Hiwatt Lead 50R no quiscient current

[lowell]

I managed to get this darn thing out of chassis... had to remove one of back panel support rails.

Anywho, there is no quiscient current on output tube pates. I am using the transformer shunt method. I have voltage checked all pins and seems fine. I even went so far as to remove power tube board and made sure each spring in each socket were indeed still soldered to their respective spot on the board. I checked the one 1k screen resistor and it is good and is supplying screen voltage. I checked the bias and it ranges from -20v to -40v when bias pot is adjusted, and bias is indeed reaching the tubes. I checked the cathodes and grounds and all check good. I'm at a loss w/ this. Could the OT be fried? That is the only conclusion I can come to... the amp DOES sound low volume and is distorted. I guess I'll check that...

[Robert M. Martinelli]

Do you have a schematic?

Hi Lowell,
do you have a schematic of the "darn thing" I could look at to be of some help?
I remember repairing an mid 80s Hiwatt and, though the preamp section was a little different from the earlier models, the final stage was much alike its elder brothers.... I guess this 50R is one of the new models, if not I beg your pardon ( I' m way into Voxes, a little less into Hiwatts... ).

Have you tried the "1 Ohm resistor method" instead of the shunt method?... What looks weird is that you stated you have the right voltages on all the pins, but no current; if the OT was fried you shouldn' t be able to read voltage on the plates, since they take their supply from the center tap of the OT....
I also have a stupid question, have you checked the HT fuse? In case you' re measuring with a digital multimeter, they' ve a very high input resistance and can pick up weird readings....another possibility is that both output tubes are blown, ( are they EL34/6CA7 like in the old Hiwatts ? ) this would explain why you have all the voltages and still no current is flowing through them....check heater filament between pins 2 and 7 directly on the tubes after taking them away from the chassis ( otherwise you' ll measure the parallel of the filaments and the 6,3 VAC sec of the supply tranny ).
Good luck
Bob

[lowell]

Bob,
thanks for your help... in testing the OT I discovered it was wired wrong. The CT and one plate wire were reversed. So then the q-current came alive. I then had no sound still so I check the preamp tubes. The +supply for the preamp plates was a cold solder joint. I retouched that and voila. Now, there's still no headroom and it's still distorted. I need a schematic for this thing because V3 looks to be wired in a different than usual way... will report back after viewing schem.

[Robert M. Martinelli]

Bias hints to avoid x-over distortion

Hi Lowell,
the distortion you' re getting could be due to the output tubes biased way too cold causing cross-over distortion - the fact is most people overlook that screen grids draw 6 - 8 mAmps, and, when reading the quiescent currents they forget to subtract the screen current from the reading - imagine you 're biasing say - at 25 mAmps, if 6 mAmps are flowing through that screen grid, then actually that plate' s current is only 19 mAmps, and cross-over distortion is likely to occur ... Biasing tubes is all about finding that "sweet spot" at which x-over distortion does not occur ( or it' s negligible ) and tubes are still well within their plate dissipation limits....

you can measure screen grids current by measuring the voltage drop across the relevant resistor, divided by the resistor value, then, putting 1 ohm resistors ( must be 1% otherwise your readings will not be accurate ) between each cathode and gnd, you' ll be able to read the quiescent current flowing through the tube simply measuring the mV across the 1 Ohm resistor
and since you have 1 Ohm, 1 mV will be equal to 1 mA.
At this point you can read the total cathode current, subtract the screen grid current you previously measured and obtain the actual plate quiescent current, which, multiplied by the +B voltage, will indicate the plate quiescent power dissipation ( be sure not to exceed 19-20 W with today' s 6CA7-EL34 )

Hope this helps
Good luck
Bob

[lowell]

Bob,
Thanks for your help. I am fairly familiar w/ biasing tubes. I have the amp biased at 17watts dissipation.

I have not ever heard of subtracting the screen current from the quiscient plate current to get an accurate reading. I do not understand how that should be brought into play when making a bias measurement. The tube is still running at 35ma if that's the current measured via transformer shunt method, why would the screen current be subtracted from that? I'm confused w/ that. Maybe I'm missing something.

Also I'm using JJ EL34s - technically their max dissipation is 25watts. You must be using the 70% or even a bit more for your max dissipation stated. Makes sense.

Anyone have a schem for this amp? V3 looks odd. There is a 470ohm resistor on cathode of second half of the tube and it doesn't seem to be grounded...?

[Robert M. Martinelli]

I have not ever heard of subtracting the screen current from the quiscient plate current to get an accurate reading. I do not understand how that should be brought into play when making a bias measurement. The tube is still running at 35ma if that's the current measured via transformer shunt method, why would the screen current be subtracted from that? I'm confused w/ that. Maybe I'm missing something.

Hi Lowell, the hints I gave you were referring to the "1 Ohm cathode resistor" measuring method, and since this method measures the TOTAL current flowing through the cathode ( which is the sum of plate PLUS screen grid current ) when using it it' s mandatory to measure and subtract the screen grid current to avoid biasing the tube too cold. If you' re still using the "shunt" method there' s no need to do so, as you' re measuring the plate current alone.
Regards
Bob

[lowell]

Oh ok Bob, I didn't know that... thanks!

[lowell]

Ok I have the scope on this thing and the input goes to pin 7...?? Weird... the signal is fine until I put scope on pin 2 of first triode... which seems to be the second grid of stage 1. There is a scooped section on top side of wave that appears at pin 2. This must be the distortion I'm hearing... or maybe not. I cannot find a suitable schematic for this amp... anyone have one? I see some odd components which I cannot tell are diodes or caps or what...

[lowell]

Alright w/ signal tracer I'm getting a good signal until the PI. The PI has a very low signal and is distorted on grids and plates... Plate supply for PI is 300v before the anode resistors. I don't have schem or voltage chart for it. Any ideas?

Pin b+ is:
1 207
2 33
3 35
6 193
7 35
8 34

[lowell]

was thinking the PI coupling caps might be causing problems. I tested them for leakage and they seemed bad. I replaced them and the signal is now good all the way until the output tube plates. I checked the bias again and see that it is 39ma on one side and 49ma on the other! I swapped the matched duo out for another just to see and it is the same. I then swapped the tubes left to right and still the same. The OT is now suspect.

[lowell]

ok ok ok... wait a minute here. In testing OT I think I found that the RED wire is indeed NOT the CT. There is RED, BROWN, WHITE on this OT. I rewired it and voila the amp sounds normal to me now. Is it possible that an OT has such odd wiring?? Is this a Hiwatt thing?

[Enzo]

No, but blue-red-brown is an American thing.

[Steve Conner]

Yup, the wire colours on British and European amps are all different. There was never any standard colour scheme here like there was in the states.

[lowell]

good to know... thanks!