Dumble Preamp (for SE amp)

[TUBEDUDE]

Sounds a little sketchy. I think a traditional fixed resistor. Why switched?

[Guy77]

I'm quikly reading that thread and I've find something that is not favourable to me

https://ampgarage.com/forum/viewtopic.p ... 09#p120509

That preamp has not enough swing for a tube as 5881 and so I think the same for the GU50 or 1625 tubes

EL84 is in the order of 11.3mA/V, 1625 is around 6mA/V and GU50 only 3-5.5mA/V

may be a 6AV6 tube is to be inserted between the Preamp and the Power Tube if the Power Tube has a "low" mA/V rate

Franco

Yes this circuit is limited to using an EL84 unless changes are made. You mentioned 6Av6 before power tube. Is there a reason for choosing this over a 12AX7 or AT7 or AY7.

Regarding the grid leak before feeding the power tube, is there a particular value that would work here in terms of bleeding to ground?

Cheers
G

[martin manning]

Regarding the grid leak before feeding the power tube, is there a particular value that would work here in terms of bleeding to ground?

Phillips data sheet (1969) says 300k max fixed bias, 1M cathode bias. Marshall 18W used 470k in cathode bias.

[Guy77]

Regarding the grid leak before feeding the power tube, is there a particular value that would work here in terms of bleeding to ground?

Phillips data sheet (1969) says 300k max fixed bias, 1M cathode bias. Marshall 18W used 470k in cathode bias.

Thanks Martin, I was just looking at the 18 watt Marshall schematic and noticed the 470K. I may build this SE amp this summer and will try with and without the Power Tube grid leak resistor.
I believe this thread should be in the Dumble section?

Thanks again everyone.

Cheers
Guy

[Kagliostro]

.... I think a traditional fixed resistor. Why switched?

I mean that the pots on the schematic, that are swapped using a switch, are acting, one at a time, as grid leak resistor

... You mentioned 6Av6 before power tube. Is there a reason for choosing this over a 12AX7 or AT7 or AY7.

I told 6AV6 because it is the same of one of the triodes in a 12AX7 tube and only one triode is required to achieve the purpose, so It will result in no unused triodes, OK, using a paralleled 12AX7 you'll use a more conventional tube and have a signal to drive the Power Tube with a bit stronger current

K

[TUBEDUDE]

Regarding the grid leak before feeding the power tube, is there a particular value that would work here in terms of bleeding to ground?

Phillips data sheet (1969) says 300k max fixed bias, 1M cathode bias. Marshall 18W used 470k in cathode bias.

I wouldn't mind a coupling cap also, as it's a direct connection to the outside world.

[Guy77]

Regarding the grid leak before feeding the power tube, is there a particular value that would work here in terms of bleeding to ground?

Phillips data sheet (1969) says 300k max fixed bias, 1M cathode bias. Marshall 18W used 470k in cathode bias.

I wouldn't mind a coupling cap also, as it's a direct connection to the outside world.

Interesting point, I see that the Fender Champ has a cap before going to the "outside world", pin 5 on the 6V6. From the sound clips I have heard of this miniHRM and from the others that have built this amp it seems to have worked fine without the grid leak and the cap.
I believe this is called the Hi Octane power section from the site AX84.

There is actually a Master volume on both the Clean and Overdrive channels acting as a variable grid leak before entering the EL84 and since I will build mine with no effects loop I will do without any other grid leaks before entering the EL84. No sure on the cap addition yet.

Thanks for everyone chiming in. This build was done many years ago and the fresh new ideas coming out may improve on an already great sounding little amp!

Sorry Kagliostro if I have hijacked your thread.

Ciao
Guy

[Kagliostro]

There Is no problem

K