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Old 22nd Feb 2015, 11:08 pm   #101
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Default Re: RGD 1046 amplifer repair

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Post 96 - I didn't think that they should get 'that' hot. It could be due to them being the 'bottle' style rather than the 'balloon' style. I think you did very well getting those replacement valves.
Yes seems too much to me but then I am not used to valves in general. A small gentle fan perhaps if using for any length of time.

I just decided to buy the valves as as the seller seemed very good, at least from feedback and previous sales. I had to pay P&P , so worked out more than I originally though at £430 but I had just sold a Quad transistor amp so used the money from that. The UU8 cost £25
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Old 22nd Feb 2015, 11:19 pm   #102
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Default Re: RGD 1046 amplifer repair

Maximum anode current should never be more than 48mA through each valve. It's strange you should say that the valves seem to be running very hot. The voltages look ok, though.
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Old 22nd Feb 2015, 11:31 pm   #103
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Default Re: RGD 1046 amplifer repair

Just thinking (always a bad thing), the cathode resistor at 360 ohms has quite a voltage across it seeing as it's a lower value than in the Dynatron. Do I seem to remember you adding a resistor in each anode to measure the voltage across it as advised by another poster, or was that with the old valves?
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Old 22nd Feb 2015, 11:40 pm   #104
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Default Re: RGD 1046 amplifer repair

The amps looking good. It shouldn't need a fan for cooling. As a final check, I would double check on the anode current for each valve just to make sure that there isn't anything funny happening - put a mA meter in line if you like, rather than add a resistor and measure voltage across it - just to be 100% sure.
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Old 23rd Feb 2015, 12:39 am   #105
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Default Re: RGD 1046 amplifer repair

Just been off for a bite to eat.

Your anode current is too high, probably over 60mA.

Those are big old valves, but they shouldn't run that hot - the rectifier is the only hot valve in there.

More investigating to be done!
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Old 23rd Feb 2015, 12:48 am   #106
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Default Re: RGD 1046 amplifer repair

I=V/R =46/360 =128mA or 64mA per valve (if they were exactly matched so it is likely one is taking more current).

W=V*I =297*0.064 =19W per valve. They are getting hot because they are being overrun. Don't run it like this. You need to make some circuit changes as the later valves have different bias requirements and you are using a different rectifier valve.

1. From the datasheet, the recommended auto-bias resistor is 1000 ohms/valve when running at 300V so change R47 to 560 ohms (for 2 valves).
2. Because you will be lowering the current, the HT will rise. Measure the anode to filament voltage when you first switch on with the new resistor value. If it exceeds 300V you will need to either find an alternative rectifier or put some small value power resistors in series with each rectifier anode.

UU7 spec: http://www.r-type.org/pdfs/uu7.pdf
UU8 spec: http://www.r-type.org/pdfs/uu8.pdf

The graphs do show that a UU8 will produce more HT at 140mA than a UU7.

Last edited by PJL; 23rd Feb 2015 at 1:12 am.
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Old 23rd Feb 2015, 12:57 am   #107
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Default Re: RGD 1046 amplifer repair

David posted the circuit in post 12 on the first page of this thread but there's no component values on it, or any voltages. He may have the rest of the data so I would PM him and ask him. You need the proper service data on this amp so you can see what's maybe happened to it in the past. The Dynatron runs around 45mA anode current on each valve, so I wouldn't run that amp for too long until the problem is sorted.

Good job you thought the valves were a bit hot!

PS, Just seen the post by PJL and he's correct, although I'm surprised that versions of the valves would be that different - I guess that amp dates from the late 1940s and I would also guess that those valves are from the same sort of date, but you never know.
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Old 23rd Feb 2015, 2:12 am   #108
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Default Re: RGD 1046 amplifer repair

The PX4 was in production over a period where there was a lot of progress in valve design. also it kept being improved for greater output power and the type number was unfortunately retained unchanged. Anode voltage and power dissipation grew in a few steps over the years. The HT voltage in this amplifier is definitely too high for some of the earlier incarnations.

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Old 23rd Feb 2015, 12:31 pm   #109
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Default Re: RGD 1046 amplifer repair

I had been running the amp for small periods with the limiter in line. Without it the valves just got too hot and were in fact hotter than the rectifier, so I gathered there was possibly a problem.

First of all I shall try and locate a UU7 as the amp did originally come with it. I have a bid on one (untested) but may be worth a punt. The UU8 seems to be a stronger valve and overkill for this amp.

I will swap out R47 for a 560ohm and measure voltages anode to filament.

Very interesting on the different variations of PX4's. I would have assumed it was just a matter f a straight swap, at least in pairs.
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Old 23rd Feb 2015, 1:43 pm   #110
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Default Re: RGD 1046 amplifer repair

Good to hear you have been playing it safe.

We need to aim to get the anode dissipation down to about 12/13W per valve. I would replace the resistor and do the measurements before worrying about the rectifier as the 560 ohms will increase the filament voltage so you might still be within the 300V. We are aiming for about 50V across the 560 ohms.

The 560 ohms needs to be 7W minimum.

The calculation of the rectifier anode resistor is (next to) impossible so you would just have to try some values. I would guess about 47 ohms for each of the resistors will drop around 40V.
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Old 23rd Feb 2015, 4:11 pm   #111
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Default Re: RGD 1046 amplifer repair

I had a 6w 560r in my spares box, so I tacked on on quickly, some 7w on order

anode to cathode 315.2v

across r47 52.4

93mA?
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Old 23rd Feb 2015, 7:08 pm   #112
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Default Re: RGD 1046 amplifer repair

Still a bit high, the total is 29.5W. If they hadn't cost £400 I would leave as is as I suspect the amplifier was designed to run the valves at the limit.

I would be inclined to try adding the 47 ohm resistor in each rectifier anode to reduce the HT voltage which will also reduce the current. It will reduce the stress on the rectifier too.

They are very strong valves!
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Old 23rd Feb 2015, 7:13 pm   #113
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Default Re: RGD 1046 amplifer repair

That sounds good, 46.5mA through each valve then. The Dynatron was 91mA and I think it had about 315volts(ish) across the valve. It seems your amp dates from around 1947, the same as the Dynatron, so I would have thought it would have been designed to work on valves from that time. I know there were some very early versions of the PX4 from the 30s that operated from a lower anode voltage, but I think from just looking at your two valves that they are not from that long ago. Those early valves have a 'look' about them and you can just tell that they're very old.

The bottom line is that your amp was designed to work 'as-is' by RGD, so why it isn't doing so now is a good question. Perhaps as David (FERNSEH) says, it's the lack of load from the radio receiver - you could experiment by adding a load and see what difference it makes. I also tend to think that the UU8 wouldn't make that much difference, but it may make some - after all, it was standard to fit an EZ81 in place of an EZ80. One other thing, are you sure the mains selector on your amp is set to the correct mains voltage and have you actually measured your mains voltage? Mine is high at 245 volts and somtimes over. I've looked at a picture of the underside of another RGD amp (one that David has posted elsewhere) and it looks the same with that green watty resistor.

Anyway, see what the others think.

PS, just checked before posting and seen PJLs post and can say that the Dynatron has 47 ohm resistors in both anodes, whereas yours doesn't have any, so worth the mod, I think.

Edit: sorry, got that wrong. PJL says to fit them in the rectifier anodes, yes, a good idea. I was saying about the output valve anodes, as in the Dynatron.

Last edited by Techman; 23rd Feb 2015 at 7:19 pm.
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Old 23rd Feb 2015, 7:37 pm   #114
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Default Re: RGD 1046 amplifer repair

Many amplifier designs run the valves at or over the maximum ratings as the greater output gave them a competitive advantage. The retailer would also benefit as the customer would be back for replacement valves every year.

The graph for the UU8 does imply it will deliver about 20V more DC than the UU7.

I agree that the mains selector needs checking as well as the AC voltage on the filaments.
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Old 23rd Feb 2015, 7:47 pm   #115
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Default Re: RGD 1046 amplifer repair

Out of interest and for comparison, here is the circuit of the output stage of the Dynatron LF59 showing component values and voltages - I hope you can see it ok. It's the 1945 prototype design with modifications as listed up to 1949/50. The PP3/250 is the same as the PX4.
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Last edited by Techman; 23rd Feb 2015 at 7:59 pm.
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Old 23rd Feb 2015, 8:36 pm   #116
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Default Re: RGD 1046 amplifer repair

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Originally Posted by PJL View Post
I would be inclined to try adding the 47 ohm resistor in each rectifier anode to reduce the HT voltage which will also reduce the current. It will reduce the stress on the rectifier too.
That's a great idea. I don't think I have any power resistors at that value but will get some. It will just take the pressure off the valves and perhaps the elderly smoothing caps.

The 560ohm resistor r47 gets rather hot. The output of this amp seems pretty impressive.
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Old 23rd Feb 2015, 8:54 pm   #117
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Default Re: RGD 1046 amplifer repair

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That sounds good, 46.5mA through each valve then. The Dynatron was 91mA and I think it had about 315volts(ish) across the valve. It seems your amp dates from around 1947, the same as the Dynatron, so I would have thought it would have been designed to work on valves from that time. I know there were some very early versions of the PX4 from the 30s that operated from a lower anode voltage, but I think from just looking at your two valves that they are not from that long ago. Those early valves have a 'look' about them and you can just tell that they're very old.

The bottom line is that your amp was designed to work 'as-is' by RGD, so why it isn't doing so now is a good question. Perhaps as David (FERNSEH) says, it's the lack of load from the radio receiver - you could experiment by adding a load and see what difference it makes. I also tend to think that the UU8 wouldn't make that much difference, but it may make some - after all, it was standard to fit an EZ81 in place of an EZ80. One other thing, are you sure the mains selector on your amp is set to the correct mains voltage and have you actually measured your mains voltage? Mine is high at 245 volts and somtimes over. I've looked at a picture of the underside of another RGD amp (one that David has posted elsewhere) and it looks the same with that green watty resistor.

Anyway, see what the others think.

PS, just checked before posting and seen PJLs post and can say that the Dynatron has 47 ohm resistors in both anodes, whereas yours doesn't have any, so worth the mod, I think.

Edit: sorry, got that wrong. PJL says to fit them in the rectifier anodes, yes, a good idea. I was saying about the output valve anodes, as in the Dynatron.
Looking at the RGD museum site this one does seem from around 1947. I have a speaker with a red magnet which I think is the original, and I read on the net while researching this stuff that it was from a 1947 radiogram.

The mains is set for 250, I have seen other pictures with it set at 230

Quote:
Originally Posted by PJL View Post
Many amplifier designs run the valves at or over the maximum ratings as the greater output gave them a competitive advantage. The retailer would also benefit as the customer would be back for replacement valves every year.
The graph for the UU8 does imply it will deliver about 20V more DC than the UU7.
I agree that the mains selector needs checking as well as the AC voltage on the filaments.
I wonder what these lovely valves cost new. I surmise it wasn't such a big deal getting a set each year. like changing the sparkplugs on a car

Voltage across mains is 239vAC.

Voltage across filaments is 4.2vAC


The lack of load could well be part of the explanation. I will try and find out what the tuner power requirements were.

I do see a lot of these amps on far east sites in pairs with no apparent mods done, so I wonder if they actually run them like that, as they may have issues with hot valves.

I shall do the rectifier resistor mod ASAP. I think I read elsewhere on this forum that someone had put Bakelite fuses on the outputs to try and protect the rectifier and or 1N4007 diodes to at least keep the amp functional if the rectifier should fail.

Thanks Techman for those scans. I must say I do love the Dynatron amps. I have only really discovered them because of the 1045. They do look stunning and i'm sure sound so.


There's a UU7 on eBay for £5.50 with 31 mins on the auction, so will get the fingers at the ready
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Old 23rd Feb 2015, 9:10 pm   #118
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Default Re: RGD 1046 amplifer repair

Quote:
Originally Posted by Techman View Post
Out of interest and for comparison, here is the circuit of the output stage of the Dynatron LF59 showing component values and voltages - I hope you can see it ok. It's the 1945 ptototype design with modifications as listed up to 1949/50. The PP3/250 is the same as the PX4.
The PP3/250 is not identical to a PX4. http://www.r-type.org/pdfs/pp3-250.pdf

Test data is different too:
valve AVO Vg/Va/Aa/Gm Mullard Vg/Va/If
PP3/250 37/300/48/5.2 30/250/30
PX4 43/300/43/6 30/250/36
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Old 23rd Feb 2015, 9:23 pm   #119
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Default Re: RGD 1046 amplifer repair

You would need a precision RMS meter to check the LT properly but it is within 5% so should be fine.

The objective here is to run your valves at below manufacturers limits in order to extend their lifetime. There will be a slight reduction in power output.
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Old 23rd Feb 2015, 10:34 pm   #120
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Default Re: RGD 1046 amplifer repair

Hi, yes, I have seen the small differences in the data on the various versions of this triode, but I stand by my statement and would refer you to the data published by 'Wireless World' in 1954 as shown below -
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