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Old 15th Jul 2018, 5:29 pm   #41
Edward Huggins
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Default Re: Beam Echo Avantic SPA11 restoration. Help please!

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Originally Posted by ChrisMarsden View Post
Wow! glad i could revive your memory!

I'll probably only use it with my EAR834 + Lenco turntable.
Is that the EAR834 valved phono pre-amp?
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Old 15th Jul 2018, 6:48 pm   #42
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Default Re: Beam Echo Avantic SPA11 restoration. Help please!

I doubt it would be for a Deram. Think they appeared in the mid 60’s. I’m sure the avantic was a 50’s product.
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Old 15th Jul 2018, 6:56 pm   #43
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Default Re: Beam Echo Avantic SPA11 restoration. Help please!

The Deram and the 9TAHC were mentioned up-thread by Edward as suggestions for how to use the Avantic nowadays. Getting any good quality ceramic cartridge nowadays seems to be problematical. I suspect it'll wind up with an RIAA box.

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Old 16th Jul 2018, 8:45 pm   #44
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Default Re: Beam Echo Avantic SPA11 restoration. Help please!

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Is that the EAR834 valved phono pre-amp?
Its a clone of the 834 that i built.
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Old 17th Jul 2018, 9:09 am   #45
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Default Re: Beam Echo Avantic SPA11 restoration. Help please!

If we are talking about Tim de Paravicini's products the 834 is an integrated amplifier ; the phono preamp is the 834P.
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Old 18th Jul 2018, 5:15 pm   #46
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Default Re: Beam Echo Avantic SPA11 restoration. Help please!

I close to my first power on. How do you suggest i approach this?

I have some cheap 8ohm speakers i don't care about and a spare set of ecl82's.

Is there any merit in powering on with out the valves fitted and measuring voltages at the valve sockets?

any pointers would be appreciated.
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Old 18th Jul 2018, 6:17 pm   #47
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Default Re: Beam Echo Avantic SPA11 restoration. Help please!

Have you checked for leakage in the DC blocking capacitors to the beam tetrodes first grids?

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Old 18th Jul 2018, 6:31 pm   #48
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Default Re: Beam Echo Avantic SPA11 restoration. Help please!

As a general rule there is no merit at all in powering on with any or all of the valves missing. The voltages will be wildly different from (often much higher than) what they would normally be and if any of the components (commonly capacitors) is close to its maximum rating under normal operation then you can risk destroying it by over-volting.

The only exception is if the amp had had a valve rectifier. Then it's sometimes instructive to power it with all the valves in place except for the rectifier e.g. for spotting on-load faults in the mains transformer. But with that exception I wouldn't do it, any more than I'd try to fire my car up with all the filters missing. And of course the SPA11 doesn't have a valve rectifier.

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Old 18th Jul 2018, 7:03 pm   #49
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Default Re: Beam Echo Avantic SPA11 restoration. Help please!

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As a general rule there is no merit at all in powering on with any or all of the valves missing. The voltages will be wildly different from (often much higher than) what they would normally be and if any of the components (commonly capacitors) is close to its maximum rating under normal operation then you can risk destroying it by over-volting.
When the valves are cold they are effectively open-circuit, there is no anode or screen current flowing. So for a couple of minutes after switch-on, the circuit will behave as if all the valves are removed. And it should have been designed to withstand that.

I am not saying it's a good idea to power the amplifier up with no valves in place, but if this damages coupling capacitors (etc) by putting too high a voltage across them then said capacitors will be over-voltaged every time the amplifier is turned on from cold.
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Old 18th Jul 2018, 7:13 pm   #50
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Default Re: Beam Echo Avantic SPA11 restoration. Help please!

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Originally Posted by Radio Wrangler View Post
Have you checked for leakage in the DC blocking capacitors to the beam tetrodes first grids?
By switch on they will have been replaced.

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Originally Posted by TonyDuell View Post
When the valves are cold they are effectively open-circuit, there is no anode or screen current flowing. So for a couple of minutes after switch-on, the circuit will behave as if all the valves are removed. And it should have been designed to withstand that.

I am not saying it's a good idea to power the amplifier up with no valves in place, but if this damages coupling capacitors (etc) by putting too high a voltage across them then said capacitors will be over-voltaged every time the amplifier is turned on from cold.
Thanks. Valves in then.
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Old 18th Jul 2018, 9:15 pm   #51
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Default Re: Beam Echo Avantic SPA11 restoration. Help please!

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When the valves are cold they are effectively open-circuit, there is no anode or screen current flowing. So for a couple of minutes after switch-on, the circuit will behave as if all the valves are removed ...
The high-voltage ends of the coupling capacitors are connected to the anodes of an ECC83 and the triode in an ECL82. Are you sure that there will be no anode current flowing in these valves for a couple of minutes after switch-on ?

Cheers,

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Old 19th Jul 2018, 5:15 am   #52
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Default Re: Beam Echo Avantic SPA11 restoration. Help please!

Perhaps 'minutes' is over-doing it, but I am pretty sure the anodes current of an ECC83 and an ECL82 are close to zero when the cathodes are cold. Given the amplifier doesn't use a valve rectifier, the HT will be present before the ECC83s and ECL82s are passing any significant current.

(Close to zero, etc to avoid pedants who insist there will be some leakage across the valveholder, etc...)

The capacitors had better be specified to withstand the voltage when no anode currents are flowing or there is a real risk of them breaking down every time the amplifier is switched on from cold.
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Old 19th Jul 2018, 5:37 am   #53
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Default Re: Beam Echo Avantic SPA11 restoration. Help please!

It's a good idea to power via a lamp limiter on first power up if you can, if not keep your hand on the off switch. Also monitor the HT, it should go up then after about 10 seconds, it should start dropping, 20 seconds later it should have dropped about 20v or more and settled, depends on the power supply, then check voltages. Any smoke or other loud noise's, sw off.

Be aware big and small caps can retain a charge and will bite if you touch them even with power off. Make a cap discharge tool, IE bit of scrap wire and a biggish 100 ohm resistor or any biggish low value R. Before working on the amp discharge caps first, check voltage at said cap, then it's safe.

Apart from that have fun, Andy.
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Old 19th Jul 2018, 8:38 am   #54
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Default Re: Beam Echo Avantic SPA11 restoration. Help please!

Thanks, I'll build a lamp limiter.

given the schematic in an earlier post where (exactly) should i measure the voltages at switch on?
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Old 19th Jul 2018, 11:16 am   #55
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Default Re: Beam Echo Avantic SPA11 restoration. Help please!

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... Given the amplifier doesn't use a valve rectifier, the HT will be present before the ECC83s and ECL82s are passing any significant current ...
In the end it boils down to the numbers.

With a view to inserting some 'ground truth' into the discussion I've made a measurement. I set up an old stock Mullard ECC83 (I63 type) with Rk = 1k2, Ra = 47k and an HT voltage of 250V as per one of the entries at the bottom of the third page of the datasheet . I used a scope to monitor the cathode voltage and the DC heater voltage and I triggered off the sharp switch-on of the latter (the HT was already on). The valve started out stone cold. The result is attached (sorry for the blotchy reflections from the scope screen - it's sunny in the workshop today !).

As you can see, the cathode voltage, and therefore the anode current, reaches two-thirds of its final value (830mV out of a final value of 1.3V) after 13.4 seconds. To be honest that was longer than I was expecting. If you'd asked me to guess I'd have guessed well under 10 seconds. But it's not 2 minutes.

As for the HT, sure, at the mains transformer secondary this comes on instantly. But the HT rail consists of a chain of electrolytic smoothers (tens of microfarads) in series with dropper resistors (adding up to tens of kilohms). The last cap, which feeds the ECC83, will take quite a while to charge up. Even without any current leaking through the valves I'd estimate it would normally be a few seconds before peak (=damaging) voltage is reached.

But a few seconds is less than 13 seconds. So you may well be right - the caps had better be able to withstand some over-voltage, albeit briefly. I guess this is why electrolytics used to have a 'surge' rating. But there is a difference between a transient switch-on surge and running the set for many minutes without valves in while voltage measurements are made around it.

There might also be an effect of the valves' cold heaters. These will load the mains transformer a bit during the warm-up phase, keeping the peak HT voltage down compared with the value it would get to if the tranny was completely unloaded.

Cheers,

GJ
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Old 20th Jul 2018, 7:37 am   #56
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Default Re: Beam Echo Avantic SPA11 restoration. Help please!

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It's a good idea to power via a lamp limiter on first power up if you can
Apart from that have fun, Andy.
Using a lamp limiter for the first time - what should the lamp do? first dim then getting brighter or initially bright then dimmer? I assume full brightness is bad = short. What are the possible state transitions for the bulb?
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Old 20th Jul 2018, 7:52 am   #57
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Default Re: Beam Echo Avantic SPA11 restoration. Help please!

A blast of brightness due to several things (DC component of turn-on transient into transformer, cold (lower resistance) heaters warming, reservoir capacitor charging) then settling down to dimness.

The light bulb's resistance changes quite a bit, as the initial surge ends, the filament cools and its resistance falls which makes it cool more which.... so it can be quite dim with a good amplifier idling.

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Old 20th Jul 2018, 9:43 am   #58
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Default Re: Beam Echo Avantic SPA11 restoration. Help please!

Will a 100w tungsten type do?
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Old 20th Jul 2018, 10:17 am   #59
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Default Re: Beam Echo Avantic SPA11 restoration. Help please!

Should do just fine.

It's worth having a few different wattages so you can move the point where the regenerative reduction of resistance kicks in. Sometimes, testing a small device doesn't get a big bulb over the hump.

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Old 26th Jul 2018, 8:18 pm   #60
ChrisMarsden
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Default Re: Beam Echo Avantic SPA11 restoration. Help please!

Any tips for valve socket cleaning? And what about potentiometers?
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