Kitty Hawk Junior redplating

[ward]

I have a Kitty Hawk Junior on the kitchentable and I'm scratching my head over it.
Here's a schematic thats very close to what I have :

The amp works well, but after some time one or the other EL34 starts redplating and the bias runs off.
Thats the first thing i noticed.
Second, with all tubes in but without highvoltages the heaters on the EL34 are at 7.2Vac.
This voltage goes to a diode bridge and some caps and the Ecc83's get about 16,5Vdc. 

Could these two symptoms be related ?
Can I bring the heater voltage down with a two way diode like this
                      |-------->I---------->I--------|
------------                                                          -----------
                      |--------I<----------I<--------|
diodes that can stand the current ofcourse.
Or would I be better to remove the DC for the preamp ?

 

[moamps]

Could you post schematic here? Your link doesn't work for me.

 

[ward]

I fixed the schematic link.
Unfortunately the power is not included in this schem.

 

[moamps]

All AC voltages will drop a bit when the HT is turned on, so you should measure them in this condition. If AC for heaters is still too high, yes, you can use anti-paralleled diodes to lower it.  And mount them on a heatsink.
Red color of output tubes means that the dissipation on them is too high.  It is sum of heater and anode power.  So you should check anode currents also and adjust the negative bias if needed.  The anode currents of both tubes should be equal.
Here is the schematic of the similar power supply which is likely used in your amp.

 

[ward]

thanks moamps,

I gave the amp a dummy load and did some measurements.
With HT applied I get

* anode voltage 458VDC  (schem says 446VDC)
* cathode current on the EL34s of 58mA and 54mA
(is that too unmatched ?)
*the heaters of the preamp are still hot at 15VDC

If my math is correct that's putting out 26Watt plus the high heaters.
The power transformer is probably wound to see 220Vac on the primary, my house is at 235Vac.
So what's the best way to reduce all voltages ?
The negative bias is non-adjustable so is it best to modify this ?

 

[moamps]

EL34 has 25W max of anode dissipation so they are at border, too much in class A/AB for a guitar amp, IMO.  I would lower the current using higher negative bias.  Add a trimmer of let say 47k in series with 47k resistor in the bias voltage divider and set the cathode current to about 20mA. Some authors use 50% setting which means  that the dissipation is set to about of 50% max value. That's good starting point.
Check out heaters voltage on output tubes. If it is too high, use diodes to lower it. If this voltage is ok, and only preamp tubes are overheating, insert there (after rectifier) a small value, 5W resistor and regulate the voltage down to 12.6V.
Stay safe!

 

[Gus]

If the output tubes red plate ALWAYS check the coupling caps to the output tubes grids.

 

[ward]

thanks,

I put a 25k trimmer extra in series with the 47K in the bias circuit and I can now set the bias for a reasonable wattage.
I have it set at 0.032A for about 15 Watts now.
Also, I put some parallel diodes before the EL34 heaters. RGP30M was what I could find.  At 3Amps max forward current they need to be replaced with something bigger.
The EL34s have 6.15VAC now, the preamp tubes still have about 14VDC, I'll look for a suitable resistor.

No more redplating, I've been noodling away for the last half hour.  The clean channel sounds pretty good!

 

[ward]

If the output tubes red plate ALWAYS check the coupling caps to the output tubes grids.

Thanks Gus, in what way do I need to check the coupling caps?  Do they short? Or fail in another way?
At work I can have acces to a RCLmeter.

 

[ward]

I changed it up a bit with parts I could find.
I removed the diodes.
5ohm 5W resistor in the preampsection gives me 6.3VDC bullseye ,
2 x 0.15ohm 5W resistor going to the powertubes gives me 6.66VAC, number of the beast.
I'm pretty happy with this.

 

[moamps]

Finally check out the temperature of the resistors after some power on time. If there is a room inside and they have ceramic body, mount them to the metal chassis.

 

[Gus]

Thanks Gus, in what way do I need to check the coupling caps?  Do they short? Or fail in another way?
At work I can have acces to a RCLmeter.

The best way is with a cap meter that can test leakage at different voltages up to the caps rated voltage.

 

[ward]

The best way is with a cap meter that can test leakage at different voltages up to the caps rated voltage.

Okay I’m gonna have to study if the meter we have at work can do that.
It’ll be in the postcorona time.

 

[radardoug]

You had 16.5 volts on the preamp tubes, so I assume as is common on d.c. heaters, that they were run filaments in series for 12.6 volts. So are you sure you should set it to 6.3 volts? Maybe double check your wiring?

 

[ward]

You had 16.5 volts on the preamp tubes, so I assume as is common on d.c. heaters, that they were run filaments in series for 12.6 volts. So are you sure you should set it to 6.3 volts? Maybe double check your wiring?

My bad, 6.3VDC referenced to ground on one half of the 12ax7.
12.6VDC if i measure over both heaters.