red plating?

[goober]

I have a newly built 6g10. It's a pretty dirty sounding amp past about 3 on the dial. I've noticed the center element in the 6v6 glows sort of orange and fluctuates in intensity with volume. Gets really bright wide open and the amp seems to compress quite a bit maxed out, more so with the negative feedback resistor connected, less compressed with it out of circuit. Using a 470ohm cathode resistor. I assume that the fluctuating glow in the center of the tube is not what is considered "red plating". My understanding of "red plating" is the large grey plates on the outside turning red. I just want a confirmation of this so I don't burn out my tube. I have about 400v on the plate and with a choke in place of the 10k about 6 or 8 volts less on the screen. Does that seem ok?

Thanks

[Wilder Amplification]

I have a newly built 6g10. It's a pretty dirty sounding amp past about 3 on the dial. I've noticed the center element in the 6v6 glows sort of orange and fluctuates in intensity with volume. Gets really bright wide open and the amp seems to compress quite a bit maxed out, more so with the negative feedback resistor connected, less compressed with it out of circuit. Using a 470ohm cathode resistor. I assume that the fluctuating glow in the center of the tube is not what is considered "red plating". My understanding of "red plating" is the large grey plates on the outside turning red. I just want a confirmation of this so I don't burn out my tube. I have about 400v on the plate and with a choke in place of the 10k about 6 or 8 volts less on the screen. Does that seem ok?

Thanks

If you're referring to the fluctuating red glow inside of the plate structure that fluctuates with volume, that's screen glow that you're seeing. Not uncommon in guitar amps as the nature of the power supply design is very hard on screens.

[Steve Conner]

Yes, that's the screen glowing. I've seen it myself when overdriving 6V6s.

How does your build compare to a real 6G10? Did you use a different OT impedance or different voltages? It would be interesting to know if the original 6G10 would do the same thing.

[goober]

Well the diagram I have shows 350v on the plate and I have around 400 with an old original 5y3. It looks like the same circuit as the 5f1 champ with a tone control and an extra 16uf filter. I'm only using one 16uf as in the champ circuit. I got the PT from an old piece of test equipment and the OT is either a Weber or Mojo Champ unit with 4 and 8 ohm taps. I'm using the 8 ohm tap with a Weber Sig12 ceramic 25 watt 12 inch speaker. I'm wondering about the value of the 22k feedback resistor and if it needs to be adjusted for the 8ohm load vs the original 4 ohm.

[tubeswell]

It looks like the same circuit as the 5f1 champ with a tone control and an extra 16uf filter.

Its exactly the same circuit as a 5F2A. Not quite sure why they called the 6G10 the "Harvard" and the 5F2A the "Princeton" tho'. Maybe the originals had different size speakers or something.

[Wilder Amplification]

Well the diagram I have shows 350v on the plate and I have around 400 with an old original 5y3. It looks like the same circuit as the 5f1 champ with a tone control and an extra 16uf filter. I'm only using one 16uf as in the champ circuit. I got the PT from an old piece of test equipment and the OT is either a Weber or Mojo Champ unit with 4 and 8 ohm taps. I'm using the 8 ohm tap with a Weber Sig12 ceramic 25 watt 12 inch speaker. I'm wondering about the value of the 22k feedback resistor and if it needs to be adjusted for the 8ohm load vs the original 4 ohm.

If anything you'd wanna increase it to keep the NFB the same as the speaker output voltage will be 1.4x what it would've been with a 4 ohm dealio. As such that would give more NFB than a 4 ohm OT would with the same stock value NFB resistor. Try maybe a 33K?

[goober]

I found the diagrams of the 6g10 and the 5f2a Princeton on the amp guide at Ampwares.com. They are the same except for the transformer numbers. According to the amp guide, the Princeton had an 8 inch speaker and the Harvard had a 10. It appears that the cabinets are about the same size. I'm running this one through a prototype 5e3 Deluxe cab I built a few years ago. I plan to build the Harvard size cab and use a 10 inch speaker.

I played around with it some this afternoon. I found a choke I pulled out of something, I'm not sure what. I put it between the 5y3 and the first filter and B+ dropped to about 280v. Putting another 16uf cap in front of the choke brings it back to 380 ~ 390v. Sounds good both ways. Softer overdrive at lower volume without the extra cap, I'm thinking about putting a switch on the cap but I'm concerned that It won't bleed down when its switched out of the circuit and I don't like touching charged caps. I got enough of that in my high school electronics class. Back then the "fun" thing to do was to charge up a 20uf cap and leave it on somebody's desk and wait for them to come along and pick it up.

[Wilder Amplification]

I found the diagrams of the 6g10 and the 5f2a Princeton on the amp guide at Ampwares.com. They are the same except for the transformer numbers. According to the amp guide, the Princeton had an 8 inch speaker and the Harvard had a 10. It appears that the cabinets are about the same size. I'm running this one through a prototype 5e3 Deluxe cab I built a few years ago. I plan to build the Harvard size cab and use a 10 inch speaker.

I played around with it some this afternoon. I found a choke I pulled out of something, I'm not sure what. I put it between the 5y3 and the first filter and B+ dropped to about 280v. Putting another 16uf cap in front of the choke brings it back to 380 ~ 390v. Sounds good both ways. Softer overdrive at lower volume without the extra cap, I'm thinking about putting a switch on the cap but I'm concerned that It won't bleed down when its switched out of the circuit and I don't like touching charged caps. I got enough of that in my high school electronics class. Back then the "fun" thing to do was to charge up a 20uf cap and leave it on somebody's desk and wait for them to come along and pick it up.

Actually what you'd wanna do in that scenario is instead of installing that cap, install a shunt switch that shorts/unshorts the choke. You'll have to find out how much voltage the choke is dropping when it's in circuit and under signal, then find a SPST switch that can handle at least that voltage across its contacts when the choke is unshorted. But this way when its shorted its out of circuit and B+ should be high, then placed back in circuit when unshorted, in which it will drop your B+ down.