please help on a silverface champ

[ericjwilliams]

I have 2 silverface champs, one sounds very good and the other still sounds good but is more shrill sounding. The voltages vary a bit between the two amps. Could this amount of voltage variation cause that much difference in sound? If so how could I get more voltage out of the other Champ?

The caps are all original in both amps, they both have the same brand of tubes (I swapped them back and forth to see if it made a difference any way) they both have weber sig. alnico speakers (I tried swapping them as well)

here are my voltages

the good champ.

12ax7
pin. VDC
1. 256
2. 0
3. 2
4. 0
5. 261
6. 0
7. 2
8. 0

6v6
pin VDC
1. 0
2. 0
3. 410
4. 419
5. 0
6. 0
7. 0
8. 24

5y3
pin VDC
1. 1
2. 424
3. 0
4. -6
5. 0
6. -6
7. 0
8. 424

here are the cap can voltages. I tried to make it how the can looks facing it

-- = 423
l = 413
-- = 391

the bad champ

12ax7
pin. VDC
1 255
2 0
3 2
4 0
5 247
6 0
7 2
8 0

6v6
pin VDC
1 0
2 0
3 386
4 395
5 0
6 0
7 0
8 23

5y3
pin VDC
1 0
2 399
3 0
4 -6
5 0
6 -5
7 0
8 399


cap. can.

-- = 400
l = 395
-- = 371

I just found this forum tonight and Im very happy to be a member. The tube chart will be very helpful in the future. I still pretty new to tube amps so go easy one me guys

[Enzo]

First things first, are the two amps identical? Yes, I realize the voltages differ and stuff, but are they identical models? Within a model name, versions, or revisions if you prefer, appeared every so often. For that matter, parts could change. Are both power transformers the same part number? (Stamped on the cover) Output transformers?

This is a simple enough amp, you can lay them side by side, and go down the part board and compare each part. There aren't that many parts. Are there any value differences? I don't mean measure them, but if a particular cap is 0.02uf on one amp and 0.01uf on the other, there is a difference you should hear. Or maybe a 470k resistor versus a 220k somewhere. And see if one amp has any parts the other lacks. DOn;t forget to look right on the tube sockets. One little cap addded for stability can sap the high end. Of course reducing the high end is the same thing as making it less shrill.]

If you are comparing voltages, pins 4 and 6 of the 5Y3 would be AC high voltage, not DC.

[MWJB]

You could try a 5V4 rectifier, or maybe even a GZ34 in the lower voltage champ. Old filter caps can cause low voltages, so changing the rectifier (if it works) is only a quick fix, I'd be looking to replace the filters & cathode bypass caps in both amps before really trying to evaluate them.

Some amps also just sound a little different to other seemingly identical amps of the same type, even after matching the measurable differences.

[Bruce / Mission Amps]

OK keep one hand behind your back or in your pocket and use an alligator clip for the other test probe!!


Something is wrong with the way you measured those voltages...
you should not have anything but filament voltage on lugs 4 & 5 and on 9.
You got some lugs goofed up in your voltage chart.

With respect to chassis ground and your 12AX7 socket, you should have high voltage DC on lugs 1 and 6, low voltage DC on lugs 3 and 8 and zero DC on lugs 4&5 and 9 (filaments). Lugs 2 and 7 should be zero volts DC too, they are the grids of the 12AX7.
On the 5y3 you should be able to read 5vac across lugs 2 and 8 and very high voltage across lugs 4 and 6... like 700vac!!!
With respect to ground, you should have high voltage DC from lug 8 or 2 to ground and high voltage AC from lugs 4 and 6 to ground.
Be real careful... I can tell you are new to this.

[coopecb1]

Re the pin #'s on the 12AX7, I see what happened. These have 9 pins, not 8, and he skipped #5.

The voltages on the first amp are right. The second is low. This might be the result of the 6V6 pulling too much current. Try swapping the tubes from the "good" amp to the "bad" amp.

The SF voltages are well in excess of the published limits for 6V6 tubes. I just got a SF Champ myself recently and discovered that any brand other than an RCA in good condition or a JJ will have problems, at least among the tubes in my collection.

[RWood]

A leaky AC-to-ground cap (aka death cap) will also pull all of the voltages down. If you could post your AC filament voltages from the rectifier (meter set to AC; one probe clipped to pin 2, one probe to pin 8) they should be 5 VAC. Filament voltages on the 6V6 (same procedure as above but to pins 2 and 7) should be within 10% of 6.3 VAC.

The idea here is that if both the filament AND the B+ voltages are down, that cap could be the culprit. And the empirical proof would be in simply clipping it out (it is between the fuse and one of the PT primary legs). In a 3-wire power cord it is not required.

RWood

[melvin]

An old 5y3 as opposed to a Sovtek will also give lower voltages. But it looks like you are only dropping 5 volts across the first dropping resistor, but a normal amount across the second. So the first cap may be bad. You could try measuring the ac at the first node...

[MWJB]

Melvin Wrote: "An old 5y3 as opposed to a Sovtek will also give lower voltages. But it looks like you are only dropping 5 volts across the first dropping resistor, but a normal amount across the second. So the first cap may be bad. You could try measuring the ac at the first node... "

The Sovtek will give slightly higher voltages but not a huge hike, because idle current draw is moderate.

5vdc drop accross the first (1K) dropping resistor = 5mA. Sounds spot on to me, a couple of mA for the screen & just over 1MA each for the 12AX7 triodes.

[melvin]

You are probably right, however, the schematic I have has 10v across that 1k resistor. So I thought it might be worth checking out.

I was just comparing two 5y3's, one old production, one new, in a tweed champ clone that produced a 20v difference. Still, it would not account for the tone difference described here.