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Radford STA15 Restoration

Postman delivered me these today...

2022-06-01_09-10-00 by Garf Arf, on Flickr

I've done a little reading around on ECF82s/6U8A in Radfords and a stumbled across this post from Will at RR and I thought I'd repost it here...

One foible with the STA15/25 is that they are quite sensitive to the choice of 6U8/ECF82. This has been noticed by a few people. Because of the dissimilar sections forming a long tailed pair phase splitter, it's quite possible for an otherwise perfectly good 6U8/ECF82 to bias up "oddly" in these amps, where one side draws too much current and throws the phase splitter off balance. This causes far more even order harmonic residual hence the wildly differing THD readings. One way I've found to quickly check for a "goodun" is to measure the anode voltages at the phase splitter, if they're within around 20v of each other it generally means it'll perform on the lower THD end of things. This is certainly easier if you don't have a distortion analyser at hand!I select for this in new and rebuilt amps of course. Generally a good one will stay balanced in the circuit and won't drift (thankfully!)A Radford with carefully selected valves will often beat the 0.1% distortion spec.

I assume the difference is between these two voltages I've drawn a very crude and particularly non-artistic ring around.

InkedST15 Series III Anodes_LI by Garf Arf, on Flickr

I did wonder what the 15V marking meant, I understand now.
 
Marchbanks said:
You and me both. After posting about the screw I remembered I hadn’t lost it, I was simply missing the clip. I had wrapped the screw in red insulating tape and put it somewhere for safe keeping. I then spent half an hour searching through the more obvious boxes and drawers. I gave up, then over a cup of coffee thought ‘hang on, something in my memory tells me it is close to the amp...’ Aha!

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Nice matching tape too :) I have a little pot for all the things I find and don't have a home right that very moment...a re-housing pot.

I'm struggling with the speaker connectors, they are just too loose for my liking. Certainly not a firm stiff connection at all. I'm using good Deltron 4mm plugs, but the sockets aren't offering much grip and just looking at your picture reminds me of not just how loose the plugs are in the sockets are, and how precarious the 4mm plugs look half in and half out. I might swap them out for the type I used on the Leak ST20.

PXL_20220420_162553093 by Garf Arf, on Flickr
 
chiily said:
I did wonder what the 15V marking meant, I understand now.
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I thought (assumed) the voltages in italics referred back to the '400mv 1 kHz' input 'test'. So 1.8v on the anode of the EF86. 15v on the phase splitter anodes and 170v on the output pair anodes?
 
snowman_al said:
I thought (assumed) the voltages in italics referred back to the '400mv 1 kHz' input 'test'. So 1.8v on the anode of the EF86. 15v on the phase splitter anodes and 170v on the output pair anodes?
Click to expand...
I'm just trying that now with a 1kHz 400mV sine wave input and measuring across the anodes of the ECF82.

Chan 1, Mullard ECF82 I'm measuring about 13VDC
Chan 2, Unknown ECF82, I'm measuring 32VDC

I've just popped in a new 6U8A into Chan2 and the inter-anode voltage is around 15VDC, an improvement indeed.
 
Bit more testing at 400mV/1kHz input and with at least 10 minutes warmup for each measurement.

I replaced the Mullard ECF82 in chan1 with a new 6U8A and the inter-anode voltage went up to 32VDC. So, I swapped the 6U8As over and the inter-anode voltages followed the valves, that is Chan 1 dropped to about 15VDC and chan 2 to 32VDC.

So, not all 6U8A are made the same it seems.

Anyway, I put the Mullard back in its original position and the low voltage 6U8A in Chan 2 and the voltages settled to about 13VDC and 15VDC respectively.

Whilst I was there I measured the voltages across the cathode resistors, to see how well matched they were.

Chan 1
V3 - 24.6VDC
V4 - 24VDC

Chan 2
V3 - 26.4VDC
V4 - 25.9VDC

They are pretty well matched in the pairs.
 
snowman_al said:
Thinking aloud - as the 1kHz is a sine wave should the italic voltages be AC?
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Good point. I shall check after lunch.

I think I was mislead by the two anode voltages 250 and 265, subtract one from the other and one gets 15V.
 
The replacement of the dodgy ECF82, whether it is just my expectation or reality, had made a difference to the sound of the amp. Much more enjoyable and evenhanded.

What is surprising is how much it sounds like my KSA50 clone, just not as dark.
 
What a lovely amp this Radford is!

I have noticed a faint "pattering" noise on both channels with no music playing. It is just audible from about 2.5m from the speakers. No hum though, mechanical or electrical. As it is in both channels and I have swapped valves around, I'm wondering what it could be?

I have bought a set of JJ EL34s, just listening now, and yes certainly brighter and cleaner than the Mullards; I'm not convinced yet.
 
chiily said:
What a lovely amp this Radford is!
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My ears are old and shot, and I think I take mine for granted - but it’s always gratifying when other folk, including pfm’ers, are knocked out by it.

chiily said:
I have noticed a faint "pattering" noise on both channels with no music playing. It is just audible from about 2.5m from the speakers. No hum though, mechanical or electrical. As it is in both channels and I have swapped valves around, I'm wondering what it could be?
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You don’t by any chance use homeplugs? I heard a similar sound on mine when I did. As soon as I stopped the data stream (iPlayer going to the TV) the noise stopped too.

I’m sure I’ve said this before - with four JJ EL34s in place mine measured 0.1% THD. With four (a mixture of xf2 and xf4) Mullards that dropped to 0.04%.
 
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I wonder like Marchbanks if it is your router, streamer or even your phone.
My squezebox touch needs to be well away from the amp or I get a tip, tip, tip noise between tracks and my phone causes all sorts of hash anywhere near the amp.
 
Thanks for the tips about homeplugs and routers. I do have a homeplug extending my WiFi to the garage. I shall try turning it off.

I'm just getting into Mingus...

KT77s...is tube rolling addictive?
 
chiily said:
Nice matching tape too :) I have a little pot for all the things I find and don't have a home right that very moment...a re-housing pot.

I'm struggling with the speaker connectors, they are just too loose for my liking. Certainly not a firm stiff connection at all. I'm using good Deltron 4mm plugs, but the sockets aren't offering much grip and just looking at your picture reminds me of not just how loose the plugs are in the sockets are, and how precarious the 4mm plugs look half in and half out. I might swap them out for the type I used on the Leak ST20.

PXL_20220420_162553093 by Garf Arf, on Flickr
Click to expand...

Either of these any good? Stripped out of old Ion kit - I kept them specifically for vintage amp rebuilds....but I never scored any :)

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Richard
 
Richard, if you're okay sending to me I would appreciate the first, squarer pair please? More than happy to pay P&P.
 
snowman_al said:
Thinking aloud - as the 1kHz is a sine wave should the italic voltages be AC?
Click to expand...
That is spot on Al. I bought a couple of other ECF82s, searching for the lowest DC between the anodes. With 1kHz@400mV on the input I'm seeing 15VAC between the anodes on both channels, pretty much spot on 15VAC in fact. DC measurement between the anodes of about 1VDC on one channel and 4VDC on the other.

And yes, the pattering noise is my mains ethernet homeplug. Thank you for the pointers. :) I wonder if there is a way to filter it on the mains input to the amp?
 
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