Fender Pro Tube Twin Low Output

[harry]

Hi there,

I'm fixing a Pro Tube Twin... output is about 43 watts before distortion. I've looked at the Fender schematic and all voltages seem to check out (although the test signal they suggest is calibrated to give about 19 watts output, so isn't much use). Getting good plate/screen voltages on the power tubes. The output wave looks good, clips a bit earlier on one side than the other, but nothing serious.

Max I can get out of it is 81 watts full-tilt. The output valves are Groove Tubes.. rated on their scale at "10", which I believe means late clipping. They were bought fairly recently. I don't have another spare set to test them out. Only have EL34 amps myself...

Injecting a 1kHz sine wave into the 'power amp in' shows similar results, so I can only conclude it's the power section not working. The schematic says a 5.6Vac sine wave should produce an 85-watt output. Unfortunately my signal generator only goes to 4.5V... so I used a solid state power amplifier slowly and carefully ramping up the volume whilst watching on the scope.. clipping arrived at the same point.. about 45 watts.

Could these tubes have been bad to begin with? It was sounding poor and fairly low output before we changed them; changing them doesn't seem to have made much difference.

I've tried new preamp valves (in PI and gain stages).. didn't make any difference.

Any suggestions much appreciated as always,

Harry

[harry]

oops - didn't think to check the heater voltages. Still quite new to this fault-finding. It appears to be 5.7V on the power tubes and 5.9V on the pre-amp tubes.

I imagine this has something to do with it.

[harry]

Although I guess now my question is - what is happening to the voltage? It appears to be 6.4v at the pilot light.. 6.2v at the first preamp tube... then fall to 5.7v at the power tubes

Is this normal? What can I do about it?

Running without the reverb and gain channel 12AX7s gives about 55 watts before clipping, 100 watts full-tilt... Running with just two 6L6 tubes and the load doubled gives about 40 watts before clipping, 80 watts full-tilt (only tried it for a very very short period). So the power transformer can't handle 4x6L6s anymore? Or is this something to do with the Groove Tubes "10" rated tubes?

[Enzo]

You have presented a sea of numbers, but we have not heard what the B+ is loaded down to, or how much idle current the power tubes are biased to.

Does the load impedance - nominal impedance - match the amp output?

Just thinking, if four 6L6 tubes draw 3.6 amps, and you are losing 0.7 volts in the wires, Ohm's Law tells me the wires have a resistance of 0.19 ohm. That doesn;t seem unreasonable to me.

[harry]

I biased the amp using the test points on the back to 60mV.. from what I can tell from the schematic this seems to be a reading taken across a 1-ohm cathode resistor shared per pair. So by that account, the idle bias is 30mA.

At B+, I see 480V with no signal. Then 460V on the onset of clipping, and 441V at full-tilt.

[harry]

I am using a resistive dummy load of 4 ohms. Using this setup, other amps in my possession have shown 100 watts rms before clipping. According to the schematic, the speaker jacks are hard-wired to the OT's 4-ohm output with no option for changing it.

Also, the results I got from using just two power tubes were in the right ballpark, so I don't think the output transformer is at fault.

[MWJB]

What happens to the heater voltages with just 2 power tubes installed? Try both pairs & combinations of all 4 tubes. 5.7VAC at the power tube heaters isn't right.

[harry]

With just 2 power tubes it goes up to 5.9Vac (well, this meter was actually measuring 5.5Vac going up to 5.7Vac with two tubes, but I have reason to believe it is less accurate).

Combination of tubes makes no difference

With no power tubes (and all 8 preamp tubes.. yes.. 8!) it measures 6.1Vac

It takes a very long time for the output voltage to reach it's maximum with a test signal applied. Probably a minute or two of slowly ramping up for that last 20%.

[MWJB]

I'd expect heaters to be around 6.5VAC under normal load, less than 6.3/6.2VAC suggests a problem, something loading down the PT secondaries. Pull the rest of the tubes & tell us what you get.

Can you get to the standby switch terminals? Unplug from the wall & connect an ammeter, set to dc<200mA accross the switch, power up - you should only be getting tens of Ma with the power tubes pulled.

Get a more accurate AC meter on it...cheapy meters can be innacurate reading low VAC, especially if you only have a <200VAC range. So we want to eliminate erroneous meter readings & stop me wasting time of yours that could be better spent elsewhere :-).

[52 Bill]

I'd expect heaters to be around 6.5VAC under normal load, less than 6.3/6.2VAC suggests a problem, something loading down the PT secondaries.

What is the incoming primary voltage reading from the mains?

[Enzo]

You beat me to it Bill, If your mains transformer connection is set up for 240 and your mains are more like 220 or even 230, that could be all it takes. A rewire to the 220v taps then perhaps.

[pdf64]

'It takes a very long time for the output voltage to reach it's maximum with a test signal applied. Probably a minute or two of slowly ramping up for that last 20%.'
That's weird. Do you mean that you turn the signal level up to onset of clipping, and then find you can turn it up a little more until you reach clipping and so on?
Or is the actual signal level increasing on it's own?
Or can you turn it up to hard clipping, and it gradually adds the sine wave peaks? Peter.

[Enzo]

Maybe that is an artifact of low heater voltage - dim tubes taking time to get fully flowing.

[saldun22]

Something from my expiriance...
Check all 100K resistors in the 12AT7 and 12AX7 tubes on the anodes legs
1 & 6!
You can check voltage and everithing sims to be OK,but power is lower than should be !
The resistors is changing value and become 300-400 K
Hope you will make Your Amp powerfool again

saldun22

[harry]

Mains voltage is 235 here.

The PT has a 230V tap, I set it to that - heater voltage went up to about 5.7V, so not much change.

The actual signal level changes as I leave the amp. Clipping still occurs at the same place. The bias is also increasing - starts at about 52mA, heads up to 65mA and stabilises.

No funny anode resistors.

Interestingly, the fuse I found on the heaters is a 4A time-delay.. Although it should be a 5A fuse according to the schematic, the owner has had this amp from new, so I presume the fuse has been in there since
new..

so 4x900mA per 6L6... = 3.6A 7x12AX7s and 1x12AT7 @ 300mA per AX and 450 per AT = approx 2.5A

This would suggest more than 6A draw if all the tubes were drawing the maximum amount of current. Yet the schematic calls for a 5A fuse, suggesting that the amp has been designed to never draw more than 5A of current.