Need help with tone controls

[Ian444]

I had an old Vase amp in for a lookover, I drew up this schematic. On final check, I found the bass control to have no effect. The "pull bass" pot switch does increase the bass, but the bass pot does nothing. I checked the pot, the cap and the resistor values and all seems fine, but no action.

Unfortunately I don't have enough theory or practice to even know if this tone control setup will work, it is not like other Vase amps I have worked on and may not be original, but the wiring, components and construction of the tone control area does actually look original. This amp was built around late 1960's. Can anyone help me with why the bass pot does not work?

Edit: the 1n and 10n caps near the "pull bass" switch are the wrong way around in the schem, need to be swapped over.

[Ian444]

I downloaded 5spice simulation software. It seems the range of bass is about 4dB at 80Hz and 8dB at 40Hz. It's a bass amp. Maybe I simply could not hear the change with a 6 string guitar. It looks like the bass control should work, and the amp has not been tampered with...live and learn. The "pull bass" switch gives it an effective range of around 18dB, that is, if the software is good. I've always been reluctant to try spice, but with no answers coming forth, I had to do something And thanks for looking at my post guys, I know it was a difficult question.

[camerongrieve]

Nice work Ian, sounds like you got to the bottom of it.

What is a "C" type pot, can I ask? I can only guess its reverse log taper?

Also, any chance you'd have a complete schematic to have a look at? Just for interests sake...


Regards,

Cameron

PS 1st post... been lurking a while, learning plenty. The level of knowledge here is really impressive.

[PaisleyTube]

C = reverse audio log

[Ian444]

I think in the 60's an A pot was linear and C was normal log. Later they changed it to A = log and B = linear.

Cameron, there is a more interesting schematic on the Vase website www.vase.com.au If you scroll down to the bottom of the main page to "Newsletter Back-Issues online for download", download the Feb 2011 newsletter, in there is a preamp schem for a 100W guitar amp, this is a good example of the old Vase guitar amps, but it seems none were really exactly the same, and apparently no written schematics were ever kept when they were built. The smaller amps used a standard LTP phase splitter, larger ones used a cathodyne followed by a driver stage like the Williamson hifi design. Those tone controls are unique as far as I know (as in they don't seem to be copied from any other manufacturer) and the amps give a nice Twangy sound, a bit raw and unrefined but in a very good way. Difficult to describe.

1st post, good on ya, yes there are some smart and experienced people here, with a remarkable lack of ego to go with it. They really know how to build amps. I'm still on my Learner plates.

[camerongrieve]

Thanks Ian, that's an unusual looking pre-amp design - or maybe it's just the way it's drawn. It takes my brain a little while to digest these things :-/

BTW, have you seen "ozvalveamps.org/vase" ?
There's a lot of info there, but not much in the way of schematics.



Cameron

[tictac]

So is there any more of that schematic you can show us?

[Ian444]

Yes there is. A LTP with 820R/10K tail, 1M bias resistors, driving 2 x EL34 through 0.1uF caps with 4K7 grid stoppers, 220K to bias supply, 220 ohm screen grid resistors, 466V B+, blow most modern EL34's to bits possibly.