Vase Bassman 100 - power amp valves and output transformer

[balmoral]

Hello - first post...   

So I'm new to the world of amp mods / building etc and I recently picked up a Vase Bassman 100 head for a steal - or at least I hope it is..        It was missing all of the output valves, and I've done the best I can to trace the circuit, but I still have a bit to go in the power supply and switching. 

Question 1:   I've been told that it could use either 4 x EL34 or 4 x KT88 valves.  Note that B+ is 500V.   The specs throw me a bit, due to the differences on pin 1.  So thoughts on this would be appreciated.   

Pin 1 appears to connect to B+ and this has a 390 ohm resistor connected to pin4.
Pin 3 Plate, to out put transformer
Pin 5 grid
Pin 6 not connected
Pin 2 & 7 are heaters
Pin 8 ground

Question 2:  I think the output transformer is a little unusual.  I believe it's a Ferguson OPM21A  Unfortunately the model number is not marked on the transformer.  Date code from 1966.  Specs below, for the only one in the catalog that matches the wiring and wattage of the amp (100w).

So I think that it's wired up for 32ohm, and there's only one speaker jack on this amp - no other options.  I've read that it's safe to run 100% under the nominal rating, so this would be fine at 16 ohms right?  It's a massive transformer, could it go any lower?   I plan on only using two output valves if this makes any difference, as I certainly don't need the power that this is likely to produce.





Schematic, or at least what I've traced so far



I know this is probably a lot for a first post, and possibly some very basic if not stupid questions - please be kind...   I'm still learning all things valve related, as I have previously spent all of my time building pedals.

Here she is - carpet has been stripped off revealing the original tolex, which is in good shape. Knobs probably aren't stock, but they look as old as the amp, so I might leave them on.  Anyone familiar with Vase will also question the green - maybe not original, but I'll leave it as is cause I like it.

[AlNewman]

You wont be able to use el34 as it's wired, because it's using pin 1 as a tie in for the screen resistors.

[uki]

Welcome to the forum

People around here love pictures of the circuit board, helps big time!

[glass54]

Hi Balmoral,
Start by having a look here  https://ozvalveamps.org/vase.html
VASE rebirthed here https://vase.com.au/
Other Info here https://reverb.com/au/news/a-history-of-brisbanes-vase-amplifiers
I've worked on a few but not recently.
Kind Regards
Mirek

[balmoral]

Welcome to the forum

People around here love pictures of the circuit board, helps big time!

Thanks and sure, here's a few more photos. 

I think I've also answered my own questions re the transformer, after brushing up on output transformers care of Uncle Doug on youtube.  Seems to have a 10:1 turn ratio, with 500V on the primary, the results start to get interesting below the designated 32ohm output. 16ohm is cutting it fine re the transformers current capability from what I can tell.





Someone has replaced the filter caps at some stage, hence the mountain of silicon & one of the valve sockets has also been changed.  It has a nice little shield / cover on the output transformer that I'm lifting up to expose the wiring underneath.

[balmoral]

Hi Balmoral,
Start by having a look here  https://ozvalveamps.org/vase.html
VASE rebirthed here https://vase.com.au/
Other Info here https://reverb.com/au/news/a-history-of-brisbanes-vase-amplifiers
I've worked on a few but not recently.
Kind Regards
Mirek

Thanks Mirek - I contacted Vase, but they said they didn't have any schematics for older models, and even if they did, they said amps varied quite a bit, depending on customer requests.  A lot of custom jobs apparently, which I'm wondering if this might be, as who has a 32ohm output on a regular amplifier?  Maybe it ran some kind of weird speaker rig?  Who knows...   

I'm in no rush on this one, so it may sit on a shelf for a bit until I decide what to do with it.  Restore to original, or mod to something useful now.  i.e.  less wattage, and more output options.   

[zircontweezer]

Hi Balmoral, you may get some answers/advice at the Australian website Aussie Guitar Gear Heads as well. The "Aussie made amplifiers" is at the top of the page.
https://www.aggh.net/discussion/index.php

[balmoral]

Hi Balmoral, you may get some answers/advice at the Australian website Aussie Guitar Gear Heads as well. The "Aussie made amplifiers" is at the top of the page.
https://www.aggh.net/discussion/index.php

I tried registering there a couple of times, but admins have not approved yet.  Not sure what's going on there...   

[Latole]

You wont be able to use el34 as it's wired, because it's using pin 1 as a tie in for the screen resistors.

Easy to mod no ?

[Merlin]

I think I've also answered my own questions re the transformer, after brushing up on output transformers care of Uncle Doug on youtube.  Seems to have a 10:1 turn ratio, with 500V on the primary, the results start to get interesting below the designated 32ohm output. 16ohm is cutting it fine re the transformers current capability from what I can tell

Indeed, I'd say you're kinda stuck with the 32ohm impedance, as the jump to 16ohms alone is a big one. I guess you could instal two output jacks in series, so you can conveniently plug in two 16 ohm speakers? You'd have to buy two speakers of course 

[balmoral]

I think I've also answered my own questions re the transformer, after brushing up on output transformers care of Uncle Doug on youtube.  Seems to have a 10:1 turn ratio, with 500V on the primary, the results start to get interesting below the designated 32ohm output. 16ohm is cutting it fine re the transformers current capability from what I can tell

Indeed, I'd say you're kinda stuck with the 32ohm impedance, as the jump to 16ohms alone is a big one. I guess you could instal two output jacks in series, so you can conveniently plug in two 16 ohm speakers? You'd have to buy two speakers of course 

I'm only in this for $100 Australian, so I may shelve it until an appropriate transformer appears.  I was thinking about a couple of 16ohm speakers in series, but then that's a pretty weird speaker box to release into the wild.  Someone will most likely screw up and hook it up where they should not - I don't want to be responsible for that.

Thanks for the advice, much appreciated.