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Howard Stone Howard Stone is offline
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Default Radford STA25 Mk 3 modifiactions

Someone is offering me one of these for a good price. HOWEVER a Radford expert says that it has been modified

1. It appears to be using HT capacitors in series with balancing resistors (probably to make up the voltage rating as the original needs 500V caps).

2. Someone has made brackets to mount the extra two capacitors on top the output transformers.

3. The caps have been changed on the main boards too though judging by their appearance they look like 80s vintage caps (as do the replaced main smoothing caps)

4. The cap for the mains voltage selector is missing, presumably the tap has been selected via a solder link somewhere within the amp.


I'm sure the amp works but what I don't know is how serious these changes are, whether they will change the character of the amp in an important way and whether.

Your thoughts much appreciated.
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Peter Wieck[_2_] Peter Wieck[_2_] is offline
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Default Radford STA25 Mk 3 modifiactions

Three things:

a) I do not own, nor have I ever used any Radford Product.
b) This is a 6550 (KT-88) based amplifier, operating at the bleeding edge of what modern-production 6550 tubes can deliver.
c) From what I can see of Radford amps contemporaneous to that one, they are reasonably well made, if a bit crowded.

So, based on my experience with 6550-based power amplifiers (Dynaco, Scott):

1. The voltage selector is the least of your worries, assuming it is correct for your location. In point-of-fact, if it is a typical rotary switch vs. a fixed plug, it becomes a weak point in the system. Good that it is gone. NOTE: Dynaco and Scott typically changed voltage by reconfiguring power-transformer windings - permanent, IOW.

2. Using dropping resistors to reduce voltage to filter caps is altogether a bad idea from many perspectives. Putting them where they are seems to involve additional leads, exposing HV B+ where it should not be - another bad idea.

3. Nothing intrinsically wrong with 80s-vintage caps - I run a number of them myself. However, and since then, the chemistry has gotten remarkably better than before. If you are going to take on this device, consider a recapping with caps of the proper voltage and screened to be close in value channel-to-channel.

Cutting to the chase: you are offered an amplifier of reasonably good repute that has some hair on it. That, overall, you would like to restore to its original reliable condition and design.

Amp costs X
Restoration will cost Y in labor (presumably yours) and Z in parts.

X - (Y+Z) would be the fair price of this amp. Writing entirely for myself, I love a challenge and would think nothing of going through the necessary restoration steps. That is not everyone's position, however.

Good luck with it!

Peter Wieck
Melrose Park, PA
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Howard Stone Howard Stone is offline
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Default Radford STA25 Mk 3 modifiactions

Cheers Peter. This is similar to the advice I got from the engineer who fixed the Krell from me, he basically said that these things shouldnt make too much of a difference and anyway they could easily be reversed at a later date. So Im going to go for it.

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Howard Stone Howard Stone is offline
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Default Radford STA25 Mk 3 modifiactions

Well it arrived and was hooked up to the Rogers JR149s!

And wonderful, I listened to it twice and both times after it had warmed up for 15 minutes the sound was just glorious! And it helped greatly make the bass on these little speakers.

And then . . .

.. . . and then A CAP EXPLODED. Very dramatically with lots of smoke.

Thank goodness for ebay buyer protection. In this case the seller wants to take it back and fix the problem and I'm going to give him the chance.

Of course it's a very old amp, and of course I expect to spend a lot of money to get it rebuilt. But I'd like it to work for more than four hours first given that it was sold as being "in good working order!"

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Peter Wieck[_2_] Peter Wieck[_2_] is offline
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Default Radford STA25 Mk 3 modifiactions

We have apples: "Good working order".

And we have Oranges: "Of course it's a very old amp...".

That it was only a capacitor is remarkable.
That the amp was modified is, likely, a contributing factor.
That it is not the seller's fault is also very most likely the case.

I will lay dollars to Krispy Kremes that the seller ran the amp for 20 minutes or so, and had no issues. That, in eBay speak is "good working order".

And, for the record, had that been my amp, it would have spent its first 8 hours or so on my metered variac, being watched very closely. Likely, I would have seen the rise in current prior to the cap going *POW*.

This is why, when introducing a new component into the herd, a certain amount of precaution must be taken.

If you have no skills personally, this will be costly, because the 'proper' fix is to restore it to its original condition. If you have such skills, less than US$100 should more than cover it in parts.

Best of luck!

Peter Wieck
Melrose Park, PA


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[email protected] dpierce.cartchunk.org@gmail.com is offline
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Default Radford STA25 Mk 3 modifiactions

On Tuesday, September 3, 2019 at 4:10:54 PM UTC-4, Peter Wieck wrote:
We have apples: "Good working order".

And we have Oranges: "Of course it's a very old amp...".

That it was only a capacitor is remarkable.
That the amp was modified is, likely, a contributing factor.


That it's made by Radford is at least as likely a factor.

In the 1970's I was an authorized service center for Radford
and made, at the time, a fair piece of change repairing their
stuff under warranty. That is until they decided to stop paying
for warranty repairs altogether. And, at that point, I stopped
accepting Radford product for warranty repairs: I'd be damned if
I was going to eat their problems.

The biggest issues were failed power supply capacitors and,
strangely enough, failed power supply transformers. There
was no apparent correlation between the two: Lots of caps
went nova, and two or three transformers went as well: but
in those cases, the caps were okay.

Beyond that, almost every one of the Radford preamps I
encountered has bad pots (all noisy, several with dead
spots).

Not the most confidence-inspiring product line, to be sure.

Best of luck!


Indeed!
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Peter Wieck[_2_] Peter Wieck[_2_] is offline
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Default Radford STA25 Mk 3 modifiactions

On Wednesday, September 4, 2019 at 9:09:49 AM UTC-4, wrote:


That it's made by Radford is at least as likely a factor.


Always a factor! However, in my experience, when any piece of tube equipment gets past its 10th birthday without incident and under normal use, failures tend to shift away from factory problems to age-and-wear problems.

Peter Wieck
Melrose Park, PA
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Howard Stone Howard Stone is offline
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Default Radford STA25 Mk 3 modifiactions

Part of my problem is that my 6 hours with the amp made me fall in love with it, so I want one!

My plan is to let the person who sold it to me have a go at repairing it. If it arrives and works for a few weeks, Ill have it serviced and restored by someone I trust. The vendor was a company called Emporium, a large UK eBay seller.

If it doesnt all work out, well just maybe Ill buy a new one. Theres a company, Radford Revival, with a good reputation, which makes them.

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Peter Wieck[_2_] Peter Wieck[_2_] is offline
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Default Radford STA25 Mk 3 modifiactions

On Thursday, September 5, 2019 at 11:11:30 AM UTC-4, Howard Stone wrote:
Part of my problem is that my 6 hours with the amp made me fall in love w=

ith it, so I want one!
=20


OK - and understood.=20

However, you should understand that a basic tube power-amp is about as simp=
le a piece of electronics as ever there is (or was). I am not sure of your =
fine motor skills or ability to follow directions in electronics, but that =
amp is crying out for your personal attention.=20

And, think of the sense of accomplishment were you to bring it to life on y=
our own!=20

Peter Wieck
Melrose Park, PA=20
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Howard Stone Howard Stone is offline
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Default Radford STA25 Mk 3 modifiactions

Well the Radford is back and it has been working well for the past week, with my little Rogers JR149s. The bass is MUCH tighter.

My plan is to hold on to it for a couple of weeks or so, just to enjoy it and get to know it better. Then I'll send it for a thorough service.

I think the big thing I've learned, thanks to my adventures with the Krell and the Radford, is that the amp really does matter a huge amount. I'd underestimated that before.


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Peter Wieck[_2_] Peter Wieck[_2_] is offline
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Default Radford STA25 Mk 3 modifiactions

On Wednesday, September 18, 2019 at 5:53:07 AM UTC-4, Howard Stone wrote:

I think the big thing I've learned, thanks to my adventures with the Krell and the Radford, is that the amp really does matter a huge amount. I'd underestimated that before.


All true, but one is an apple, one is an orange, and they are substantially different beasts. Not to suggest that one is better or worse, just different. You will notice that most around the edges - when driven to/near clipping and/or on passages with a wide peak-to-average with significant transients.


Peter Wieck
Melrose Park, PA
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Trevor Wilson[_3_] Trevor Wilson[_3_] is offline
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Default Radford STA25 Mk 3 modifiactions

On 18/09/2019 7:53 pm, Howard Stone wrote:
Well the Radford is back and it has been working well for the past week, with my little Rogers JR149s. The bass is MUCH tighter.


**Than what? The Krell or the unrepaired Radford? The Krell possesses an
almost perfect bass response and low output impedance, thus bass will be
as good as it gets. The Radford, although a decent enough amplifier
cannot hope to match the Krell numbers. That said, the high(ish) output
impedance, common to most valve amplifiers, can lead to a pronounced
bass 'hump' at the resonance peak/s of the speaker system. The bass peak
may be preferred by some listeners.


My plan is to hold on to it for a couple of weeks or so, just to enjoy it and get to know it better. Then I'll send it for a thorough service.

I think the big thing I've learned, thanks to my adventures with the Krell and the Radford, is that the amp really does matter a huge amount. I'd underestimated that before.


**Of course it matters. On one hand you have a 'blameless'
amplifier (Krell) and, on the other hand, you have an amplifier that
insinuates it's own sonic character. The Krell exhibits a ruler flat
frequency response, excellent phase response, inaudible levels of THD
and is load invariant, down to below 2 Ohms. The Radford is the
opposite. It's frequency response varies with the applied load
impedance, it's phase response is not flat at frequency extremes and
distortion is high(ish).


--
Trevor Wilson
www.rageaudio.com.au

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Howard Stone Howard Stone is offline
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Default Radford STA25 Mk 3 modifiactions

I havent ever paired the Krell with the 149s, the Krell is with a pair of Spendor SP1s. My plan is to live with the Radford/149 combination a few more days, just to really get used to it and know it, and then swap amps - I.e. put the Radford with the spendors and the Krell with the 149s, and then compare and contrast.

And then, in October, I get the Radford serviced/rebuilt.
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Howard Stone Howard Stone is offline
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Default Radford STA25 Mk 3 modifiactions

On Thursday, 19 September 2019 11:00:48 UTC+1, Trevor Wilson wrote:
On 18/09/2019 7:53 pm, Howard Stone wrote:
Well the Radford is back and it has been working well for the past week=

, with my little Rogers JR149s. The bass is MUCH tighter.
=20
**Than what? The Krell or the unrepaired Radford? The Krell possesses an=

=20
almost perfect bass response and low output impedance, thus bass will be=

=20
as good as it gets. The Radford, although a decent enough amplifier=20
cannot hope to match the Krell numbers. That said, the high(ish) output=

=20
impedance, common to most valve amplifiers, can lead to a pronounced=20
bass 'hump' at the resonance peak/s of the speaker system. The bass peak=

=20
may be preferred by some listeners.
=20
=20
My plan is to hold on to it for a couple of weeks or so, just to enjoy =

it and get to know it better. Then I'll send it for a thorough service.
=20
I think the big thing I've learned, thanks to my adventures with the Kr=

ell and the Radford, is that the amp really does matter a huge amount. I'd =
underestimated that before.
=20

=20
**Of course it matters. On one hand you have a 'blameless'
amplifier (Krell) and, on the other hand, you have an amplifier that=20
insinuates it's own sonic character. The Krell exhibits a ruler flat=20
frequency response, excellent phase response, inaudible levels of THD=20
and is load invariant, down to below 2 Ohms. The Radford is the=20
opposite. It's frequency response varies with the applied load=20
impedance, it's phase response is not flat at frequency extremes and=20
distortion is high(ish).
=20
=20
--=20
Trevor Wilson
www.rageaudio.com.au
=20
---
This email has been checked for viruses by Avast antivirus software.
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Re the Radford, I can't see the frequency response curve online. The guy wh=
o rebuilt my Krell said (before he's experienced the KSA50 -- so that may h=
ave changed his perceptions) "The Radford STA25 is probably the best power =
amp I've ever heard at any price.. in the top few anyway. Coincidentally (h=
a!) it happens to be the best in electronic engineering terms, design, and =
measured performance, with lower distortion, wider frequency response and h=
igher damping factor than any other valve power amp I've measured."=20
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Trevor Wilson[_3_] Trevor Wilson[_3_] is offline
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Default Radford STA25 Mk 3 modifiactions

On 20/09/2019 7:45 pm, Howard Stone wrote:
On Thursday, 19 September 2019 11:00:48 UTC+1, Trevor Wilson wrote:
On 18/09/2019 7:53 pm, Howard Stone wrote:
Well the Radford is back and it has been working well for the past week, with my little Rogers JR149s. The bass is MUCH tighter.


**Than what? The Krell or the unrepaired Radford? The Krell possesses an
almost perfect bass response and low output impedance, thus bass will be
as good as it gets. The Radford, although a decent enough amplifier
cannot hope to match the Krell numbers. That said, the high(ish) output
impedance, common to most valve amplifiers, can lead to a pronounced
bass 'hump' at the resonance peak/s of the speaker system. The bass peak
may be preferred by some listeners.


My plan is to hold on to it for a couple of weeks or so, just to enjoy it and get to know it better. Then I'll send it for a thorough service.

I think the big thing I've learned, thanks to my adventures with the Krell and the Radford, is that the amp really does matter a huge amount. I'd underestimated that before.


**Of course it matters. On one hand you have a 'blameless'
amplifier (Krell) and, on the other hand, you have an amplifier that
insinuates it's own sonic character. The Krell exhibits a ruler flat
frequency response, excellent phase response, inaudible levels of THD
and is load invariant, down to below 2 Ohms. The Radford is the
opposite. It's frequency response varies with the applied load
impedance, it's phase response is not flat at frequency extremes and
distortion is high(ish).


--
Trevor Wilson
www.rageaudio.com.au

---
This email has been checked for viruses by Avast antivirus software.
https://www.avast.com/antivirus


Re the Radford, I can't see the frequency response curve online.


**The frequency response into a dummy load resistor will likely be
pretty reasonable. Where almost all valve amps fall down, is when
driving real-world loudspeakers that present a varying impedance to the
amplifier (which is most of them). Some exceptions include Magnepans and
those speakers which have been designed with Zobel networks to flatten
the impedance curve. Consider the two frequency response graphs. Focus
on the frequency response of two amplifiers (one a valve model and one
an SS model), when driving a simulated loudspeaker:

https://www.stereophile.com/content/...r-measurements

https://www.stereophile.com/content/...0-measurements

Depending on the two amplifiers, those frequency response errors may be
lesser or greater in different amplifiers. They do, however, illustrate
the fundamental differences between most valve amplifiers and most SS
amplifiers. And, make no mistake: It is the errors in the frequency
response that makes the most audible differences between two different
amplifiers.

The guy who rebuilt my Krell said (before he's experienced the KSA50
-- so that may have changed his perceptions) "The Radford STA25 is
probably the best power amp I've ever heard at any price.. in the top
few anyway.


**Perhaps he hasn't heard many power amps. Perhaps he has a preference
for that particular amplifier. I don't know and, I suspect, neither do
you. Thing is, your Krell KSA100 is, essentially, blameless (ie:
Perfect) from the standpoint of audible flaws. I should add that I have
listened and measured both the KSA50 and KSA100 amplifiers. I felt that
the KSA100 was a truly excellent product. The KSA50 was lacking in my
system, but it was reasonably respectable through other speakers.
Consider an 'ideal' amplifier:

https://www.stereophile.com/content/...r-measurements

Such an amplifier is (or should be) the amplifier to judge others by.

Coincidentally (ha!) it happens to be the best in electronic
engineering terms, design, and measured performance, with lower
distortion, wider frequency response and higher damping factor than any
other valve power amp I've measured."


**Can you supply those measurements? The output impedance is the figure
I am most interested in. I would doubt that the figure would be much
below 1 ~ 1.5 Ohms. I very much doubt his conclusions. I hasten to add
that I have never examined the STA25 on my own bench. I have, however,
had literally hundreds of valve amps cross my bench. ALL display the
kinds of audible flaws I've been discussing to a greater or lesser
degree. And, depending on the load impedance of the speaker, those flaws
may or may not be audibly significant. I should also add that I have had
a large number of SS amplifiers cross my bench that also exhibit audible
flaws,


--
Trevor Wilson
www.rageaudio.com.au
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