩ (or raise to 5MΩ) and replace the "magnetic phono input" with a 1/4" jack.

[PRR]

> Is "-195v" really missing a decimal point?

Or, like my meter, the 'mV' indicator is too small to see casually?

[casssax]

"If you leave the grid-leak biasing, yank out C4 & R5, leave R6 at 3MΩ (or raise to 5MΩ) and replace the "magnetic phono input" with a 1/4" jack." --  HotBluePlates

This is what I did. I have two input jacks, hi and low, just like on a princeton reverb. I'll try to post some updated specs when I get a chance. All the changes I made involve the first tube (V1). The only thing I changed after V2 is to remove the treble control because that is now in the tone stack between the first two stages.

"Or, like my meter, the 'mV' indicator is too small to see casually?" -- PRR
That's a possibility, my meter does some strange things sometimes. I'm going to try to take those readings again and see if I get the same results.

[casssax]

Is "-195v" really missing a decimal point? Perhaps it's actually "-1.95v" or "-0.195v"?

I double checked this tonight and it is in fact -0.195v

[PRR]

The published schematic shows -0.5V, you see -0.2V.

For one, the schem also shows Vp=100V, you observe 75V. The way this bias works, you expect the grid-leak self-bias to scale *roughly* with plate voltage. So if Vp=75V is acceptable (it works), you expect -0.375V at grid.

This grid-leak scheme is VERY variable tube to tube. 2:1 differences are reasonable.

This voltage is at a VERY high impedance. The schem says "vacuum tube voltmeter", no spec (typically 10Meg to 22meg). Modern DVMs may be 10Meg, though some are 1meg. Not-knowing, I'd guess that some of the difference in reading is just difference of meter.

IMHO, that stage is working near-enough as-designed.

However it was designed for magnetic phono or dynamic microphone, nominal level 0.005V, peaks hardly over 0.035V. Your guitar is likely to peak 0.2V to 0.5V. As the bias is 0.2V, your loudest strums WILL distort.

The published schem is in error at V3B, cathodyne driver. The 45V 100V 290V notes do not add-up correctly. Pin 6 is probably nearer 240V. Your observations differ.

Meter loading is an issue for the higher-Z nodes. Would be good to know what your meter is, and look-up the specs for DCV input loading.

I hate to say it, but I bet many of the resistors have drifted far-far off of spec. With power OFF, about 90% of them can be checked in-circuit. Start with the big dropper resistors on the main filter cap can. First short each cap-lug to chassis with an insulated-handle screwdriver tip. (Even non-lethal voltages can blow an ohm-meter.) These should read very-low at first (showing the cap absorbing charge), then rise to nearly the marked/noted values 3K 3K 50K.

And for geetar, if there is any (or no) doubt about R13 50K, just replace it with 22K or 10K. Guitar needs higher voltage to the preamps.

Go through the signal stages too. A "500K" plate resistor drifted to over 1Meg will be bad. And drift in carbon-comp resistors strongly suggests hiss and outright bad contact (crackle). I'd also bet there is an almost-good solder-joint in there, starting to come-apart after only 57 years; look for it/them.

For an all-purpose guitar amp, you would rip-up V1 and re-do it as cathode-bias with gain control after the first stage. The "MIC" input must be overloading when hit hard, even normal. This may be useful, if your fingers adjust to the happy/wild level of the preamp, but different guitars or passages may not fit the preamp so well.

[casssax]

Ok, I checked out the values of the resistors. Most of them are very close to the original value.

That R13 did go up a little bit to 52.5k

R6 and R9 are the 3M grid-leak resistors and I couldn't get a good measure of them so I'm guessing I would have to take them out of circuit.
There is one resistor, R22, that I couldn't get any reading at all on.

I found something interesting while I was doing this.
R12 and R18 seem like they have been reversed. It looks like someone went in and tried to replace R12 (there is a 473, slightly newer resistor that doesn't look like all the rest) but they mixed the two up and put them back in the wrong place.

So I guess I'll have to replace R22 and either swap or replace R12 and R18. Then I'll start thinking about changing from grid-leak to cathode-bypass.

The meter I've been using is an AideTek VC97+ (I think they also sell the same one under the name Vishy)
http://aidetek.com/mm5/merchant.mvc?Screen=PROD&Store_Code=A&Product_Code=VC97&Category_Code=DMM

I picked up a VTVM on ebay (don't have it yet). It's a Heathkit V-7A. I'm hoping it works, but it has all the probes with it.

Code: [Select]

AF-12 Resistor values
(all values in k ohms)
Resistor Original Value Current Value Difference
6 3000 33.5 -2966.5
7 100 102 2
9 3000 226.5 -2773.5
10 100 101.9 1.9
12 500 101.2 -398.8
13 50 52.5 2.5
14 5 5.09 0.09
15 0.1 0.0965 -0.0035
17 1000 1036 36
18 100 473 373
19 5 5.34 0.34
20 100 102.8 2.8
21 3 3.72 0.72
22 500 No Reading #VALUE!
23 500 503 3
24 0.25 0.249 -0.001
25 3 2.987 -0.013

I created a new schematic of what I've done to the amp since I've been working on it.

[sluckey]

The grid leak bias for V1A and V1B will not work properly with the changes you've made. For grid leak bias to work properly you need to put a coupling cap just to the left of each of those 3M resistors. Look at the original schematic. BTW, with that coupling cap in place you will be able to accurately measure those 3Ms without removing them.

But, rather than make the grid leak work, since this preamp is now looking very much like a typical Fender blackface preamp, I'd just clip out both the 3Ms and put a 1.5k/22µF between the cathode and ground of V1A and do the same for V1B. This will convert to cathode bias and I'm confident you'll like the results. Some people like to use smaller cathode bypass caps.

[trobbins]

Perhaps some basic protection parts would be in order first?  The schematic has no primary or secondary side fuses.


There are a few other basic additions that are highly recommended for any old amp, like 6V6 athode sensing, and PI and output stage stoppers.

[casssax]

I didn't add it to the schematic above, but I replaced the two prong plug with a three prong and a fuse. I also added a separate power switch and led. Originally the power switch was on the bass pot.

[casssax]

Code: [Select]

User Name Amp Type File Date Notes File Name
casssax realistic af-12 08-07-2015 Yes 18515

- Valve Pin 1 Pin 2 Pin 3 Pin 4 Pin 5 Pin 6 Pin 7 Pin 8 Pin 9
V1 12AX7 75 -.19 0 * * 75 -.15 0 *
V2 12AX7 150 .003 1.56 * * 255 15.6 82 *
V3 6V6 * * 324 305 .004 * * 18.7 *
V4 6V6 * * 324 305 .005 * * 18.7 *
V5 5Y3 * 334 * 340 * 340 * 334 *

I replaced R12,R18, and R22 and took new readings.
Things look closer to what the specs say.

The voltages on V2 pins 1 and 6  basically reversed themselves.
(Those are the pins with R12 and R18 attached to them)

I checked the source voltages and they are:
340V = 334V
300V = 305V
290V = 296V
200V = 183V

The published schem is in error at V3B, cathodyne driver. The 45V 100V 290V notes do not add-up correctly. Pin 6 is probably nearer 240V. Your observations differ.

And for geetar, if there is any (or no) doubt about R13 50K, just replace it with 22K or 10K. Guitar needs higher voltage to the preamps.

You were right, pin 6 is close to 240V now.
Do you think the 82V is correct where the schematic says 45V at pin 8?

Only getting 75V on pins 1 and 6 for V1
I sounds like I should try some smaller values in R13 to get this closer to the original 100V.



I ordered some parts to do the cathode bypass so that will be next.