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Old 28th October 2002, 02:18 PM   #1
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Default Scott LK-72 kit tube amp

I have a Scott Kit LK72 amp. It is in poor shape with dry solder joints and corroded chassis. The tubes seem to work fine. they are 7591x4-7199x2-12AX7x4.

The pots are noisy and the switches are also noisy. Would it be better to rebuild it as a new power amp( Transformers are OK) or clean up and rebuild the unit with replaced parts.

It sounds OK but not great !
Thanks.
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Old 28th October 2002, 04:42 PM   #2
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Default Re: Scott LK-72 kit tube amp

Quote:
Originally posted by ashok
I have a Scott Kit LK72 amp. It is in poor shape with dry solder joints and corroded chassis. The tubes seem to work fine. they are 7591x4-7199x2-12AX7x4.

The pots are noisy and the switches are also noisy. Would it be better to rebuild it as a new power amp( Transformers are OK) or clean up and rebuild the unit with replaced parts.

It sounds OK but not great !
Thanks.
Depends on what you're trying to acheive and your budget (time and money). I beleive the Scott OPTs are quite respectable, so it would be possible to build a nice PP amp with them, esp one that sounds better than the Williamson topology the Scott uses, and have a better amp than the stock one, for about the same effort as compltely rebuilding it. 7591's are very good trioded, but only put out about 6W class A.

Restoring an old amp, esp if it's in poor condition is a real pain, unless you really love it and your willing to have a labour of love. PSU and signal path caps, many resistors and other passives will need to be replaced, probably more work than a scratch build, and far more limiting in your options.

I have a Fisher receiver that really could use a chassis strip and full rebuild, but it works perfectly, so I'm just using the OPTs in an experimental amp, and will restore the faceplate and have a replica timber case made so you can't see the (mild but ugly) chassis corrosion and semi-restore it as an office unit later.

Cheers
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Old 28th October 2002, 04:59 PM   #3
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Default Re: Re: Scott LK-72 kit tube amp

I bought a Scott LK48 (PP EL84/7199) expressly to rebuild it as a power-amp. Mine came on an aluminum chassis and is point-to-point wired. I am going to use Allen Wright's topology for the front end.

dave
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Old 28th October 2002, 05:17 PM   #4
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Default Re: Re: Re: Scott LK-72 kit tube amp

Quote:
Originally posted by planet10
I bought a Scott LK48 (PP EL84/7199) expressly to rebuild it as a power-amp. Mine came on an aluminum chassis and is point-to-point wired. I am going to use Allen Wright's topology for the front end.
Dave,

Won't that have a tad too much gain? For an EL84 you'll only need about 1/3 the swing for amy output wrt the EL34 o/p stage. Full power sensitivity of the amp would be about 300mV
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Old 28th October 2002, 05:41 PM   #5
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Default Re: Re: Re: Re: Scott LK-72 kit tube amp

Quote:
Originally posted by Brett
Won't that have a tad too much gain? For an EL84 you'll only need about 1/3 the swing for amy output wrt the EL34 o/p stage. Full power sensitivity of the amp would be about 300mV
That was always in the back of my mind, but i also had a conversation with Allen about the gain on the (very similar) phono stage of the RTP, and he indicated that it could be dialed in, so i made the assumption i could do the same on the EL84 amps.

I'm just a rookie in this thou, i could be out in left field. Gotta actually get some more things built and get some experience points.

dave
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Old 28th October 2002, 05:46 PM   #6
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Default This should be under 'Tubes'

I goofed this . It should have come under tubes.
Thanks for your opinions.

Hi Brett, I thought that a power amp without the tone controls would be best. I also have a set of EL34 tubes which have never been used. How does the 7591 compare with the EL34 and which can give more power in PP mode? I am not looking at Class A as my speakers are not very efficient.

Hi Dave, What is Allan Wright's front end topology ?

Have you looked at the differential tube input power amp ? Output and driver stage being transistors. Any comments on that scheme?

Should I repost this thread on the 'tube ' forum ?
Thanks,
Cheers.
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Old 28th October 2002, 06:04 PM   #7
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Default Re: This should be under 'Tubes'

Quote:
Originally posted by ashok
I goofed this . It should have come under tubes.
Thanks for your opinions.

Hi Brett, I thought that a power amp without the tone controls would be best. I also have a set of EL34 tubes which have never been used. How does the 7591 compare with the EL34 and which can give more power in PP mode? I am not looking at Class A as my speakers are not very efficient.
I wouldn't bother reposting it.

I don't have the two sets of datasheets in front of me, but for a similar B+ of around 400V there shouldn't be a lot in it between the power output of two types. The EL34 can take a higher B+ so could possibly put out a bit more power but I don't think it would be significant. I think both types are in the low to mid 30W range based on the published specs for Dyna and Fisher gear. To be honest, with a Willaimson type topology and 20dB+ of feedback the particular sonic characteristics of the tubes are rendered much less. Both are octal so it shouldn't be much effort to swap them and see what you like best.
Quote:
Hi Dave, What is Allan Wright's front end topology ?

Have you looked at the differential tube input power amp ? Output and driver stage being transistors. Any comments on that scheme?
For the first part look at http://www.vacuumstate.com/schematics.htm especially http://www.vacuumstate.com/images/PP-1C_a.gif .

I don't understand what you mean by the second sentence, but I presume you're talking about a hybrid with the output stage being SS. If you're going to put sand in the circuit that's one of the better places.
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Old 28th October 2002, 06:11 PM   #8
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[QUOTE]Originally posted by ashok
What is Allan Wright's front end topology ?

Following schema is from the Vacuum State website:

Click the image to open in full size.

Quote:
Have you looked at the differential tube input power amp ?
Allen's topo is differential.

Quote:
Should I repost this thread on the 'tube ' forum ?
I just moved it there.

dave
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Old 28th October 2002, 06:13 PM   #9
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Default Re: Re: Re: Re: Re: Scott LK-72 kit tube amp

Quote:
Originally posted by planet10


That was always in the back of my mind, but i also had a conversation with Allen about the gain on the (very similar) phono stage of the RTP, and he indicated that it could be dialed in, so i made the assumption i could do the same on the EL84 amps.

I'm just a rookie in this thou, i could be out in left field. Gotta actually get some more things built and get some experience points.

dave
It was mentioned more as an FYI. I have a similar problem with the PP1C as it has way too much gain for my needs, and I want to tone it down a bit. I need to email Allan about something else so I'll ask him. A PP1C style EL84 or 12B4A amp appeals to me too.

Cheers
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Old 29th October 2002, 01:58 AM   #10
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Default 12B4A

Brett,

Given the question I'ld choose the the 12B4A over any 6BQ5 and moreover since you've not built an OTL yet I'd definitely recommend this tube. (mini 6AS7GA)
Earopenener.
Any of you sitting on a stock of ED8000's ?

Cheers,
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