Cornford hurricane Blown Mains TX 2nd time

[blindboybenton]

Hi all again

The story of the cornford continues

Voltages are On the tx 240v ac with 312v ac out and 6.46v heaters ( taken with all valves in and no signal )
Tone control set to middle for all
Amp meter connected though the 312v current draw of 78ma and matched set of sovteks EL84 the tad bias master say 29ma ea
with output tubes removed 9.5ma

With a signal gen in at about 1k with the output turned to minimum and the amp up full current draw 132ma and tubes are at 50ma and 56ma
Monitoring the output with a scope and turning gain up with master on full drawing 83ma just before the sinewave clips

EL84 pin out voltages at idle Pin 2 - 0.015v - Pin 3 - 13v Pin 7 - 389v Pin 8 - 0.015v Pin 9 - 338v " same for both tubes "

EL84 pin out voltages at full on Pin 2 - 0.015v - Pin 3 - 27.9v Pin 7 - 360v Pin 8 - 2.2v Pin 9 - 268v
EL84 pin out voltages at full on Pin 2 - 0.015v - Pin 3 - 23v Pin 7 - 357v Pin 8 - 2.5v Pin 9 - 263v

Pre amp tubes without any signal
V1 Pin 1 - 160v Pin 3 - 0.03v Pin 8 - 0.8v
V2 Pin 1 - 135v Pin 3 - 0.7v Pin 8 - 0.8v
V3 Pin 1 - 152v Pin 3 - 0.03v Pin 8 - 0.8v
V4 Pin 1 - 230v Pin 3 - 53v Pin 8 - 53 v

Under full load Pin 1 of V1/2/3/4 all drops to 130v

Hope somebody may be able to read into this and let me know.. I aint got any hair and what i got is being pulled out.. Now this jobs is going back on the shelf while i sort out other jobs

many thanks all of you lovely people who have been tring to help

John

[bob p]

You must be meaner than the owner himself.

I like that. If you really want to torture test the amp, then you need to test with hard clipping. You need to be prepared for real world owners who like to square-wave their amps. but that's going to be hard on a marginally-rated power supply.

they are not the same mA.

As I read BBB's post, I thought he was telling us the DC current flowing through the Pi filter, so that we could make an apples to apples comparison when looking at Kirchoff loops for the different stages of the amp. You're right -- knowing the AC current coming out of the mains doesn't really help us, it's an apples to oranges comparison that doesn't really help with troubleshooting.

As a matter of being thorough, I would still take a look at a complete voltage table to be sure that there's no additional current sink that you're not aware of. It's just good practice. Actually, when I'm working on an amp I like to go even further than that. I'll record on the schematic all of the actual measured values for the resistors, especially if the amp has Carbon Comp resistors in it -- it's all too common that you'll find one out of spec in the power supply. then I'll record voltages across each power supply element and cathode element on the schematic, so that I can calculate where the power is going. If you're thorough, you'll often find subtle problems need correcting while you're in there.

edit: if you've got an LCR meter or a cap tester, it makes sense to test the caps as well.

[bob p]

What I use (no kidding, I'm serious) is to play a few AC DC's albums, full power, sound horribly distorted (somehow AC DC is bearable under those conditions, that's why I use it) into a real speaker (no resistive load).
You'll need a speaker which can take it and if too unbearable, point it towards a wall or better into a clothes closet.
In short, reproduce what it will suffer at a rehearsal room, 4 or 5 hours in a row, balls to the wall.

I use a very similar technique that I started using for speaker break-in. It works well for amp burn-in as well. I feed a music signal via a CD-player into an amp that's located at the far end of the house, that has a pillow and a blanket thrown over the speaker cabinet. Then the CD-player will auto-repeat some AC/DC, RHCP or Led Zeppelin.

As you noted, volume is a problem, even with the speakers located at the far end of the house and some bedding thrown over them to muffle the sound. Although this setup is a necessary evil for speaker break-in, I'm thinking that a silent reactive load like a Weber MASS might be better for amp burn-in. If you have to do it often, it might justify the purchase.

[R.G.]

Hmmm...
Intermittent short/leak in the filter across the cathode resistor for the output tubes would un-bias them and possibly overload the mains TX with current. I would examine the resistor/cap at the two output tube cathodes very, very carefully and replace on suspicion with bigger-rated parts.

In power supply work, a soft short like unbiased output tubes is often deadlier than a hard short because it doesn't trip protection circuits til too late.

If it were me, I'd consider fusing the mains tx secondary at about 150ma slow blow/time delay. That will protect the secondary from soft shorts better than the primary fuse can possibly do.

[bob p]

No surprises that you'd recommend fusing, RG!

I linked to your fusing article (Immortal Amp Mods) in BBB's old thread about this amp's transformer-eating problem.
http://music-electronics-forum.com/t33039/#post302573

It doesn't sound like the fuses are in yet.

[R.G.]

No surprises that you'd recommend fusing, RG!

Ok, Ok, so I'm a one-trick pony

I do hope he looks at the cap on the cathode resistor though. Having that short is just like having the bias voltage filter cap short in a fixed-bias amp, except that it's nearly always mounted nearby the toaster-oven-hot cathode resistors. Easy to age them before their time.

[blindboybenton]

No surprises that you'd recommend fusing, RG!

I linked to your fusing article (Immortal Amp Mods) in BBB's old thread about this amp's transformer-eating problem.
http://music-electronics-forum.com/t33039/#post302573

It doesn't sound like the fuses are in yet.

Yes chaps the fuses are in place. When i replaced the first tx. i tested the amp seems to be fine. It was a day or so later it eat the tx. Since then ive had a 160ma fuse in place
Its been ok with this test tx ( higher rated ) Im just concerned about the replacement tx waiting in the wings to be fitted.is not going to be able to cope with the amp with what seems to be high current draw

I shall over come this problem!!

BBB

[bob p]

Ok, Ok, so I'm a one-trick pony

I've got a dog that will run over to a cooler, flip the lid open with his nose, pull out a beer, bring it back to me and hold it in his mouth whenever I ask him for a beer. It always gets a laugh at picnics and he's won every pet trick contest he's entered. Sometimes one good trick is all it takes.

[Enzo]

I taught my cat to play bridge. She's pretty good at it too. I just can;t get her to stop trumping my aces.

[Mick Bailey]

John,

I'd monitor the B+ line for current draw, instead of the AC feed into the filter caps. That way your'e seperating two sections. Because you're reading current into the entire amp you won't know if it's the PSU or subsequent stages that are drawing current if you're only monitoring the AC feed off the secondary. As this amp is fairly easy to interrupt to take measurements you can break it down into sections.

My approach would be to measure the DC current going into the OPT centre tap and establish if this is ok.

[bob p]

Enzo will be performing two shows per evening on weekends from Memorial Day to Labor Day.

The problem with my dog is that he wants to fetch beer whenever he sees a cooler. Not like that's really a problem though.

[bob p]

Back on topic...

RG makes a good point. Bias caps are troublemakers.

I've never worked on a hurricane, but I have to admit that every cathode biased amp that I've ever had to work on has had a problem in the bias circuit. Either the bias resistor has drifted out of value, or just failed by cracking from the heat, or the bypass cap has suffered damage from heat from being mounted too close to the bias resistor. Those bias resistors do get hot, and most of the time amp manufacturers don't build them the way that you might build them if you were sparing no expense to over-build an amp for yourself. Although I keep the bias resistor and the bypass cap far apart from one another, many builders will just put them right next to each other. In the most extreme cases I've found them snugged right up against one another and tied down to the same turret or eyelet. IMO that's just asking for trouble. Almost all of the time a manufacturer will use a single resistor with just enough power rating for the job, where a DIY builder might use resistors with much higher power ratings. A DIY builder might even use separate bias resistors, one for each tube. The point of this is to say that the bias caps and resistors are a common failure point, and that if by some chance you haven't measured them, doing so would be a good idea. That's part of what I alluded to earlier about measuring caps and resistors to assure that everything in the amp is in-spec.

I'm at the point that where I'll just replace the bias resistor and bypass cap with a power tube change. For a couple of reasons. first, I like to tune the bias resistor to the tubes that are going in; second, I like to replace low-power resistors with higher power resistors if needed; third, i don't trust old bypass caps that have been subjected to heat; fourth, the parts are cheap and it's not worth my time to have to go back into the amp a second time. i guess that's four reasons, not two.

Now that I have an LCR meter I'm more inclined to test caps than to just replace them, but a low voltage meter doesn't always give you an accurate read on dynamic high voltage performance in an amp. Replacing bias caps prophylactically makes sense to me.

[Mick Bailey]

I've converted a few cathode-bias amps to fixed bias, which I prefer. Never had one back to restore the original configuration.

[Mick Bailey]

Many thanks Mick

To be fair this is the first EL84 amp ive had troubles with. Ive only had easy jobs on this type of amp. tube changes or cap jobs. Put me off the damm things for life.

Going off subject abit

where are you from. Hows business ? I do it on the side at the present but seem to be getting busy every month.
I will hopefully go part time in my day job.. Add the amp repairs and bands + alittle Bass teaching im busy

BBB

I'm from North Staffordshire, near to Leek.

Business is highly variable, but mainly just too much work and pressing deadlines. I don't just do repairs - lots of people do that. I get the sounds for people that they want from their equipment and this involves going to see them play live and listening to their work. I try to add value to what I do, but sometimes this takes a lot of time which I have to invest up front. The payoff is further down the line - not on that immediate job.

If you're thinking of doing this as part or all of a living, it's tough. I worked Christmas day for 14 hours and New Years day for 6. Bands in the spotlight need reliable work done very quickly. I got three pieces of gear dropped off the other day - a broken compressor, analogue delay and a guitar that needed new pickups, rewiring and setting up. Dropped off at 14:00, collected at 17:00. Sound checked on stage at 19:00.

Similarly I got a bass amp with a failed fan and burnt PCB tracks and failed components dropped off in the morning for a TV show the next day.

When you take on a job, someone is handing you their problem and walking away from it. You have to shoulder that responsibility, whether it's a kid with a snapped socket on a starter amp, or a pro musician that lives by their gear. To them you're the end of the line and if you work by yourself there's no backup, only this forum and the web, along with your own experience and understanding.

As technology moves along, so you have to. Vintage amps and classic designs are in the minority. In fact, guitar amps are easily outweighed by PA systems, bass amps, effects, keyboards, mixing desks, studio gear and DSP. As soon as you repair one piece of kit you become the port of call for everything else - electronic or electrical. Much of this stuff is SMD and you can't afford to turn it down.

If you imagine a good employee treated badly by a mean boss, then you're both of these at once. Corporate life is much easier and you can walk away at 5.

[blindboybenton]

Hi All

Well the story of this amp is now almost closed

Shes working ok a new set JJ's EL84, Changed the Bias cap and 220r resistor. Biased much colder now then before. Found a possible bad output tube base, so i changed that
Left the uprated tx in place

The amp is ready to go but i noticed its abit hissy. In my opinion too much. Spoke to customer. he did say its a noisy amp

Its noisy about 2/3 of the way on master and gain. The tone of the hiss is effected by tone control. If you removed V1 amp is quite. Ive checked and replaced components in v1 stage

nothing seems to affect it.

If this is just the way the cornford is ??. Is there any mod to removed some of the hiss or any component to look it


http://music-electronics-forum.com/t...4356/#poststop

bbb