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Old 23rd Feb 2015, 11:20 pm   #121
PJL
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Default Re: RGD 1046 amplifer repair

Equivalent lists are misleading. Take a look at this datasheet for an early AC044 which is nothing like a late PX4: http://www.r-type.org/pdfs/ac044.pdf.

At 250Va/40Vg AC044=40mA PX4=20mA PP3/250=7mA. These are not interchangeable. I have been caught by this, I got a Pye set with a PX4 for £5 and thought it was low emissions but it was just the bias was wrong.

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Old 24th Feb 2015, 1:05 am   #122
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Default Re: RGD 1046 amplifer repair

Quote:
Originally Posted by PJL View Post
Equivalent lists are misleading. Take a look at this datasheet for an early AC044 which is nothing like a late PX4: http://www.r-type.org/pdfs/ac044.pdf.

At 250Va/40Vg AC044=40mA PX4=20mA PP3/250=7mA. These are not interchangeable. I have been caught by this, I got a Pye set with a PX4 for £5 and thought it was low emissions but it was just the bias was wrong.
Yes, that can certainly be the case. That data sheet you linked to is for a very early version of that valve, I note the max anode voltage of 200 volts and the lower heater current of 0.7 amps. this goes back to the changes from the very early days to the late 40s and 50s as the other David was saying a page or so back. It's often the case that one type of valve can be used to replace another, but sometimes not the other way round.

Just having a quick scan back through this thread and I had forgotten the info posted by the other David in post 40. He gives the various voltages and currents for this amplifier, so worth another look just as a reminder.

I've had the 'Wireless World' data book for more than half a life time and it's what I use for nearly all the early(ish) stuff. I've posted the actual data for the three types of valve from the book below - note the ACO44 then has a higher anode voltage and heater current by this time, matching the other valves.

I think the OP has done really well with this amp and has so far made a very good job of the repairs. As I said, I have never worked on the RGD myself, so I've been learning a lot about them too. I think it would probably be ok with just that bias resistor increased in value, but that's easy to say when it's not your own amplifier. As has been said, those are tough old valves and there normally is nothing to go wrong with them other than the heater connections in the base. When the amp is finished, it would be worth rigging up a circuit with some power supplies and meters to see what you can do with the one original valve that still partially works - I did this on Saturday as a bit of an experiment with a power triode - I'll maybe post some pictures just for interest sometime.

First published February 1949 & updated 1954 Wireless World data on the three valves, just for reference, below -
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Old 24th Feb 2015, 11:43 am   #123
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Default Re: RGD 1046 amplifer repair

I had a re-read over post 40 and it's a wealth of information.

Quote:
Originally Posted by FERNSEH View Post
According to the information in Volume II of the Radio Servicing book the voltages present at the base of the PX4 triodes V8 and V9 are: anodes 340volts. V8 & V9 Grid bias -38.5 volts. anode currents V8 & V9 48/50mA.
The voltage at the cathode of the UU7 HT rectifier is about 375V. Add 20 volts if silicon diodes are used to replace the UU7. Not recommended. You need 450 volt elcos
Resistor R47 (360ohms) is the common bias resistor for both output valves.
Bear in mind that the bias voltage developed across this resistor was the function of total HT current of the amplifier and radio receiver.
I'd reckon the HT current of the radio receiver will be in the order of 15 to 20mA.
So it follows because there is no radio receiver connected to the amplifier the bias for the PX4s will be a lower value than it should be.
Seems we are aiming to compensate for the lack of the radio receiver and also run the valves at a lower level than stock due to the now silly replacement prices.

I think just to sum up this is where i'm at now.


without limiter/10k's

was
anode to cathode 297v
R47 as 360 ohm 47v
128Ma 64 per valve

now
anode to cathode 315v
R47 as 560 ohm 52v
93mA 46.5 per valve

valves are still running hot, but not as bad as before, so I have been using the lamp limiter. I will maybe try a cheap temperature sensor I have just to see what it reads on the glass. Perhaps a bit pointless as there's no reference to measure against but might give some indication

I will fit the 47 ohm resistors in the rectifier anode out's once they arrive.

That's a very good idea Techman about measuring the mA from valve anode to OP transformer. I missed that, so will measure it tonight.

I missed out on the auction. The valve was untested and went for over £30 so didn't bother bidding more in the end
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Old 24th Feb 2015, 1:04 pm   #124
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Default Re: RGD 1046 amplifer repair

I don't think the bias voltage is directly affected by the HT current as only the output valve current passes through R47. However, the radio would have taken HT and LT current resulting in increased resistive losses in the primary of the mains transformer reducing all of the secondary voltages slightly. The filament voltage is measuring a little high although within measurement tolerance so it might be beneficial to reduce all voltages rather than just the HT.

You are already on the maximum mains setting but you can use a low voltage mains transformer to reduce the mains primary input voltage. A 10-12V transformer wired as in the circuit would reduce all secondary voltages by about 5%. The transformer only needs to be small, 1A output would be more than enough.

The output can be Mains+12V or Mains-12V depending on the phase of the 12V output, so measure the AC voltage input to the amp before switch on.
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Old 24th Feb 2015, 3:53 pm   #125
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Default Re: RGD 1046 amplifer repair

That's a pretty neat way of reducing the input voltage. I believe I have a 15/15 1A in the house, would this do? voltage will reduce a little more 6-7% ish?

I can get a 12v if needs be. Thanks for doing up the diagram. The advantage of this is that it can be in the mains lead rather than the amp body
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Old 28th Feb 2015, 5:59 pm   #126
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Default Re: RGD 1046 amplifer repair

just as an update

did a few comparisons:


with lamp limiter:
AC in 171
rectifier out: 260v
Anode to Cathode 244
Across R47 32v

with transformer in mains lead:
AC in 212
rectifier out: 357v
Anode to Cathode 286
Across R47 46v

full power
AC in 238.6
rectifier out: 390v
Anode to Cathode 311
Across R47 51.6v


I have not put the resistors on the rectifier yet.

Valves are seriously hot on full power. I attached a cheapo temperature probe an d voltage was over 70 degC after a few minutes

With limiter, the valves are hot but no where near as hot as before

With tranny inline,, Hot, i.e couldn't touch the side of the valves.

Still seems hot to me with the transformer inline. But voltages and current seem within spec with this setup.

Painted transformers gloss black, really looks the part now.
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Old 28th Feb 2015, 7:05 pm   #127
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Default Re: RGD 1046 amplifer repair

With the transformer it is running at 23.5W for the pair so this is well within the 30W max. If the filament voltage is within 5% I would use it like this. There will be some output power sacrificed but the valves should last much longer.
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Old 28th Feb 2015, 8:47 pm   #128
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Default Re: RGD 1046 amplifer repair

Hi yes this seems like a very nice compromise between power and longevity. I did notice a very small rich blue glow on the glass of one of the vales like a 2mm line. Upon doing a bit of searching, this seem to be fluorescence.

The amps has a hum which is greatly reduced when a smoothing cap can is touched so I was guessing an earth issue. I read I think that an isolating transformer may be used to produce an earth. Some good post on this forum which I am currently reading.

I was hoping to attach the earth directly to chassis. I measure 1.6vac between earth and neutral. Is it possible to use an appropriate capacitor to earth the chassis. This seem to reduce the hum but again I was unsure if this necessary or recommended.

cheers
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Old 28th Feb 2015, 9:40 pm   #129
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Default Re: RGD 1046 amplifer repair

You should earth the chassis for safety reasons. If the mains transformer insulation fail this will ensure the fuse blows rather than you get electrocuted.
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Old 28th Feb 2015, 9:47 pm   #130
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Default Re: RGD 1046 amplifer repair

absolutely, I was reading a post on a RGD1050 I believe where the poster immediately earthed the chassis. I just was not sure if it was a matter of earthing direct;y to chassis of if there was more to it
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Old 28th Feb 2015, 11:48 pm   #131
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Default Re: RGD 1046 amplifer repair

If the power supply is isolated from the mains via a transformer - as yours is - you can earth it directly.

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Old 1st Mar 2015, 12:22 am   #132
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Default Re: RGD 1046 amplifer repair

Good safety practice says it should be earthed and soundly enough at that to take out fuses.

Problems come in the shape of earth loops. Speakers get connected to amplifiers and nowhere else, so that end of things is loop-free. Turntable cartridges can be earthed via the amplifier, with a separate, from its plug, earth to the metalwork of the turntable.

The fun comes when you have a separate preamp, but in the classic valve amp days preamps were routinely powered with HT and heater supplies from the power amp.

Most CD players are double insulated and floating, so they take earth potential from the amp they're connected to.

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Old 1st Mar 2015, 2:45 am   #133
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Default Re: RGD 1046 amplifer repair

That's great I have ordered a 3 pin bulgin type plug similar to what is on the unit already.

Fitted the 47 ohm resistors on teh rectifier anodes, basically no difference in voltages though
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