Custom Base For VTA ST-120

Hi Guys, I am new to the forum and thought I would post a couple pictures of my recent ST-120 build with a custom walnut base. I designed this base to allow the amp to slide in without any modifications other than attaching a ground lug to one of the front octal socket screws.

nicely done indeed!!

Thank You Guys.

That's a beautiful looking amp. You did a hellava nice job putting that amp together. DrewK

Nice job. Looks something like Skizo's work.

Question: By using the bias meters, can you omit the test points on the front? I was thinking of using meters as well.

Captain Coconut wrote:Nice job. Looks something like Skizo's work.

Question: By using the bias meters, can you omit the test points on the front? I was thinking of using meters as well.

Actually, it was Skizo’s beautiful work that inspired me to build this one.
I put the test points in the front so I can calibrate the analog meters with my Fluke DMM.

Ah, never thought about calibrating the meters.

I thought it looked a bit familiar too ... ;-}

Beautiful work, eh.

As far as biasing using just the meters ... I can get close just using the Mark One Eyeball - within a point or two anyway - but the VOM is much more accurate. Great for quick checks when I fire the amp up - I used the same switching setup for the meters, so just flip thru the positions and read them for ready. I still use the VOM at the test points when needed.

Just one word of caution ... the meter circuit does add resistance, so you need to verify that the readings are the same at the tubes. Had a pretty major discrepancy here with a direct hookup.



Ended up going with center off switches for the front/rear selectors and added some small VRs (200 if I remember right) to each test position that allow me to match the meters to what I get on the VOM with the meter out of line. I also had to cut the internal shunt resistor on the meters to convert the meters I used from mV to mA ...



Still gotta get in there one of these days and clean up the wiring a bit ... things got a bit messy after version three or four of the circuit ...

sKiZo wrote:I thought it looked a bit familiar too ... ;-}

Beautiful work, eh.

As far as biasing using just the meters ... I can get close just using the Mark One Eyeball - within a point or two anyway - but the VOM is much more accurate. Great for quick checks when I fire the amp up - I used the same switching setup for the meters, so just flip thru the positions and read them for ready. I still use the VOM at the test points when needed.

Just one word of caution ... the meter circuit does add resistance, so you need to verify that the readings are the same at the tubes. Had a pretty major discrepancy here with a direct hookup.



Ended up going with center off switches for the front/rear selectors and added some small VRs (200 if I remember right) to each test position that allow me to match the meters to what I get on the VOM with the meter out of line. I also had to cut the internal shunt resistor on the meters to convert the meters I used from mV to mA ...



Still gotta get in there one of these days and clean up the wiring a bit ... things got a bit messy after version three or four of the circuit ...

Well, you know what they say about imitation..........

Thanks for the heads up. I will check the voltage at the tubes with a DMM and see if it coincides with the analog meters. If it does not I will try the VR’s you suggested. I did use center off SPDT switches because I didn’t want the meters in-circuit when I wasn’t actually checking the bias, although I’m not sure if that even matters. I was able to find mV panel meters so I did not have to do the conversion.

I knew sKiZo would weigh in on this, I thought he might sue you for copyright infringement! But then again, is anything original design??
It's ALL been done before by someone, somewhere

Very nice work!! I envy those who have the skills to build such beautiful custom bases.
Enjoy the amp....it does sound wonderfull

I think you guys just talked me out of using meters!

Captain Coconut wrote: I think you guys just talked me out of using meters!

the ONLY meters worth while adding are VU!!

Thanks again to sKiZo. I did have a discrepancy with the meters although not as much as he had. My DMM was showing 5mV less with the analog meters in-circuit. Not that big of a deal for me as the meters I am using are 0-1V with a 20mV resolution. I just have to remember to set the bias at 545mV when using a DMM at the test points.

If I had to do it over again I would have added 4 test points instead of 2 so I could bypass the meters with the switch in the center position.

Figured it was worth mentioning ... happy to be of service!

Stands to reason you'd have less discrepancy as your test points and meters are a lot shorter wiring runs than mine, and they're also closer together. Even 5mV is certainly enough to make a difference though. Difference is night and day here on grunt running the KT120's at 60mV instead of 55mV.

Adding a couple test points looks like it'd be easy enough somewhere down the road, and then you could add the fine tuning VRs to adjust the meter circuit for each tube socket while you're at it. Mine isn't "done" yet, as I still plan to replace the existing bias pots with some multi-turn units as the one's I'm using are kinda twitchy to dial in tight. Rock steady one I get them set properly, so no great rush.

This is a really nice cabinet that the standard VTA-120 pretty much drops right into.

1. Beautiful work.

2. Ever think about providing the pieces in kit form? Perhaps finished or unfinished? Hint Hint!

3. In lieu of analog meters for bias, how would one go about wiring up some form of analog vu meter? Read the output voltage across 600 ohms or maybe some other more appropriate point?

Just a thought on the resistance issue in those analog meters changing the readings:
Using digital meters would likely solve this problem as they only sample the bias voltage. That's of course if you don't mind the digital aesthetic of them.

Use nixie tubes for the readout?

What's strange is the scarcity of decent analog meters at a decent price. Anyone into that sort of thing ... might be time to stock up!

The ones I used are doing a bang up job for $10 each. Picked up a spare set while I was at it, but if I'm lucky, I won't need them. I like em because they're big and easy to read. Us old farts like that kind of stuff.





Just a reminder for anyone thinking of using them ... they ARE mA meters, so you have to clip an internal shunt for mV readings. I also added trimmer pots to each to match the offset the aforementioned resistance issue. That would vary with each installation depending on wire length, gauge, etc ... mine were a good 6mV off when compared to the bias test points at the tubes. I also used center off switches so they're not in line during play. Not necessary, but less wear and tear on the meters that way.

PS ... if you look close, the "55mV" tick isn't ... each tick is actually 4mV. Just works better visually to have the needle fall on a mark for quick peeks. That's all I use the meters for ... I still use the VOM for actual adjustments.

Hoffsgarage wrote:Hi Guys, I am new to the forum and thought I would post a couple pictures of my recent ST-120 build with a custom walnut base. I designed this base to allow the amp to slide in without any modifications other than attaching a ground lug to one of the front octal socket screws.

Very creative way to incorporate a custom base to your ST-120 ... nicely done and it looks great

Very nice work. I'm trying to improve my woodworking skills and on the hunt for nice VU meters myself.