6L6 vs 6V6 in Leilani by Gourley (Probably really by Dickerson)

[Resonator Guy]

I added a 25/25 bypass cap to the cathode of the 6V6 with no appreciable difference. I found a faulty ground from the OT and corrected that with some difference. It is now slightly louder but darker.
I am now attempting to draw up a sort of schematic. Reminds me too much of system flow specs that I used to do back when and I didn't enjoy doing them but I will persevere.
One or two further questions. In the Dickerson schem there is a 1000 ohm resistor with a 10/25 bypass cap off the cathode of the 6SJ7. Mine does not have that. A 5C1 Champ doesn't have that either. Would this make any appreciable difference to gain? I have a 57 Supro Super with the same tubes and a 6X9 speaker that I have never turned up past 6. Should this amp work similarly? It is like the Supro on 1 or 2.

[g1]

In the Dickerson schem there is a 1000 ohm resistor with a 10/25 bypass cap off the cathode of the 6SJ7. Mine does not have that. A 5C1 Champ doesn't have that either.

This is why I asked about the bias cell and why the schematic is essential. Shouldn't be tough to draw, grit your teeth and dig in .

[madkatb]

The 5C1 Champ has grid contact bias where the bias for the 6SJ7 is developed across the large grid load resistor and a cap on the input. The 1000 ohm resistor (with or without the 10/25 cap) would be used to set the bias. Chances are that if your amp doesn't have the 1000 ohm resistor it probably never did. But... it would sound better with that resistor as there would be less noise. Check for a cap on the input and a large (4 Meg plus) resistor to ground from the grid (pin 4) of the 6SJ7.
These two schematics, while both different, may be close to what you have. They both show different treatments for the input.

AIRCASTLE 79A SCH Service Manual free download, schematics, eeprom, repair info for electronics
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[Resonator Guy]

Alright, here it is. Not beautiful but it may serve. I was listening to a piece on the radio about how people, especially men, over 45 have trouble learning new things, but I got tired of it and stopped listening.
I originally added the cap and resistor (A) on the input to try and replace the bias cell in an effort to get -1.2 volts like the bias cell is supposed to provide. Tried a number of sizes of resistor without a great deal of success.
The big resistor off pin 4 of the 6V6 going to the 5Y3 was unreadable but since it was a big plastic coated perhaps wirewound that smelled good I left it there.

Now my thinking, thanks to some further advice from madkatb and comparing the 3 schematics for similar non Fender amps, is that I should rework the input resistors to something similar to those schems and add a 10/25 or 25/25 cap and a 1000 ohm resistor to pin 5 of 6SJ7. I think I should break the link from pin 5 to pins 1, 2 and 3 and reground them.

How does that sound? That should provide proper bias to the 6SJ7 and then send a stronger signal to the 6V6???

[madkatb]

I guess I maybe should have called that "grid-leak" bias. Basically the signal develops a voltage in the small capacitor that then makes the grid more negative than the cathode. So your cathode voltage reading would be ground or zero but your grid voltage, after signal was applied long enough to develop a voltage, would be a small negative value. I'm not sure if you're trying to make the amp "original" or "workable" but cathode biasing is probably easier to manipulate to get the voltage and frequency response you may want...change the resistor to change the bias voltage and change the capacitor to get the frequency response. And remove the small cap.
I notice that the heater circuits are referenced to ground (6sj7 pin 7 and 6v6 pin 7 via pin 1). If you can find the other leg of the heater winding from the transformer that is going to ground and wire the heaters with a twisted pair you'll get rid of a lot of hum. Right now the heater voltage at a fairly high current is creating a ground loop through the chassis.

[g1]

I was listening to a piece on the radio about how people, especially men, over 45 have trouble learning new things, but I got tired of it and stopped listening.

The big resistor off pin 4 of the 6V6 going to the 5Y3 was unreadable

What does it measure?
The 6V6 cathode resistor is shown as 100, you said it was 200?
What is the value of the resistor going from 6SJ7 pin 6 to 6V6 pin 4 ?
The resistor from 6SJ7 pin 4 to ground is 3K ?

[Resonator Guy]

What does it measure?
The 6V6 cathode resistor is shown as 100, you said it was 200?
What is the value of the resistor going from 6SJ7 pin 6 to 6V6 pin 4 ?
The resistor from 6SJ7 pin 4 to ground is 3K ?

6V6 cathode resistor is 200, my error in transcribing.
Resistor from 6SJ7 pin 6 to 6V6 pin 4 is 1000K
The resistor from 6SJ7 pin 4 to ground is 3K

I will have to measure the resistor from pin 4 of the 6V6 to 5Y3 tonight.

[Resonator Guy]

That resistor, a big dogbone type that looks like a plastic covered wirewound reads 2.06K. I lifted one end to read it.

[Resonator Guy]

Well, after studying all the available schems on similar amps and reding a good deal more in a number of books I have, I decided to abandon my attempts to apply a bias of -1.2 v on the control grid of the 6SJ7. That is where the (removed) bias cell was hooked up. I took out the cap and resistor I had added there to replace the bias cell and just ran a wire straight from the input to pin 4. Then I added a 1000 ohm resistor, bypassed with a 25/25 cap, from the cathode to ground and tested it.
It works! Pretty good tone, nice amount of volume, very little hum. So this one may be done.
Thanks to Jazz P Bass, g-one and Madkatb for your help and advice.
Three more on my workshop shelves plus a fix to the tremolo switch on a Gibson GA-5-T and all my amps will be working perfectly. I am going to try and draw a sort-of schematic for that Dukane chassis I have been talking about in another thread. Then it is on to a Teisco Checkmate 50 and a 6L6 powered homebrew (made by someone else).
Here is a pic from the back of the Leilani. It shows the on/off switch I added. This was the only non-reversible change I made to the amp. I really didn't like just plugging it in to turn it on.