Cornford hurricane Blown Mains TX 2nd time

[blindboybenton]

Hi All

Ive posted a tread a week ago on this subject. Thought i would start a fresh one

A quick recap Cornford Hurricane came in with blown mains tx. Got the original maker of the tx to make me one. Fitted it with a new set of Sovtek EL84 . Tad Bias meter
had them at 30ma each

Customer happy. He called me 12 hrs later tx blown again.

I got the amp back. 500ma mains fuse gone and tx burnt out. Spoken to the maker. he told me to send them both back. He checked them out. he didnt say they were faulty and it was the fault of his tx's ... But did replace it and send me an earlier cornford tx as well which is higher rated (Orginal cornford spec is 130ma for 295vac )

he told me to hook up that tx and run the amp flat out and monitor

I fitted a 160ma fuse and fitted the tx( Up rated tx ).Running flat out with a siginal gen on input and 4 ohm 100watt load on output
With the TAD Bias meter . Without siginal .. tubes are 30ma each at Flat out the tubes at 53ma and 57ma
volt meter set to ~ ma flat out 142ma (contacted though ac 295v ) with a guitar playing though it its about 120-130 ma

What im concerned about is that something it still drawing to much current. Im a little out of my comfort zone here.

Does anyone have any idea's or thoughts. Also im looking for some written info on testing the wattage of valve amps.


Blind boy benton

[Mick Bailey]

The bias at 30ma needs referencing to the anode voltage of the EL84s to give the total dissipation. Your secondary voltage is probably running quite a bit higher than expected and you may need to reduce the 30ma to compensate, though this in itself is unlikely to be the cause of the TX failing unless a valve is shorting. A secondary AC voltage of 295v loaded would give a B+ of 417v (not allowing for losses). What's the screen voltage? Are the screen resistors cooking?

A 500ma mains fuse is too high to safely protect the transformer secondary in the event of a short. Is the 160ma fuse fitted to the secondary as per your previous suggestion? Good idea to do this.

You could have a shorting filter cap, rectifier, or socket short. Check the amp doesn't draw current with the valves removed. The tag boards are standard RS and can break down between tags under high voltage, sometimes on the back side. I don't know if this layout has HT on adjacent tags - worthwhile checking to see if it does.

So, check;

The B+
Anode and screen voltages of the EL84s
Rectifier diodes
Valve sockets - ensure good separation of the solder tags and wiring
Tag boards
I'd also check the heater voltages.

[mikeydee77]

I had a similar story last year... my details are a bit half baked because I took on the job from another tech that was suffering with health problems.

Basically my story was that with a hurricane where the original transformer had been replaced with a proper replacement only to have that fail after some hours use. It came to me to fit the second replacement transformer (cant remember the supplying company but I understand it was the original as the client had some email conversations with Martin Kidd; the amp's designer confirming it).

I also installed a secondary fuse and I seemed to recall that it would blow if the electrostatic screen was connected. It might be worth seeing if omitting that connection effects the current draw.

[Mick Bailey]

The original transformer is a TECM323A - I don't think this has a screen. Anyhow, if a screen is grounded and it blows an HT fuse then the transformer would be faulty as the screen shouldn't connect to the windings.

The easiest way to check an amp for excessive current draw is to use a series lightbulb on the mains feed. That way you're not blowing fuses and can see during playing if there are any momentary bright flashes indicating a short. With a 100w bulb you'll get a bright glow at switch on as the caps charge, then a dull orange glow which will modulate as you're playing. Your amp may need a slightly lower wattage - 60w. A fault condition will give a bright glow that stays that way. I keep a number of bulbs handy depending on the amp.

Did you check the current draw without the valves (with your meter)?

We need to see what those anode and screen voltages are - current draw on it's own doesn't give the full picture.

[blindboybenton]

Hi Mick

Thanks for the info. Ive got a layout of the cornford. no schematic. This transformer did have a screen . The tx im using for test is the TEC323A
Using this tx im having no troubles with the amp Its just ive got the replacement one and its only rated at 130ma.
The picture of the tx's the one with the white label is the replacement

Amp on with No signal 79ma ( EL84) in place ... With El84 removed 9.5ma
Heaters are 6.5v ac with all valves in

Voltages are on one EL84
Pin 2 = 0.02v
Pin 3 = 12.9v
Pin 7 = 389v
Pin 8 = 0.02v
Pin 9 = 338v

Many thanks for your help

John

[bob p]

The easiest way to check an amp for excessive current draw is to use a series lightbulb on the mains feed.

Can you still buy incandescent 100W bulbs where you live? I can't. I have to resort to a series resistor and an ammeter.

[blindboybenton]

Hi Bob

Nope i can only get energy saver types. I aint yet got round to sourcing some.
With this test tx in the amp runs fine. But with the replacement one in it may or maynot burn up. My plan is to find the cause of why or what is causing it. Do you think that 140 ma current draw is too high for a amp of this type Or is the way im checking the current wrong ? Ie Ive conected a meter though the 295v ac and on 200ma setting at full output on a dummy load. and using a signal gen its drawing 140ma. I know the customer wont run it flat out. nothing looks bad or cooking or checks out bad.. Idea's !!

John

[Enzo]

Wire two 60 watt bulbs in parallel. Leave one half unscrewed (or add a switch) for 60 watt bulbing, and screw in the second one for 120 watt bulbing where necessary.

[bob p]

Yes -- An array of low power bulbs would work well. So I guess I could use a string of the old C7 Christmas-tree lamps when they decide to phase out the 60W incandescents and make us use CFL bulbs.

I've found that the series resistor and ammeter combination works well. You don't get that instant qualitative feedback of "bright vs dim", but you do get a quantitative answer.

[bob p]

Ive got a layout of the cornford. no schematic.

You should re-read your old thread. The schematic was posted there 3 weeks ago.

[bob p]

look here:

http://music-electronics-forum.com/t32953/

edit: notice that at the bottom of this page, there is a box that lists "similar threads." sometimes that box contains some really helpful info. in this case, it saves you from searching, because the links are right there for all of the relevant threads for your amp.

[blindboybenton]

Being a bit of a thick englishman ( You chaps call it Dumb) How would using the bulbs help me find out what is causing this amp to take out transformers?. Or am i missing something here
I dont really want to fit the replacement tx until i found the cause. I understand using the bulbs will stop the amp from blowning the tx

Question. The amp at full on power with 4 ohm dummy load ( 100watt ) and a signal gen for signal and a amp meter connected though the 295v ac is drawing 140ma
and the orignal tx is rated at 130ma. It seem to me that the amp is drawing too much current. Accept that customer is not going to run a full power.
The amp is working as well as it can with a test tx with a higher rating.

I could just fit this tx and send it out, But im concerned that there is somthing else wrong ?
Question Is 140ma too high for a amp of this type ?
I think, well maybe that the tx is just to lower rating and just cant cope with any more current, that may arise over time. Have been hearing about cornfords with same problem and a bigger tx going in solved the problem

Thanks for your help on this, Im tring to learn myself a new brain Just brought the Dan Torres inside valve amps book today That should help

John

[blindboybenton]

You should re-read your old thread. The schematic was posted there 3 weeks ago.

[ATTACH=CONFIG]23493[/ATTACH]

Hi Bob

Sorry I just been so busy just must of missed it

[bob p]

John, I can't say that I'm too enthusiastic about seeing an amp drawing 140mA on the PT when it's only rated at 130mA. Granted, those are test conditions and the customer is never going to run 1 kHz sine waves through the amp, so in the real world that level of current should not be a problem. But 140mA is a lot of current.

It's hard to say much about the amp's operating conditions, because you've provided insights into the operating conditions, but not a full set of operating parameters. For example, you've posted a pin chart of voltages for the EL84, but not for the entire amp. And you've posted a snapshot of idle conditions, but not the conditions under full load. For the sake of completeness, it might help to see two tube pinout charts -- that include every tube in the amp. one chart at idle, one under full drive conditions. right now we're only looking at the power tubes. having a complete set of voltage pinouts would help to determine that there's not a problem hiding elsewhere in the amp.

Looking at your idle data, it does look like you've got the EL84 biased on the hot side, though people do commonly do this in cathode-biased amps, as the voltage rise is somewhat self-limiting. but Vk = 12.9VDC at idle seems kind of high to me. with 220R Rk, that looks like 58.6 mA for the pair, or 29.3mA per tube.

I don't remember what you've written in the other threads, so I'll ask it here -- when the power transformers are blowing up, are the power tubes and the transformers both dying at the same time?

[Mick Bailey]

The restriction on 100w and above incandescent bulbs is only for domestic use in Europe, so the shops now stock industrial ruggedised filament ones to get round it.

Aldi were selling boxes of 4 100w 'heavy duty' bulbs marked up as "not suitable for household room illumination" (green pack). They're also clear which makes them even better for amp use. The glass is probably made from orphans' tears to make you feel even more guilty about buying them.