Champ clone, 6V6, cathode bias, what voltage?

Chalky

Dear Amp Experts,

Got a no-name 6V6 clone with a 369EX power transformer that gives around 250v plate voltage, just like the Ampmaker N5X kit specifies when running a 6V6. I also read about typical Champs with bigger PTs running 350v-400v on the 6V6 plate, using a similar 470r cathode resistor.  Why the big voltage difference? What difference would I hear in moving to a bigger PT and raising the voltage so much?

Thanks!

ecc83

Morning Chalky and a HNY! I know gitamp makers   often run the A off valves but the limiting values given for the 6V6 G are Va 315, Ia 35mA  and Pa 12W although there are many variations of the V6 and lots of different specs.

I found a (French) data sheet for 2 valves in p-p. Va =285, Ia (no sig) = 70mA total which gives an Rk of 256 Ohms. The power for 3.5%thd is given as 14W. Anode dissp' is a sensible 10W.

Generally if you want early breakup, go for lower HT voltages. Raw power, 300V + but do remember that I suspect 6V6s are now NOT the valves I found to be virtually indestructible in old radios!

Dave.

Chalky

ecc83 said:

Morning Chalky and a HNY! I know gitamp makers   often run the A off valves but the limiting values given for the 6V6 G are Va 315, Ia 35mA  and Pa 12W although there are many variations of the V6 and lots of different specs.

I found a (French) data sheet for 2 valves in p-p. Va =285, Ia (no sig) = 70mA total which gives an Rk of 256 Ohms. The power for 3.5%thd is given as 14W. Anode dissp' is a sensible 10W.

Generally if you want early breakup, go for lower HT voltages. Raw power, 300V + but do remember that I suspect 6V6s are now NOT the valves I found to be virtually indestructible in old radios!

Dave.

Happy New Year to you too Dave!

Ah right, so as a home player it sounds like the 250v plate voltage is preferable, as early breakup is nice and I don't need maximum volume.  The 6V6 i have is marked 6V6GTA which suggests its a more modern variant than the oldies but goldies you're talking about Thanks!

ecc83

No worries C! I recall now that I fixed a couple of amps in Bell&Howell projectors. They used the very compact 6V6, 2 in ppull. Cine projjies got hot and the valves were subject to a lot of motor vibration but they rarely failed, usually it was caps wot went both eletros and foil.

BTW if you want an even quieter amp, fit a triode operation switch.

Dave.

ICBM

Higher voltage gives more power, more clean headroom, wider frequency response and a sharper breakup, generally.

250V is too low really, that will give quite a middy, mushy sound with very early breakup and not enough clean headroom in my opinion. (Although you may want that.)

If you have a scope it’s probably worth experimenting with cathode resistor values to see if you can centre the waveform between forward and cutoff clipping better than just going with 470 ohms, if the breakup is too early.

ecc83

ICBM said:

Higher voltage gives more power, more clean headroom, wider frequency response and a sharper breakup, generally.

250V is too low really, that will give quite a middy, mushy sound with very early breakup and not enough clean headroom in my opinion. (Although you may want that.)

If you have a scope it’s probably worth experimenting with cathode resistor values to see if you can centre the waveform between forward and cutoff clipping better than just going with 470 ohms, if the breakup is too early.

Not  sure if Chalky has a scope  but he IS an IT man so a PC scope progg' should be easy for him to sort!

You could use a 500 Ohm 2W pot to tweak the bias and then measure it and make it up from fixed Rs.

(that French spec I found (385V) seems about on the money IC. I have the .pdf)

Dave.

RiftAmps

FWIW, I run my SE Tweeds (5F1/5C1/5A2) like this:

Va-k -  300v
Vg2-k - 290v
Vk - 20v
Rk - 470r 5w

PT is 275-0-275 @ 90mA

Chalky

ecc83 said:

ICBM said:

Higher voltage gives more power, more clean headroom, wider frequency response and a sharper breakup, generally.

250V is too low really, that will give quite a middy, mushy sound with very early breakup and not enough clean headroom in my opinion. (Although you may want that.)

If you have a scope it’s probably worth experimenting with cathode resistor values to see if you can centre the waveform between forward and cutoff clipping better than just going with 470 ohms, if the breakup is too early.

Not  sure if Chalky has a scope  but he IS an IT man so a PC scope progg' should be easy for him to sort!

You could use a 500 Ohm 2W pot to tweak the bias and then measure it and make it up from fixed Rs.

(that French spec I found (385V) seems about on the money IC. I have the .pdf)

Dave.

Luckily, I have access to a scope from my mate who's a radio chap. @ICBM what shape am I trying to achieve on the scope?

RiftAmps

Chalky said:

ecc83 said:

ICBM said:

Higher voltage gives more power, more clean headroom, wider frequency response and a sharper breakup, generally.

250V is too low really, that will give quite a middy, mushy sound with very early breakup and not enough clean headroom in my opinion. (Although you may want that.)

If you have a scope it’s probably worth experimenting with cathode resistor values to see if you can centre the waveform between forward and cutoff clipping better than just going with 470 ohms, if the breakup is too early.

Not  sure if Chalky has a scope  but he IS an IT man so a PC scope progg' should be easy for him to sort!

You could use a 500 Ohm 2W pot to tweak the bias and then measure it and make it up from fixed Rs.

(that French spec I found (385V) seems about on the money IC. I have the .pdf)

Dave.

Luckily, I have access to a scope from my mate who's a radio chap. @ICBM what shape am I trying to achieve on the scope?

With a 1kHZ 150mV input, I'd be looking for 2% THD at rated output - 4-5w in this case. Because of the SE design, you'll only achieve a perfect sine wave at 3w output power.

ICBM

Chalky said:

Luckily, I have access to a scope from my mate who's a radio chap. @ICBM what shape am I trying to achieve on the scope?

Ideally, look for symmetrical clipping. It's not really critical if you can't get it, it just gives the most clean headroom (although as RiftAmps says, still not that high with this circuit) and probably the most even overdrive.

Chalky

Thank you all for your kind help.  Ampmaker does a 0-190-270 transformer that can be set up for switchable B+. I wonder if thatwould be an interesting way to explore raising the plate to a higher voltage. Or would you think I am more likely to just leave it at the higher voltage once its installed?

RiftAmps

Chalky said:

Thank you all for your kind help.  Ampmaker does a 0-190-270 transformer that can be set up for switchable B+. I wonder if thatwould be an interesting way to explore raising the plate to a higher voltage. Or would you think I am more likely to just leave it at the higher voltage once its installed?

You can't run a valve rectifier with that TX so would need to switch to a SS setup. Switchable B+ is great, but you'll find the lower setting somewhat lacking in tone and probably leave it at full whack.

Chalky

Thanks @RiftAmps - I thought as much. As @ICBM said, it is rather middy at the moment. I think the only other PTs I have are too beefy so it sounds like buying a new transformer will be needed.