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A DIY Radford STA25 restoration

Yank said:
Can someone explain the diff amp? How do you get a balanced signal when one half is a pentode and the other is a triode? I kept looking to see if it was secretly a floating paraphase and I had somehow missed the feed to the triode grid, but it's grounded (at audio frequency) so it must be a diff amp.
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With the given anode resistors (33k / 39k) and the operating point of the circuit, it just so ends up that the balance ends up fairly good - better than you'd expect looking at the circuit anyway! There's no inherent way the circuit maintains perfect balance (like it having a cathode CCS and matched anode resistors for example), the design just centres things for that to be roughly the case most of the time.
 
Yank said:
Can someone explain the diff amp? How do you get a balanced signal when one half is a pentode and the other is a triode? I kept looking to see if it was secretly a floating paraphase and I had somehow missed the feed to the triode grid, but it's grounded (at audio frequency) so it must be a diff amp.
Click to expand...

It's a cathode coupled amp (still a diff amp). The pentode gives excellent bandwidth, as it does not suffer from Miller Effect of a triode, and it drives the triode via the linked cathodes. The triode acts as a common grid amplifier as it's grid is grounded at audio frequencies, but shares input biasing via the 1M resistor. Now, as it's operating in common grid it too has little Miller Effect and a good bandwidth... plus the Ra of the triode section is no longer extant, leaving both triode and pentode as, in effect, current sources. The anode resistors then derive an output voltage from this current. The unequal anode resistors equalise the difference in transconductance between the triode and pentode sections and also the slight imbalance from the lack of a constant current tail load for the cathodes.

I see Will posted as I was typing also!
 
Since my last post I've fitted the mains transformer and choke. I have also replaced much of the wiring loom, keeping the original colour coding throughout.

I haven't been able to finish the boards because I'm still waiting on the F&T dual section capacitors that I ordered from Modulus.

P1110651 by Michael Pickwell, on Flickr

As has been mentioned a few times upthread most of the wiring around the output impedance selector switches was missing/non-standard when I got the amp. The wiring around the input RCA sockets had also been messed about, at least to some extent. Below you can see the 1/4w metal film resistors (22K) and ferrite beads which are obviously non-standard. Note the 100K resistors are 1/2w on one channel and 1w on the other.

P1110627 by Michael Pickwell, on Flickr

P1110626 by Michael Pickwell, on Flickr

As far as I can see the extra resistors just needed removing and replacing with wire links to put it back to how it should be. However, since this part of the circuit had been messed about with I'd be grateful if someone could cast their eye over it to reassure me I've got it right.

P1110650 by Michael Pickwell, on Flickr
 
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Do you think a previous owner was using those ferrite beads as a (strange) way to insulate bare, exposed leads like the resistor legs?
 
booja30 said:
Do you think a previous owner was using those ferrite beads as a (strange) way to insulate bare, exposed leads like the resistor legs?
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I think they were just a tweaker. The impedance selector switches had been bypassed which smacks of someone trying to make marginal improvements.
 
The ferrite bead thing is some of the most absurd bodge mentalism I've ever seen. I wonder if it was a mistaken attempt to fix a hum or something?

Have you made any sense of the other “mods”?

PS At least they didn’t go drilling holes in the case!
 
Tony L said:
The ferrite bead thing is some of the most absurd bodge mentalism I've ever seen. I wonder if it was a mistaken attempt to fix a hum or something?
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Could be. Bodger may have thought that the ferrite beads conferred some magic screening properties.
 
Mike P said:
I think they were just a tweaker. The impedance selector switches had been bypassed which smacks of someone trying to make marginal improvements.
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Unless they didn’t work/were intermittent. You said they were filthy...
 
Those ferrites can indeed safely go, if you have enough RF at the input to need a chain of ferrites like that you have bigger problems!!
 
The F&T electrolytic caps finally arrived this lunchtime and I wasted no time getting them fitted and then connecting the boards up.

P1110669 by Michael Pickwell, on Flickr

P1110670 by Michael Pickwell, on Flickr

P1110673 by Michael Pickwell, on Flickr

And so it was time to switch it on. I don't have a variac so instead I have knocked up a dim bulb tester with a 100w filament bulb.

Well, I just flicked the switch and the 100w bulb lit up brightly and stayed bright. On the plus side there were no bangs no magic smoke and no nasty noises through the speakers. The internal fuse hasn't blown.

So, I turned it back off again and have come on here to seek advice. :eek::oops::confused::(
 
Personally I would start by taking out all the valves, measuring AC voltages at the power transformer, then move out to the DC voltages on the power rails. Be very careful, but you know that.

Looks fantastic, BTW!
 
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Thanks Leigh.

I'm back on childcare duties now but will report back when I've had a chance to look at it.
 
Mike,
Nice work there. Don't despair yet your bulb is too small...
Figure, heaters take 40+ watts and HT 80+ watts. So 120 - 130 watts draw minimum. Enough to keep the bulb lit. Check the voltage drop across your bulb (or amp) to be sure.
 
snowman_al said:
Mike,
Nice work there. Don't despair yet your bulb is too small...
Figure, heaters take 40+ watts and HT 80+ watts. So 120 - 130 watts draw minimum. Enough to keep the bulb lit. Check the voltage drop across your bulb (or amp) to be sure.
Click to expand...
Thanks Alan.
 
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