Power Amp Compression.

[gingertube]

I have recently completed restoration of a vintage Oz Goldentone "Bassmasetr 60".
The power Amp uses 6DQ6 sweep tubes.
B+ = 430V
Vg2 = 220V
Bias = -40V
Raa = 2K6
It looks very much like this:
http://www.ozvalveamps.org/playmaster/pm117cct60w2.gif
Except - 12AX7 for a common cathode amp + concertina splitter rather than the 6BL8 shown. Also no vibrato in the preamp.
My mods - 47K grid stops on the output tubes.

I had a couple of the local guitar heros give it a "thrash" on the weekend just gone and they both raved about the power amp compression it exhibits. I want to reproduce this characteristic in other amps.

Has anyone an opinion as to if this lovely power amp compression is due to the sweep tubes used or is it characteristic of low Vg2? That is, could we expect to get the same power amp compression with 6L6,EL34,EL84, 6V6 etc. run with lower Vg2?

Cheers,
Ian

[jazbo8]

I think/speculate the compression would be similar... if you ignore the grid voltage scale as well as the Eg2 voltage steps in the picture below, you can see that the curvature and spacing of the transfer characteristics of the 6DQ6 (black) and 6L6GC (brown) are similar. So the output "compression" should occur in the 6L6GC as you lower the screen voltage, but of course the output power would decrease as well, since it was not specifically designed to work with low screen voltages.

[img:766:348]https://dl.dropboxusercontent.com/u/132 ... 06L6GC.png[/img]

[matt h]

(deleted)

[jazbo8]

Here is the transfer characteristic of 6DQ6 compared with the EL34, again, the result is similar, but of course, just because the "look" is similar does not mean they will react or feel the same way - the slopes of the curveature are still different between the various tubes, since I had to stretched the curves to make them look similar to the 6DQ6 in the first place.

[img:857:597]https://dl.dropboxusercontent.com/u/132 ... 20EL34.png[/img]

[pdf64]

I suggest checking the plate and screen supplies at max sine and then max square output conditions, to see if the compression is due to either / both sagging as demand increases.
Assuming that you haven't done that already?
For myself, I find that it's best to be methodical, rather than be distracted by something shiny!
Keep us updated!
Pete

[Reeltarded]

The small FB crowd will recognize this as another conversation from just days ago and here some time earlier. I just never asked like that!

This is the question to my answer. ^^^ up there. I ordered a bunch of values to test out the idea. I can swap headroom for squeeze and be really happy here.

[martin manning]

I'm with Pete here. The screen supply is derived in an unusual way, and that might have something to do with it.

[stephenl]

Try adding powerscaling/vvr to only the screens for testing.

I've tried KT66, 6V6, and 5881 with the screens 100v - 125v lower than the plates and liked all three variants. KT66 was my favorite.

[stephenl]

At a glance, I don't understand how the screen supply works... Isn't it missing a rectifier? Looks like it's putting 190vac across the cap. It should at least have a single diode for half wave rec. maybe I'm missing something.

[Stevem]

I see that also!
That schematic must be wrong as that 190 volts feed off the PTs center tap has a filter but no recto to make DC voltage, what gives?

[martin manning]

At a glance, I don't understand how the screen supply works... Isn't it missing a rectifier?

No, in a FWB rectifier the CT will have positive voltage, with ripple, centered at about half of the B+.

[darryl_h]

That power supply topology was quite common in Australian guitar amps in the 1960's and 1970's.

It was an economical power supply for output valves which could be operated with a screen voltage 50% of the plate voltage, such as the 6DQ6 ( 380V & 190V ), 6550 (600V & 300V ) and 6CA7 ( 800V and 400V ).

[stephenl]

Thanks for the education on that Martin - I learned something new.

[stephenl]

A different take on the lower screen supply:

[dobbhill]

Steve:

I just had to comment: I love the use of chokes in that supply! It's quite unlike the typical classic guitar amp power supply, but a beautiful example of something old and different.
Thanks for sharing that one.

D