73 Vibro Champ : No Sound

[keithb7]

Hello kind people,
I have a 1973 Vibro Champ on my bench tonight. No sound was the complaint. Tubes light up.
I look around and first off, see an 8 ohm speaker hooked up. OT on the VC is supposed to see 3.2 ohms.
I tested all tubes in my tube tester, seem to work as they should. I then took some simple ohm load measurements
on OT as I suspect it may be a problem with 8 ohm speaker hooked up.

I am still learning here, more on every amp I touch, so your patience is appreciated.

I see a pair of blue and red wires going into one side of transformer. I believe this is the input side.
I measure 289 ohms. I see a pair of black & yellow wires coming out of the OT going to
the speaker. I get 0.3 ohms across these windings. With my continuity beeper setting on
my DMM, it does beep across the black & yellow wires.

This is a link to a replacement transformer from Classictone with some specs:
http://www.classictone.net/40-18030.pdf

I have little experience with output transformers. Does this original '73 OT appear to be suspect? Thanks, Keith

[Enzo]

The 8 ohm speaker there will do absolutely no harm and would not cause a loss of sound. Assuming it is a good speaker. No the transformer does not sound suspect. Transformers are the lowest things on the list. 3.2 ohms is the impedance, not the resistance. A OT secondary will always have a very low DC resistance.

If BOTH heaters are glowing in the small tubes and the one in the power tube, then we want to know all voltages are present. Is there high voltage on all the tube plates?

[Chuck H]

I suspect Enzo is right, but .3 ohms read at the secondary does seem low. Low enough to possibly be the meter default reading in a given ohm range setting. But even with a secondary short there would probably still be some sound out of the speaker because currents there are very high.

With the power tube in and the amp ready to play, what DC voltage reading do you get from pins 3 and 8 on the power tube socket?

[keithb7]

Thanks guys. Pins 3 and 8 on 6V6GT measure:
Pin 3 393 VDC
Pin 8 23.42 VDC

Enzo, both heaters are lighting up in 12AX7 tubes. Plates are pins 6 & 2?
Pin 2 jumps around all over the place. Mostly 3.0 Mv to 0.1 mV
Pin 6 measured 0.2 mV

Took a few more measurements:
370V, 375V to 394V at 3 points at cap can.
6.5V AC at heater wires
Red wire from board into OT 394V

[Chuck H]

Plates on a 12ax7 are pins 1 and 6.

Your pin 6 measurement of .2mv is probably getting closer to the problem than the OT secondary reading. Check VDC to chassis at each lug on the big can capacitor.

[Jazz P Bass]

Here is a VibroChamp schematic & layout with the expected pin voltages.

champ_vibro_aa764.zip

[keithb7]

12AX7 pin 1 -24.2 mV DC

3 points to ground at cap can:
425V 410V, 409V

Both plate resistors measure in at close to 100k.

[Leo_Gnardo]

Your pin 6 measurement of .2mv is probably getting closer to the problem than the OT secondary reading.

Gots to be open or disconnected plate resistor to pin 6, I'm all out of other suspects. A rarely seen condition that will certainly silence your prize, but you'll have it fixed in a minute I'm sure.

[keithb7]

Off pin 8 of 6V6 power tube, there is a wire that runs down to 1W resistor with a 25v/25uF cap in parallel. I measured 23.4 VDC off pin 8.
The resistor is supposed to be 470 ohm, I measured it at 800k. A previous hack, removed the stock 25/25 cap there are put in a 500V 22 uf cap.
Measuring voltage out of said resistor, I get .4 mV. Same voltage I am getting at pin 6 on 12AX7, as wire from resistor travels to pin6.

Suspect I change 1W resistor to proper value and install proper 25/25 cap. Am I getting close?

[Jazz P Bass]

If you have 23.4 Vdc on pin 8 of the output tube, that is fine.
Leave it be & move on.
The 22uf/500 cap is fine. A bit overkill on the voltage rating but still, it's fine.

Why a wire goes from the output tube cathode to pin 6 of the 12AX7 is confusing to say the least.

Please look at the schematic & layout that I uploaded.

[keithb7]

Here is a photo of the amp guts. I traced the wire path in question with the green painted line.
Pin 8 on 6V6 through cathode resistor and cap, the 100k plate resistors then plates on 12AX7.
This seems to be a wiring error. I will study further.

[Jazz P Bass]

I have seen first tube heaters elevated by being tied to the output tube cathode.

Miswiring?

I would simply make it stock.

[52 Bill]

Miswiring?

I can't tell what is going on where the large cap has been added, but if that big cap is wired across the 470 ohm resistor, then it is wired in backwards. The negative end should be grounded.

The red wire from the two 100K plate resistors should not be connected to the end of the 470 ohm resistor. That end of the resistor should be grounded, is it? The red wire should be connected to end of the 10K dropping resistor that is part of the power supply.

[g1]

Can you post that picture without the green drawn in?

[Enzo]

If you have 20 some volts on pin 8 of the 6V6, then the resistor is fine. Your 800k reading is likely due to residual voltage confusing your meter.

Make your first job getting voltage back on pin 6 of the preamp tube. Without that you will never get sound. In your picture, pins 1 and 6 are wired to the board where a pair of 100k resistors meet in a Y shape. The 100k on the right is either open or a connection to it has failed.