Carlsbro 50/Express hybrid/monster!

[greatwhitemark]

‘Total novice biting off more than he can chew’ alert!

Hi folks, I recently acquired a Carlsbro 50 Top amp head which isn’t really suitable to my style, so I thought why not turn it into an Express. Easy peasy! Main problem being I’ve never built an amp before but I can follow instructions and use a soldering iron.
The Carlsbro uses 12ax7 and EL34 tubes and reputably has Drake/Partridge trannies like old plexies so I thought everything should work out ok(very scientific).

[IMG586]http://img715.imageshack.us/img715/2118/carlsbro50topboard.jpg[/img]
[IMG1638]http://img209.imageshack.us/img209/7046/carlsbro50top.jpg[/img]
So I ordered the Expression boards from Ceriatone which took a staggering 4 days to arrive to uk from Indonesia, quicker than the parts I ordered from 40miles down the road. I drilled a few new holes for the preamp tubes to help with layout and stuck it all together(whilst meticulously reading and re-reading Ron Worley’s excellent build guide).

As the PT secondaries has no centre tap I changed the rectifier to a ‘bridge’ type so I could take a ground from that. I also used the dedicated transformer winding for the bias circuit and ended up removing the 220k resistor to get correct voltage after testing without tubes.
[IMG652]http://img227.imageshack.us/img227/4328/50topwrecked.jpg[/img]
All the voltages looked good when connected with a 65w bulb limiter, slightly lower than on the schematic so I thought lets go for it.
I plugged in my guitar and WOW! It works!! Sounded really nice, a bit bright but after messing with the controls I got a really creamy Express-like tone(there’s no bright switch as yet). The noise levels we’re also very low. I could quite easily max out the volume in a home setting as it wasn’t that loud, I put this down to the bulb limiter so decided to give it a go without the light bulb.
I turned it on without the bulb and volume at zero and after a couple of seconds it just started squealing, horribly and very loud so I quickly measured a few voltages and turned it off before anything melted. B+ voltages we’re way higher than what I was expecting.
Mains 243v
Heaters 6.1v
PT across secondaries 360v (375v and 371v from each side to ground)
Bias range -39v to -34v
B+1, 500v
B+2, 410v
B+3, 327v
B+4, 308v
B+5, 290v

So I’m thinking, if I can bring the B+ voltage down I may be able to cage the beast and get that sweet tone back. I’m also confused as to why the B+ is so high, not that I really know what I’m doing or fully understand the electrical laws of the universe.

So with this, I’m appealing to the charitable side of you forum members for any guidance or simple instructions that could get me to the next stage in taming this beast. Sorry I can’t give you more info on the transformers used. Thanks in advance........

Mark

[Cliff Schecht]

Sounds like you need to replace your power transformer with a beefier one that has a lower HV secondary tap. 370-0-370 is very high for an Express and puts you at about 100V too high.

Also, your lead dress worries me. As I and others have found, messing with KF's build formula usually results in a screaming mess..

[greatwhitemark]

Thanks Cliff.

A different PT would certainly help the situation but I was looking for any tips that may bring the voltage down without a need for new one.

Lead dress issues will be dealt with when I reach that hurdle.......

[ampgeek]

Not bad at all for a beginner!!

I would wager that your squealing is due to your NFB (presence) circuit.

Disconnet your presence feed wire from your OT 8 ohm secondary, tape it off and fire it up again. If it behaves well, you will need to switch around your OT primary wires on the EL34s to permanently resolve that issue.

Then...on to voltage reduction if you really need to.

Again, nice job mate!
Dave O.

[paulster]

You can always use the zener diode in the centre-tap trick to bring down your B+. You've got to bring it down a long way, but it will work at least without having to replace your PT.

[Cliff Schecht]

If he says the amp worked with a limiter then the output transformer leads probably aren't flipped.

Does the flipped leads cause a higher or lower frequency oscillation? I thought it was lower in the audio spectrum although I guess it can potentially oscillate at a pretty wide range of frequencies.

[selloutrr]

damn!

that's really sad to see a nice amp it's own right get hacked.
the carlsbro were really a sweet amp! and could hold there own.

just so you know... if your amp worked as a carlsbro it was worth more then the express kit prebuild.

[ampgeek]

I hear ya! Not sure of the typical frequency though. Only that I have heard it a couple of times on my builds (with all volumes on 0) and tried to forget it...fast!

Thinking that low volume = low voltage at the secondary. Perhaps below the threshold required to make it oscillate after passing through the presence network?

Easy enough to investigate and eliminate from the list if not the cause. Seems like the 50:50 odds of it being a problem in new builds (when left strictly to chance) make it worthy of consideration.

We shall see...I hope!
Dave O.

[greatwhitemark]

Thanks all, I will try disconnecting the presence feedback and see if it makes a difference.

Will the el34s stand up to 500v ok? I'm a little concerned about it frying the few that I've got.
Does anyone have any experience of the zener diode trick? I've seen a few express inspired schematics that use 5x10v zeners going to ground, is there a limit to how many can be used, any pros or cons?

selloutrr - I totally get where you're coming from in regards to messing with old amps. I've kept all components safe, front panel untouched and can be easily restored in the future - when I'm over my amp builder fantasy!

Thanks again

[ampgeek]

500 V should be way OK on the plates. Looks like 500 V max on the screen though.

http://www.duncanamps.com/tdsl/show.php?des=EL34

Do bias within the ~70% of max plate dissipation (25W) though for starters. Tweak up or down to taste being mindful of redplating.

Good luck!
Dave O.

[greatwhitemark]

Thanks ampgeek, it appears your diagnosis was spot on.

Removed the feedback wire and hey, it worked. So I swapped the OT wires and replaced the feedback wire and hey, hey, it still works!

Didn't have time to correctly measure everything but it's still at the 500v mark with everything connected and running. Measured the screen at 483v which is a little too close to the 500v max for my liking.

I'm heading to rehearsal room now and shall give it a few minutes appraisal before looking at changing the voltage.

Cheers

[ampgeek]

Excellent Mark!
We all get lucky in our "remote diagnoses" every once in awhile.

I am about to embark on an express build so I will be very interested in how yours works out.

Getting the screens and subsequent B+ nodes down to the proper levels shouldn't be tough at all. Just need to jigger the dropping resistors a bit. I whipped up a quick spread sheet for just that but don't have it here with me at the moment.

Glad I could help,
Dave O.

[JamesHealey]

I converted a Carlsbro 60TC (same chassis and transformers) into a wreck and used a 75w 100v Zener diode with a 0.1uF 500v Polyprop cap in parallel on tail of the Bridge Rectifier to get the voltage right.

Worked a treat

[bancika]

One thing about zeners - they can die easily, even with loads lower than rated. Their power rating deteriorates very fast as temperature increases so it's not a bad idea at least to put it somewhere where air flows and not to cut leads too short because they will dissipate heat too.

Cheers

[paulster]

Very true, which is why I always prefer to use the correct PT.

They also want a healthy cap across the dropping string to stop switching noise getting into your audio path.