Cathode resistor value..Zenith phonograph to 5D3 Deluxe

[Blindog]

I've been playing around with this old Zenith chassis I had lying around. Got to looking at the the schematic, and saw I could easily turn it into a 5D3 Deluxe circuit, since it had a paraphase inverter. Below is the original schematic, and schematic showing the changes I made. My question has to do with the 6J5 octal. What value cathode resistor and bypass cap would you suggest? It sounds pretty good right now, although it breaks up (with humbuckers) really early, with almost no headroom if you keep turning up the volume. Any suggestions? I'm going to put bypass caps on the pi input and the output tubes bias resistor like the 5D3 (or 5E3) circuit.

Thanks for any input you all might have.

Mark

[Andy Le Blanc]

paraphase inverters make a lot of gain..... it might be the inverter
reduce the values of the plate resistors there with an eye towards getting
equal or balanced signal for push pull

increasing the cathode resistor will reduce gain in the pre and the bypass
cap can be to taste start with small values or try it without...

you might want to put a bypass cap on the cathode of the power tubes...
you can also change the responce of the amp by sharing one resistor
for the cathodes of the inverter.....

[jjman]

I would be looking for which stage is the offender. Is it possible that the 6J5 is being overdriven?

For the PI you could also convert to cathodine (which is also not fully balanced.) Many desireable amps used it.

[Blindog]

Thanks for the input guys. I put 22uf 50v bypass caps as in the 5D3 schematic below. I also put in the 100pf cap between the plates of the PI.
The most dramitic effect was the PI bypass cap. It created more gain, but a more pleasant frequency response (better upper mid and highs). I also think bypassing the output tubes helped tame it down some. I tooks some voltage measrements, and something does not look right on the PI and the ouput.

6J5
Plate 84V

12AX7
Plate A 72v
Plate B 37.5

6V6
Plates 286v
Pin 3 317v (this is from the 37.5v section)
Pin 4 241V
Cathode 24.5V

Any ideas what's going on here? I must say it starting to sound pretty damn good (although isn't that what everyone says right before they blow their amp up? )

[Blindog]

Can anybody smarter than me figure out what thes voltages mean, if any thing? Something does not look right on the PI and the ouput.

6J5
Plate 84V

12AX7
Plate A 72v
Plate B 37.5

6V6
Plates 286v
Pin 3 317v (this is from the 37.5v section)
Pin 4 241V
Cathode 24.5V

Thanks!

Mark

[Andy Le Blanc]

on a 6v6 pin 3 is the plate.......pin 4 is G2

the plate V in the pre are relatively low.... but dont change them it will really
change the amps responce if you like it as is....

balance the output of the inverter if you can it will be another improvement
the value of the plate resister of inverting stage can be adjusted to match
plate V with the input stage of the inverter..... keep the floating arrangement
it provides a form of feedback so you dont need to apply any global feedback

[Blindog]

I'm sorry Andy, I meant to say that the grid of one 6V6 was 314v and the other was 241v. The plates are 286v. The 6V6 that was running high is from 12AX7 section with the 37.5 plate voltage. I think I will try some different tubes in that spot to see if that affects the voltage. All the tubes are the original Zenith tubes. Thanks a million for the advice.

Mark

[Blindog]

I've been tweaking this old hunk of iron some more. Put new filter caps in, and bypassed the cathodes of the 6J5, PI, and 6V6 with 22uf, 50v caps.
You can see the revised schematic below. I also posted some voltages in the schematic. Everything look OK?

One wierd thing happened when I lowered the plate resistors to the PI to 100k (like a Deluxe) It sounded good for about 5 minutes, then started to fizz really bad. Didn't matter what volume setting (amp or guitar) I changed back to 220k and it went away. Any ideas what caused this?

Also, do the PI plate voltages look wierd?

Thanks!

Mark

[Andy Le Blanc]

double check and may be replace the filter cap that supplies the inv. ....
those voltages are low.... and a higher draw seems to affect operation....

[Blindog]

Thanks Andy. I was getting those low voltages before I replaced the filter caps as well. New filter caps didn't seem to help at all. I'm at a loss! Could bad coupling caps to the output tubes cause situations like this? They are still the original wax coated .1 uf caps. Also, is it normal for the all the voltages to drop after about 15 minutues of warm up? The first node drops (slowly) over 10volts during warm up. Thanks again for your input!


Mark

[Andy Le Blanc]

ya .... check for leaky caps ...... positive voltage on the wrong side of the cap
it should have a degree of stability as far as supply volts goes.... if you
havent yet.... swap the original rectifier tube...

[Blindog]

I'll check those coupling caps, may just go ahead and swap them out. There's only two. I did put a 5AR4 rectifier in there knowing it would make more DC. The cathode resistor on the output tubes started smoking, so I took it out! I'll have to go and pick up a 5Y3GT. Are the Sovteks any good? I can get them from Parts Express close to where I live.

Thanks!

Mark

[Andy Le Blanc]

it sounds like the bias resistor needs to be looked at... bias 1-o-1.......
go down to a radio shack (my wife calls it radio sack) and get a pack of 1ohm
10 watt resistors..... their a low tolarance but it'll do ....
install one between the chathode resistor and ground.... this gives you something
to measure milliamps across..... static tube dissipation is a WATTS figure
and is one of the limiting factors of tube operation...
to find watts ..... A x V = W....... the ma. measured across the 1ohm......
converted to amps (move the decimal 3 places to the left) times the plate volts
equals static dissipation in watts..... 6v6 has a limit of 14w (16 counting the screen grid)
youll end up with say.... .088 x 317 = 28 (w) for two tubes.. or 14w idle or static dissipation for one tube
replace the chathode resistor with a higher wattage..... every 50 ohm increase
in resistance bring the dissipation figure down a couple three watts.....
always double check and re-measure every time you make a change
the plate volts can be measured from plate to chathode right on the socket
or to ground..... just be consistant and safe...

[Blindog]

I think I have some 1 ohm high watt resistors left over from another build.
So I solder it right across the bias resistor like a bypass cap?

Both coupling caps from th PI were leaky. I put some new ones in and that solved the low voltage on the PI, and got rid of some crackling I had upon power up. With 100k plates i'm getting about 150v per side now, Much better! But now there is a new problem. The breakup used to be fairly crunchy and uncompressed at about halfway on the volume control. Now it sounds like a almost like a fuzz pedal when pushed. Could this be a result of the bias resistor? Am I right in assuming this is not a preamp/inverter problem?

Thanks Andy

Mark

[Wayne]

So I solder it right across the bias resistor like a bypass cap?

NOOOOO!!!!! Your tubes won't like that at all!

They go from the most negative (probably grounded now) end of your cathode resistor to ground, in series with your cathode resistor.

W