MODULUS BASSMAN 5F6A BUILD

[Marcomel79]

Looks really nice! Hope you'll be able to crank it soon.

I put an order for 500V rated filter caps as it has 450V rated caps at the moment, and i get 450V on the plates off of standby with 7581A's..

But yeah, im so curious. I dont have a 12AY7, so ill use a 12AT7, which should be pretty close...

 

[Marcomel79]

With KT66 the heater current would be 3.8A, so it seems fine.
With the lower voltage HT taps and a 5U4, cathode biasing may turn out ok

Its on the higher voltage HT taps and the lower ones are tucked away. The owner does have a nice Mullard 5U4, so maybe he can use that....

 

[BlueX]

I put an order for 500V rated filter caps as it has 450V rated caps at the moment, and i get 450V on the plates off of standby with 7581A's..

But yeah, im so curious. I dont have a 12AY7, so ill use a 12AT7, which should be pretty close...

Did you buy this as a kit, and it came with underrated caps? Or did you make any mods to run at higher voltage?

 

[Marcomel79]

Did you buy this as a kit, and it came with underrated caps? Or did you make any mods to run at higher voltage?

A guy i do work for bought it from a guy who bought the kit but never opened the box. So thats how it came.... i always buy 500V rated caps no matter what....

 

[Pete Farrington]

Its on the higher voltage HT taps and the lower ones are tucked away. The owner does have a nice Mullard 5U4, so maybe he can use that....

Ok, for cathode bias perhaps try it with the cab load impedance doubled, so as to put 8k on the output valves.
That should help to reduce the squish (Aiken's term for the rise in cathode bias voltage at high power outputs) that might otherwise degrade the response.

 

[Marcomel79]

Ok, for cathode bias perhaps try it with the cab load impedance doubled, so as to put 8k on the output valves.
That should help to reduce the squish (Aiken's term for the rise in cathode bias voltage at high power outputs) that might otherwise degrade the response.

So i tried the amp full tilt earlier and theres quite a bit of ghosting, especially using the neck pickup... that pesky ghosting..... the tone though, is glorious with a ecc81 in V1!

 

[PelliX]

Curious, did that Ground switch ever get connected, or is it just there to be aesthetically correct?

 

[Marcomel79]

Curious, did that Ground switch ever get connected, or is it just there to be aesthetically correct?

The latter...

 

[Marcomel79]

Ok, for cathode bias perhaps try it with the cab load impedance doubled, so as to put 8k on the output valves.

So, thats achieved by using the 2ohm output from the amp, into a 4ohm cab, correct? The OT primary is 4K....

 

[Pete Farrington]

So, thats achieved by using the 2ohm output from the amp, into a 4ohm cab, correct? The OT primary is 4K....

Yes, that's it.

 

[Marcomel79]

Little update. I did the Rob Robinette mod for switchable fix/cathode bias and it works great! It was very tight but i got it in there...


I also swapped the 22+22uF 450V caps for an F&T 47uF 500V one for piece of mind. Hopefully the owner will like it

 

[Marcomel79]

few more pics in the head the owner built himself

 

[pietro.castelli]

Very nice work as usual Marco! Especially considering the challenges with the narrow chassis and the mods you have been able to squeeze in. Congrats!

The fixed/cathode bias switch is very interesting.
I might try it in the future, as I really would like to play with KT88s sooner or later.

I’m probably late to the part but would like to comment on this:

Wanna ask a question about grid stopper on the power tubes for this build. I have two 1.5K resistor left over from what csme with the kit. Now, from the original schematic, and the layout supplied by Michael at Modulus there are no grid stoppers. Should i skip them or is there any benefit in using them?

I should also add that the owner wanted it to be cathode bias, so he could basically just plug and play. Im doing the switchable fix/cathode bias mod from Rob Robinette, so he can also run KT66 in it. @Pete Farrington?

As far a I know, grid stopper resistors are not really advisable on a 5f6-a as they are known for changing the overdriven tone. I have no direct experience with this circuit yet, but wouldn’t install them unless the amp you have built oscillates or shows other kinds of issues.

 

[Marcomel79]

Very nice work as usual Marco! Especially considering the challenges with the narrow chassis and the mods you have been able to squeeze in. Congrats!

The fixed/cathode bias switch is very interesting.
I might try it in the future, as I really would like to play with KT88s sooner or later.

I’m probably late to the part but would like to comment on this:



As far a I know, grid stopper resistors are not really advisable on a 5f6-a as they are known for changing the overdriven tone. I have no direct experience with this circuit yet, but wouldn’t install them unless the amp you have built oscillates or shows other kinds of issues.

Thank you Pietro! Indeed it was a challenge with such narrow chassis. The owner of the amp seems to be very happy with it, so it will stay like that for the moment. Your amp is coming along nicely!!