RGD 1046 amplifer repair

[PJL]

That was quick.

Using the 10K resistors in the anodes might be a good place to start. This way you will reduce the risk of damaging anything and you can get an accurate anode current measurement by measuring the voltage across the 10K resistors. Then measure the grid voltages which should be near 0.

A quick check against the datasheet will confirm the valve condition. http://www.r-type.org/pdfs/px4-1.pdf

[PJL]

http://www.datasheetarchive.com/dl/S...s-00180049.pdf this one is better.

[mindburner]

Originally Posted by PJL

That was quick.

Yea Royal mail usually miss the special delivery window so it was great to get them before Monday.

Well the good news is that the amp works and actually sounds great. I had already powered her up with the inline lamp, audio source and speaker.

Even with the lamp in line, the quality was pretty good, although with some low end distortion perhaps due to the limiter

There is a very slight hum, like an earth hum. It fades slightly if I touch the top metal surface of the amp. This may be due to the amp not being grounded. I might get a bulgin 3 pin plug and investigate how to earth it. A good idea from a safety point of view.

Both valves appear to work. They both get quite hot even with the lamp limiter going. One gets hotter than the other. I ran the amp for a few minutes at a time. The lamp limiter increased in brightness to a reasonable level and stabilised nicely.

I checked the anode voltages to chassis - 250v

With some slight confidence I bypassed the lamp limiter and powered up.

Anode voltages now measured 351v. So 50v above the specs. What would be causing this?


I will fit the 2x 10k resistors and measure voltage drop across and report back.

But overall I am very happy with the result. This amp is a nice bit of kit and it's great to see it perform.

I'll have a look at those data sheets you kindly listed too.

[mindburner]

fitted 10k's

V drop across

with lamp limiter 182.7 & 186.9
w/out lamp limiter 257.6 & 260.1

seems pretty good?

thanks

[Radio Wrangler]

The HT doesn't look high considering the voltage written on the mains transformer tags, if it is a capacitor input filter. It's higher than the valve data.

Marconi's data sheet (link above from PJL) says that automatic bias must always be used, and that it is preferable to have separate bias systems for each valve of a push-pull pair.

Maybe RGD didn't read it, or they wanted their radiogram to go up to 11

David

[mindburner]

yes it seems that having just the one bias resistor is not the preferred way. They say on the data sheet. that it seems to be ok to use a common bias resistor if the valves are operated well below max voltage. The RDG sitting on 250v across the 10k.

So is that 25mA for the anode current (250/10000)

data sheet says 116 in a push pull

I could have misunderstood this

[Station X]

The series resistors mean that the anode voltage is reduced by 250V, so the anode current will be lower.

Even if you put 500V straight across the 10K resistors, the current wouldn't exceed 50mA. The resistors might get hot though.

[mindburner]

grid voltages 0.001/2 & 0.002

[mindburner]

Originally Posted by Station X

Even if you put 500V straight across the 10K resistors, the current wouldn't exceed 50mA. The resistors might get hot though.

hi yes they are pretty toasty after 5 minutes.

[Herald1360]

RGD may have been counting on the largeish bias voltage to reduce the actual Vak to nearer the PX4 limits. Later versions of the PX4 were good for 300V Vak and required about 43 volts bias which here will be reducing the Vak by that amount.

[PJL]

The valves were very expensive so it would be a good idea to do a check on the total anode dissipation. The results with the 10K resistor indicates they are a good match. I assume you have removed the 10K's now.

1. Measure the filament voltage from chassis in a DC setting on your DMM. It should be about 31V according to an earlier post. This will give us the total current for both valves as we already know the resistance is 370 ohms from your earlier measurement.
2. Measure the anode voltage from chassis of both valves.

The spec says it should not exceed 15W per valve and the anode voltage should not exceed 300V. If it is running at 84mA then this would equate to around 12.5W which would allow a little for missmatch between the valves.

[Techman]

There's been so much done over the last pages that I may be getting a bit confused about what components have been added/altered or not as the case may be.

The amp should be left as designed by RGD. The only difference now is that you don't have the radio receiver connected now. This may have taken an extra 35 mA (or so) from the HT, so if you want to experiment, you could calculate what (watty) resistor you would need to connect between the end of the HT line and ground to 'waste' some power and see if the load drops the HT at all - it probably won't do a lot.

An anode voltage to ground of 350 volts is perfect for this amp, remember, you've got another 40 odd volts across that cathode resistor, so if you measure across the valve then you'll probably have not much over 300 volts which is the spec for the PX4. I'm not personally familiar with the RGD amplifier, but I am familiar with the Dynatron LF59 and I can tell you that those voltages are near enough exactly the same.

[mindburner]

Originally Posted by PJL

The valves were very expensive so it would be a good idea to do a check on the total anode dissipation. The results with the 10K resistor indicates they are a good match. I assume you have removed the 10K's now.

hi yes 10's now off

filament voltage to chassis is 46v (that's with no limiter or 10k's)

anode voltages 344.5 & 344.5

[Techman]

Voltages are perfect.

[mindburner]

I calculated the cathode current as per this post
http://www.vintage-radio.com/repair-...ve_valves.html

0.012/.360 (bias resistor)
=0.033

[mindburner]

Originally Posted by Techman

There's been so much done over the last pages that I may be getting a bit confused about what components have been added/altered or not as the case may be.

Hi yes there is 46v across r47 (360ohm)
297v across valve anode filament/cathode

I think I was getting confused as I was looking at anode to chassis voltages and not checking across the valve (vak?). I am also a little paranoid as my knowledge is basic and my investment not quite high and I don't want to run the valves hard. They do get mighty hot though

[Techman]

The Dynatron LF59 is virtually the same and to quote from the original Dynatron service sheet - Anode volts of PX4 = 350 volts. Cathode volts = 41 volts.

I had an actual amp in front of me a while ago and checked the voltages and they were correct to within a volt or so.

Edit: our posts crossed. Yes, that sounds about right. Good result, I think.

[Techman]

You'll have to remind me - is that the original value of the cathode resistor?

The Dynatron is a bit higher at 450 ohms.

[Techman]

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.

[mindburner]

Originally Posted by Techman

You'll have to remind me - is that the original value of the cathode resistor?

yes it's 360ohm 5W. It's actually hand written on the side of the resistor.

It's reassuring to know that the new valves at least are no going into a seriously out of spec chassis. Hopefully the amp has not been changed that much and keeps some of it's authenticity.

A few pics of it now.

Just to say again how much the help has got me to this point. Without it I would have made some silly and possibly costly decisions. Now I know that both original valves are non functional as least for this amp.

So faults were..

all TCC coupling caps leaky, new 400v Vishays fitted
smoothing caps reformed (that was a great knowledge building experience)
4 replaced resistors, heading upwards of 20% tolerance
8uf cap replaced
dodgy/short on AC voltage selector
and 2 bad PX4 valves
dead UU7 rectifier

Hopefully I will mange on to damage this beauty with any more fiddling.

Might just respray the transformer housings but may just leave them as is