Building tube amps (whilst waiting for thesis comments)

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Building tube amps (whilst waiting for thesis comments)

Postby tubeswell » Wed Jan 21, 2015 9:11 pm

Pics of a build underway here at present

I got an old Jansen Bassman 50 Carcass of local website auction a couple of months back. Here's how it looked then.

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Then I gutted the chassis (salvaging some mustard caps in the process), and wire-brushed all the rust of the the chassis

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and primed it with rust guard

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Then I rigged a dummy load (8 paralleled 64R 10W resistors to give a draw of 1.5A on the 12.6VAC heater winding) to test the original PT

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And tested the HT winding too (It needs a voltage doubler - The original Jansen Bassman 50 was built with voltage doublers everywhere)

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The I wired in the heaters and built the voltage doublers for the HT and the bias

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Then I wired in the screen supply node (using a 4k7 screen-supply dropper) (I decided later-on not to go with an unbypassed 3k3 screen grid - too much screen current feedback)

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Then I wired in the inverter (but later I ditched the Sprague Atom cap shown here in the mock-up (see later shots)

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How it looked after I changed the inverter supply cap to 2 x 47uF 450V in series (and began to mock-up the pre-amp)

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Here's where I'm up to at present ('61 bassman normal channel wired in and bass channel partly wired in - I ran out of salvaged mustard caps at this point dammit)

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Re: Building tube amps (whilst waiting for thesis comments)

Postby Enzo » Thu Jan 22, 2015 6:28 am

Wow, you're a wild man.

I cannot see, but can I assume you removed the paint on the inside around the pot holes, so the bushings will ground?
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Re: Building tube amps (whilst waiting for thesis comments)

Postby Heid the Ba » Thu Jan 22, 2015 9:05 am

I have no idea what any of that means but I admire quality workmanship. Good effort fella!
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Re: Building tube amps (whilst waiting for thesis comments)

Postby Lianachan » Thu Jan 22, 2015 9:11 am

Heid the Ba' wrote:I have no idea what any of that means but I admire quality workmanship. Good effort fella!


Exactly this.
A-nis bidh fios aig daoine nuair a tha mi a 'mionnachadh aig dhaibh.
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Re: Building tube amps (whilst waiting for thesis comments)

Postby tubeswell » Thu Jan 22, 2015 6:24 pm

Enzo wrote:Wow, you're a wild man.

I cannot see, but can I assume you removed the paint on the inside around the pot holes, so the bushings will ground?


yup
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Re: Building tube amps (whilst waiting for thesis comments)

Postby Arneb » Thu Jan 22, 2015 6:59 pm

I am in awe. This is something I diregarded when I was young, and which I now miss deeply: Being able to build your own stuff.
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Re: Building tube amps (whilst waiting for thesis comments)

Postby Enzo » Thu Jan 22, 2015 9:58 pm

As someone with a great deal of knowledge of these things, I can assure you that tubeswell is doing very nice work.
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Re: Building tube amps (whilst waiting for thesis comments)

Postby Мастер » Sun Jan 25, 2015 11:32 am

Arneb wrote:I am in awe. This is something I diregarded when I was young, and which I now miss deeply: Being able to build your own stuff.


Yea, I want to be able to build my own nuke.
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Re: Building tube amps (whilst waiting for thesis comments)

Postby Lance » Sun Jan 25, 2015 6:52 pm

Мастер wrote:Yea, I want to be able to build my own nuke.

I built one many years ago when I worked at a machine shop for a while. It was cool. I was the only person on my block who had one.

I used it as a door stop.
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Re: Building tube amps (whilst waiting for thesis comments)

Postby Arneb » Sun Jan 25, 2015 8:32 pm

Explains the way your daughter looks on that avatar photo.
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Re: Building tube amps (whilst waiting for thesis comments)

Postby Lance » Sun Jan 25, 2015 8:49 pm

Arneb wrote:Explains the way your daughter looks on that avatar photo.

Huh?
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Re: Building tube amps (whilst waiting for thesis comments)

Postby Lance » Sun Jan 25, 2015 8:57 pm

Perhaps an explanation of my logic is in order here.

A gun with no ammunition is completely safe, but still a gun.
A car with no gasoline is immobile, but still a car.

Many years ago I worked for a while at a machine shop where I built a small nuke in my spare time. It never contained any fissionable material so it was never "loaded", but it was still a small, crude nuke.

</thread hijack>
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Re: Building tube amps (whilst waiting for thesis comments)

Postby tubeswell » Mon Jan 26, 2015 12:07 am

Well I chucked some EL34s in it (and a bunch of pre-amp tubes) and fired it up.

Result

B+ 617 (655 on std-by)
Heater elevation 53V
Plates 616V @ 28.4mA
Screens 380V
Grids -34V
Suppressors -24V (pinned to Bias supply)
PI supply node 346V
Normal Ch supply node 324V
Bass Ch supply node 236V
CF (V1b cathode) 133V

FWIW I discovered that the trouble with the 6G6B preamp is that the Bass Channel is prone to self-oscillation when the vol pot is about 2/3 of maximum rotation if built as per the schematic. I found that this was able to be cured by putting a coupling cap between the CF cathode and the junction of the 1M/250pF (I used 22nF with good results).
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Re: Building tube amps (whilst waiting for thesis comments)

Postby tubeswell » Mon Jan 26, 2015 12:08 am

And my thesis supervisor has comments for me now - meeting with him on Friday
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Re: Building tube amps (whilst waiting for thesis comments)

Postby Enzo » Mon Jan 26, 2015 2:32 am

How many stages in the bass channel and are they all off the same B+ node?
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Re: Building tube amps (whilst waiting for thesis comments)

Postby Dragon Star » Mon Jan 26, 2015 2:41 am

I do some HVAC work from time to time, and that looks like a nightmare to me.
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Re: Building tube amps (whilst waiting for thesis comments)

Postby Мастер » Mon Jan 26, 2015 3:34 am

Lance wrote:Perhaps an explanation of my logic is in order here.

A gun with no ammunition is completely safe, but still a gun.
A car with no gasoline is immobile, but still a car.

Many years ago I worked for a while at a machine shop where I built a small nuke in my spare time. It never contained any fissionable material so it was never "loaded", but it was still a small, crude nuke.

</thread hijack>


What do you think the odds are that it would have worked, had it been loaded?
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Re: Building tube amps (whilst waiting for thesis comments)

Postby Lance » Mon Jan 26, 2015 4:39 am

Мастер wrote:
Lance wrote:Perhaps an explanation of my logic is in order here.

A gun with no ammunition is completely safe, but still a gun.
A car with no gasoline is immobile, but still a car.

Many years ago I worked for a while at a machine shop where I built a small nuke in my spare time. It never contained any fissionable material so it was never "loaded", but it was still a small, crude nuke.

</thread hijack>


What do you think the odds are that it would have worked, had it been loaded?

Properly loaded? I think highly likely. They are simple devices once you get the fissionable part right. Of course, I did not fabricate any kind of triggering device either. Basically it was just the shell.

Dear NSA: It was not, nor has it ever been my actual intention to create a working nuclear device. Having an "empty" atomic bomb as a doorstop was for amusement purposes only.
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Re: Building tube amps (whilst waiting for thesis comments)

Postby tubeswell » Mon Jan 26, 2015 4:59 am

Enzo wrote:How many stages in the bass channel and are they all off the same B+ node?


Its the 6G6B preamp up to and including the inverter. The bass channel has two dual triodes (4 stages) all fed from the same filter cap. V1 is a DC-coupled pair incorporating a CF which drives the bass tone stack and channel volume (gain), and then V2a drives the treble control, and V2B feeds into one of the 470k channel mixing resistors.

In the original schematic there is no coupling cap after the CF stage in the bass channel (I ended up adding 22nF here). Before I added the coupling cap there was a low frequency oscillation that appeared with the vol pot cranked above about 2/3 of maximum rotation which became worse as the pot was dimed. Had me puzzled for a while thinking it was primarily because it was not properly decoupling in the power supply (which was probably a contributing factor). Anyway when I pulled V2 and there was no oscillation, I realised that it must have been to do with the coupling to the DC-coupled pair in front. Putting a coupling cap between the CF and the bass tone stack fixed it.
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Re: Building tube amps (whilst waiting for thesis comments)

Postby Enzo » Mon Jan 26, 2015 7:57 am

I think your cure was brute force. My working theory is not enough power rail decoupling, as in too many stages on one filter cap node. Maybe?
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Re: Building tube amps (whilst waiting for thesis comments)

Postby tubeswell » Mon Jan 26, 2015 10:09 am

You could well be right about the filtering node at the end on the bass channel. I'm using the same number of filter nodes as the 6G6B but without any choke.

50uF reservoir (in the voltage doubler) - 15k dropper - 50uF screen node - 5k dropper - 22uF PI node (supplying V4) - 5k dropper - 22uF normal channel node (supplying V3) -27k dropper- 22uF bass channel node (at the input jack end) which supplies V1 and V2

Galactic ground with returns to each respective cap ground follows the same line to the input jack where its grounded.

Come to think of it, the Bass channel is such that the signal inverts at V1 and then inverts again at V2a and inverts again at V2b (which means that V1 and V2b are in phase, and opposite to V2a) so that there are two stages on the bass channel node 'pulling' the HT in one direction and only 1 stage 'pushing' it in the other direction.
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Re: Building tube amps (whilst waiting for thesis comments)

Postby tubeswell » Mon Jan 26, 2015 10:20 am

Dragon Star wrote:I do some HVAC work from time to time, and that looks like a nightmare to me.


I prefer the term 'workman-like'. As Enzo noted, I'm a bush man. The layout is carefully calculated and meticulously planned in an methodical-yet-happy-go-lucky fashion from the back end to the front end. :-)
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