No Cathode Current / No Sound

[Gibsonman63]

I recently converted my 1972 Fender Super Reverb to Blackface (AB-763) configuration. All of the voltages check out except for the vibrato circuit, which should not be switched in without the footswitch. I dialed the bias supply voltage to -52Vdc to match the schematic. I installed my BiasKing meter and power tubes, but I am getting no cathode current and no sound from the amp and the power tubes do not get very warm.

I haven't used the amp in several years and now I wish I had fired it up before making the modifications.

I have changed all of the electrolytic capacitors and double-checked polarity on all of them, except for the filter capacitors. I will pull the pan and re-check those tonight. I did find that I installed the 50uF/50Vdc capacitor on the little board with the diode backwards. I re-installed it with proper polarity, but no change.

All of the tubes are new except for the vibrato tube, but I understand that is not a critical tube for sound.

Suggestions would be appreciated.

[jmaf]

Gibsonman63, at what point did you measure the voltages? If you haven't, then do measure them straight on the tube pins(careful not to let the probe slip and short the HT) and let us know what you find.

[MWJB]

"All of the voltages check out "...well, if you have gone to the trouble of recording them, then you may as well post them. May be some clues there?

Replace that 50/50 bias cap with 100/100.

The filter caps are the important ones to change, use 2x100uf/350v, or 450v, instead of 70/350.

It's possible that the amp is lower voltage (400-ish B+?) & running too little plate current & therefore not making a sound? If there was no/little plate current the tubes would not get hot. Set plate current by mA (mV if using the cathode resistor method), negative voltage only gets you so far. See what happens if you adjust the negative voltage to late 40's.

Obviously double check speaker connections, if yuo accidentaly plugged them into the "Ext Jack" you will get no sound.

[Gibsonman63]

JMAF,

Thanks for the reply. I measured the voltages both on the tube pins and on the circuit board. Everything seems to be within tolerance except for the tremelo circuit.

I would assume most of the voltages would be slightly higher (for the US) since the original design was for 110VAC, single phase power and our current standard voltage is 120VAC.

[MWJB]

But you have changed the power tubes, these will not likely draw the same current as the previous tubes...for example I have seen folks change a rectifier in a brand new 59 Bassman and have the amp stop working because, with that rectifier & those tubes, there was NO plate current. Likewise a Concert that ran 40mA with the original tubes ran just 6mA with some new tubes.

You have told us that you have -52vdc at pin 5 of the power tubes, which should be fine (at least produce sound), in 9 out of 10 cases, if the B+ is over 450vdc. Are your new tubes Russian (Sovtek/EH)?

[Gibsonman63]

MWJB,

Thanks for the suggestions. I will have to post the voltages later when I get home. My B+ is at the other end of the spectrum. I am running at 508Vdc. Everything else is running 20 to 30 volts higher than the schematic, other than the tremelo circuit. My heater voltage is just a smidgeon above 6.3Vac.

I have increased the filter capacitance from 70uF to 100uF per Gerald Weber's advise. (Kendrick Amplifiers)

Interesting suggestion about the extension speaker output. I will have to double-check myself this evening, but I am pretty sure I had it correct.

[Gibsonman63]

MWJB,

The new tubes are Russian made, but I started with the old tubes for fear of frying a new set of power tubes. I get the same result with either set of tubes.

[MWJB]

Double check that there is no chance that the power tubes can be oriented wrongly in the probes heads (it happens) & that the heads are oriented correctly in the tube socket. What is the most (biggest number) negative voltage that you can dial in on the bias pot (no tubes installed)?

I'm assuming you have rewired the bias balance pot as bias adjust, double check connections & post a pic if you can.

[jmaf]

MWJB,

Thanks for the suggestions. I will have to post the voltages later when I get home. My B+ is at the other end of the spectrum. I am running at 508Vdc. Everything else is running 20 to 30 volts higher than the schematic, other than the tremelo circuit. My heater voltage is just a smidgeon above 6.3Vac.

I have increased the filter capacitance from 70uF to 100uF per Gerald Weber's advise. (Kendrick Amplifiers)

Interesting suggestion about the extension speaker output. I will have to double-check myself this evening, but I am pretty sure I had it correct.

Gibsonman63, is the amp absolutely mute? No static even at high levels?

[Gibsonman63]

It is completely mute. Not even a small pop when I flip the standby switch to play.

[jmaf]

It is completely mute. Not even a small pop when I flip the standby switch to play.

Gibsonman63, I fear a totally silent amp with all voltages OK at volume 10 is not good news.

A shorted primary on the output transformer would make the power tubes extremely hot and even destroy the tubes, you said they're warm. So let's assume the primary on the OT is fine.

Assuming all voltages ok(except the trem as you noted) and no sound at all at volume 10, we can pretty much narrow this down to:

1) As suggested above you may have plugged the speaker on the wrong jack.

2) You have an interrupted wire from the OT to the output speaker jack. Disconnect the speaker terminals and measure DC resistance on the terminals of the cable coming from the amp, you should have a low DC resistance there but not zero (and not too high or open either).

3) Measure the speaker cabinet's DC resistance at the input jack. If it's an open cabinet, disconnect one terminal at a time from speakers and measure the speaker DC resistances, connect them right back so you don't forget. They should be low as well, not zero or infinite/open.

4) I fear you have a bad secondary on that OT. I left this for last, hoping it's not the case, as that Fender vintage tranny is gold...

EDIT - PS. First try lowering the BIAS voltage a bit, see if you get static or hiss, it's unprobable that -52 VDC would totally shut down the tubes but it's possible, worth a try. If they're 6L6's, give them -46 or -47 VDC and watch the tubes for any sign of excess blue emition or a reddish plate, if they tubes hold up ok at lower BIAS, turn the amp up to 10 again without an instrument. You should hear some static.

[Enzo]

Isolate the problem.

Does the speaker work? test it. Unplug it from the amp. Touch a 9v battery terminals to the terminals on the speaker. A working speaker will make a little pop with each touch. Now touch the battery to the tip and sleeve of the plug on the speaker wire. Still pop? Or for that matter plug the speaker into some other amp for a test. ANd connect this amp to some other speaker.

Once we have verified a working speaker, plug it into the amp making sure to use the main speaker jack NOT the extension speaker jack.

NO sound at all?

Pull the power tubes and fire it up. Measure voltage in each power tube socket. Is there the B+ 500v or whatever on BOTH pin 3 and pin 4 of every power tube? Is there about -50v on pin 5 of each? Is pin 8 grounded on each? (Or 1 ohm away if you added a 1 ohm resistor) Take these measurements from the top so you are probing the actual socket pins that grab the tube pins. Don;t take them from the underside. REmember the pins count the other direction from the top.

Until there is proper power supply everywhere it needs to be, the tubes won;t function.

If ALL voltages are at all power tube, AND a known good speaker and good wiring is conected, then install tubes. Fire up the amp. Touch pin 5 of each power tube with a small screwdriver or maybe a meter probe. ANy sound?

if not, check the output transformer per RG's tip over at Geofex.

I'd bet my lunch money that you have something basic wrong, not some esoteric bias problem.

[Gibsonman63]

Bad OT?

Enzo,

Thanks for you help. I checked the secondary of the output transformer where the transformer leads connect to the output jacks and it reads zero ohms, so I would assume that it is shorted.

[MWJB]

No, the secondary has a very low DCR (half an ohm-ish), it possibly reads as low as the default lowest setting on your meter. A Short will most likely be accross the primary. Measure ohms from pin 3 of one power tube to the point where B+ rail & upstream end of the choke meet. Then measure from the other pin 3 to the same point, readings should be within 10%.

[jmaf]

Enzo, maybe one of the 2 output jacks are shorted to the chassi?

Worth a try unsoldering one or two of the OT's leads from the output jack and hooking the speakers up there directly?