Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: LooseChange on October 30, 2011, 06:42:23 am
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This amp was built in 1964 in England for American consumption.
I'm going to restore it for my own use.
What I am looking for is a Standee Resistor. Specifically a 100r (5 watt). Look at the picture below. They are the three white things standing up in the middle. On mine it is gone.
Thanks!
Schematic with the part circles attached too.
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Great classic amp, well worth a careful restoration.
The supply R is a problem in this amp. They are rated for 5w and have to dissipate 6w or more. Kaboom. The cathode Rs usually suffer the same fate.
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The supply R is a problem in this amp. They are rated for 5w and have to dissipate 6w or more. Kaboom. The cathode Rs usually suffer the same fate.
Well darn! The power supply Standee is the difficult one to stuff inside. I guess I'll need a 10watt at least.
What do you think about changing that 100r to a choke? The setup is a simple Pi filter anyway. It feeds everything and does not even split the plate and screen supply like we do with Guitar amps.
Thanks!
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The power supply Standee is the difficult one to stuff inside. I guess I'll need a 10watt at least.
You can find some 10w radials. The ones pictured here are 16 x 12 x 36 mm - say roughly .6 x .5 x 1.4" - with a 7.5 mm - .3" - pitch. They could fit with a lil' trimming, or be encased in white-painted aluminum tubes filled with fine sand in order to keep a similar appearance.
What do you think about changing that 100r to a choke? The setup is a simple Pi filter anyway. It feeds everything and does not even split the plate and screen supply like we do with Guitar amps.
I wouldn't. Not for technical reasons - Leak used chokes in a lot of amps, including the ST-60 (an iteration of the ST-50) but for historical accuracy. The ST50 is a classic selling in pristine condition for more than two grands. Modding it wouldn't be a very good idea.
BTW, here's the original schemo in OK condition. You may compare it to the Sams Photofact version.
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What is with the Brits and their one-off parts... My Major has these "door knob" resistors that are long gone too....
Good luck, that is a cool project!
Jim
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Those amps just drip classic, and they are built beautifully, as well. They fetch very high prices from the fetishists. With those, I would urge originality if there is any way. You'll find those resistors if you keep looking. I could see rigging up some more normal 10-watters, either square ceramic axial or the older style wirewounds as a temp measure or if you planned to use it yourself. Whatever you do...no chassis drilling!
That Radio Craftsman I showed some time ago ultimately sold for $685 which completely blew me away, as it was just a single mono amp, with a replacement power transformer whose legs were slightly bent to fit the chassis holes (but it was operationally fine) I didn't do a thing to it electronics wise other than to check bias and check how well the (original) Genalex KT-88's matched, and oh boy, they were amazingly well matched. Those right there might have been a $250 pair of tubes.
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What is with the Brits and their one-off parts... My Major has these "door knob" resistors that are long gone too....
Must be whatever RadioSpares was blowing out at the time.
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Nice. I've got the EL84 version, Stereo 20. Got two - one brown, one later black one that sounds better for some reason. Don't often see the Stereo 50.
I used to have a pair of the EL34 monoblocks - TL/25's. The ultimate though was the TL/50 - KT88's. They go for big money, if you can find them.
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Leak used slightly different components and circuit variations during the ST-20 lifetime. OTs were model 3921 on champagne and gold versions, model 8778 - improved - on later versions.
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I thought the circuit was pretty much the same - possibly a miller effect cap added on the first stage. But I didn't know about the OT change, that's really interesting. I've heard other people saying they think the black ones sound better - first real plausible reason I've heard, thanks!
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The Audiophile guys say I should use Russian PIO caps. In my Guitar tube amps they sound dead. What's your opinion?
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Well, I'm from the double blind school of subjective testing and have never found a iota of difference between properly used broadly similar caps. Russian PIOs are well built, very reliable and cost very little.