The 'Mule Project'- learning about valve/tube amps on a B25.

[pentode]

Greetings everybody!

Firstly, apologies if I've upset anyone thinking that I'm needlessly butchering one of Ampeg's classic vintage amps...

This particular example has willingly donated it's body for research - in fact one or two of you may have seen this one before. After I got it I saw a thread where somebody was asking if they thought a B25 in a shop was worth buying and one of the replies had words along lines of, "Is that the one in (whichever shop it was) with its guts hanging out the back of it with some bits missing?"

It was, and the OP hadn't bought it, but I had - and not only that, I had it carted all the way to the UK.....!!! Mind you I got it for a song and there were over two dollars to the pound at the time which helped.

When it arrived it was looking a little sorry for itself. One of the 6L6GC valves had a cracked base and it was generally rather tatty. I replaced both of the output valves, cobbled together an adjustable bias pot and very nervously fired it up. I set the bias as closely as I could but the B+ was all over the place, almost certainly down to what appeared to be the original caps in it.

I powered it down, drained the caps and that was it for a few years as life took over.

A couple of weeks ago, after finishing another project I decided to have another look at this fella and get some experience with valve/tube amps by turning it into a learning project. I had a few ideas and preferences I wanted to try and felt I could implement them on this and gain some experience at the same time. I've relatively little knowledge of electronic theory but maybe I can pick something up on the way and hopefully somebody else will gain something too.

As it was -





I replaced the two 30uf 600v dynamite stick caps with four F+T 32uf+32uf 500v caps on a piece of circuit board mounted on the output transformer bolts.






Underneath the new board.





The 40uf+40uf cap was replaced by an F+T 50uf+50uf 500v in the same position as the original with a plastic clamp.

That's it for the moment. I've already had some much appreciated help from BassHog on another forum. Any constructive criticism is most welcome and thanks for putting up with my waffling post.

All the best
Grahame

[pentode]

So along with replacing the smoothing caps, all components in the bias circuit were sorted as well as a tidier bias adjustment setup.


Here's the 50uf+50uf cap and bias bits. There's also a new power resistor connected between the 16ohm tap and ground.







A quick look at the inside with new caps, adjustable bias and slightly tidier mains wiring.
I've also removed the polarity switch and put the standby switch in its place.








A close up of the adjustable bias'







Some of the other things I'd like to try on this project are -

KT88s.

Sockets for monitoring bias and plate voltage with separate bias for each valve.

Switchable valve/SS rectifier.


Cheers
Grahame

[pentode]

Well I had a chance to play through it earlier and while channel two was reasonably quiet and sounded okay at low volume, it quickly farted out big time when I cranked it up.

By the way, I'm playing bass through it and the cab is a fEARful 15/6.

Channel one had a lot more hiss but it had a 12AX7 as opposed to the 12AU7 in channel two.

Again, channel one seriously farted out at higher volumes, I mean really bad.... The 15" speaker was really suffering so I backed right off....

BassHog queried as to whether I had the output valves biased too cold - they were...... My bad.

I adjusted the bias, with a high plate voltage of 530v I set the current at 39ma and that tightened the sound up a bit. However, it was still distorting badly.....

The 15/6 isn't a sealed cab which won't help with poor damping issues I know.

Cheers
Grahame

[Hangman]

I'm not sure how a 12au7 would perform in the b25... have you tried putting the 12ax7 in channel 2 to see if that channel behaves when it has the right tube in it?

the circuits in both channels are the same, so if channel 2 works with the 12ax7... its likely that the b25 doesn't like the 12au7.

[pentode]

Hi Steve,

Thanks for your input!

I popped a 12AX7 into channel two and it became quite 'gainy' and noisy, rather like channel one.

I tried using a pair of 12AU7's and whilst there was less volume it sounded a lot cleaner which suited the bass better.

There was still a speaker ripping distortion near full volume. Now is this just a trait of the B25 or is there an issue with this example?

I thought I'd take measurements at the valves and see if there was anything obviously amiss.

There was.

The two preamp valve voltages were wildly out. Why? Because I still had the 12AU7's in there...!!

Changed them over to the proper 12AX7's and things looked much better in the preamp section. However things seem to be out on the 7199 PI valve. The B+ is a bit high but even so....

Here's the schematic -

http://www.kbapps.com/audio/schematics/t...es/b25.gif


The voltages on mine -


B+ - 523

Nodes

C - 456
E - 353


V1 - 6L6GC

1 - 523
4 - 522.3
5 - -61.3


V2 - 6L6GC

1 - 523
4 - 522.3
5 - -61.4


V3 - 7199

1 - 326.5
2 - 200.4
3 - 81.6
6 - 2.6
7 - 0
8 - 134.8
9 - 111


V4 - 12AX7

1 - 239.7
2 - 0
3 - 2.3
6 - 251.7
7 - 0
8 - 2.4


V5 - 12AX7

1 - 233.8
2 - 0
3 - 2.4
6 - 245
7 - 0
8 - 2.4


So, any thoughts?

Many thanks
Grahame

[pentode]

The next thing I thought I'd try is replacing the components around the PI valve in case some of the values had drifted.
It's to Ampegs credit that virtually all were very close, a couple were quite a way out but most I think would've been fine as they were.

Measuring around the 7199 again revealed no change in the voltages on the pins...

Not to worry, the old B25 was sounding quite nice as long as I didn't push it too hard. Maybe I'm expecting too much from it.

I have a personal preference for Speakon sockets because of the way they lock. Sooooooooo out with a suitably sized punch and away we go -






Two sockets, one for 8ohms the other for 16ohms - really wish there was a 4ohm tap......







Slightly rushed wiring here but I hope to come back and tidy things up a bit.






That's it for the moment folks.

Thanks for looking by!
Grahame

[pentode]

So, as we're on the subject of sockets, time to put some nice ceramic valve sockets in as the originals were feeling a little loose and I hoped that I could tidy things up a bit.


One of the old output sockets -








So whipped it out (after taking a few pics of the wiring) and cleaned the chassis around it -








The new sockets were slightly larger so rather than grind or file the hole bigger and fill the chassis with metal filings I thought I'd try using a punch even though my engineering background screamed, "Don't be an idiot, you'll screw it up!"
I had to line the punch up very carefully over the existing hole as there was only 1mm / 0.040" all round to remove.....







Taking it carefully left a tidy hole with only a slight burr. Oh and three neat slivers of chassis! -








Repeat with the other socket and fit the new shiny ones in complete with valve retainers - remember, KT88s in the future! -









Next exciting instalment is wiring it all back up again.

Cheers
Grahame

[pentode]

Right, wiring the sockets up again.

As I wanted to use KT88s at some point, I believe 1K 5watt screen resistors are the way to go. Had a bit of tag strip handy and thought it would be useful so positioned it in a convenient place.






Resistors in place and wired up.






Heater wiring.






All done!






Fired everything up and had a quick play and it definitely seems to sound a little better! Maybe it's just because I've spent some time on it but I'm happier with it.

[Dave1966]

Nicely done!

[pentode]

Most kind Sir!

Things will slow a little as I've had another project foisted upon me.....

I could do with checking the voltages on the power valve sockets before trying the KT's.