SC120 transformers

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SC120 transformers

Postby burnout on Thu Jul 26, 2012 6:33 pm

Can the SC120 partridge transformers (PT and OT) be run at 100w? I'm rebuilding my problematic and previous-owner-butchered 120 into a Hiwatt DR103 clone per the Mark Huss design. I've seen goldtop built one with the SC power section but I feel more comfortable just going by the MHuss diagrams.

My question is can the 120 transformers be safely run with 4 EL34s at 100w or am I going to add stress on them?
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Re: SC120 transformers

Postby gldtp99 on Fri Jul 27, 2012 12:13 pm

The transformers will be fine--- no extra stress on them--- but you'll have problems with the EL34 (or any other) output tubes because they will be running into a much lower impedance (primary impedance of the OT) than the spec---- i (re)built one SC 120 into a Soldano SLO lead channel preamp and i used only four output tubes, EL34's----- I had problems with EL34's failing until I realized what was going on. It never really sounded like I thought it should, either.
I've compensated by running the amp set at 8 ohms into a 16 ohm cab and the output tubes are more stable--- this is a temporary fix until i can get back to the amp and configure it to six EL34's----- that's the way the output circuit is designed and the way it works best.
With four EL34's the amp still was massively powerful so i wasn't gaining anything by using four EL34's except for the lower cost of the tube set. And by ruining some EL34's by running it this way i didn't save any money at all--- it was more expensive in the long run.
i'd suggest just setting it up with six EL34's and going for it--- that's the way these amps sound the best------ an SC 120/Hiwatt conversion with EL34 x 6 is an impressive sounding beast--- it has more depth of tone than a DR103. As far as overall volume goes--- who runs a DR103 fully cranked anyway ? (Deaf or soon to be deaf people)-- so the extra power provides more detail and extra headroom, in real life use.
M. Huss has written about trying an SC 120 OT in a DR103 clone (EL34 x 4)--- he said it didn't sound as good as the Heyboer Partridge clone DR103-type OT---- I'd have to agree with this from my own experience-------- but put the SC 120 PT and OT together pushing EL34 x 6 and you get a glorious sounding amp with the Hiwatt preamp--- it seems to fit between a DR103 and a Hiwatt 200 watter.
If you really want to run EL34 x 4 then you should get a Heyboer Partridge clone DR103-type OT---- then the combination will be matched and give great sound and reliability: http://www.mojotone.com/amp-parts/transformers-hiwatt/Hiwatt-100-Watt-Output-Transformer
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Re: SC120 transformers

Postby burnout on Fri Jul 27, 2012 2:01 pm

So basically the tubes would be wired like this (adapted from the M. Huss diagram) and the output section wired accordingly?

I'm going to keep going with this diagram (full output section) for my own reference but it seems like it should be this way?

Image
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Re: SC120 transformers

Postby Mr. Foxen on Fri Jul 27, 2012 6:47 pm

There was a guy claiming that running less valves didn't require any impedance changing/cause negative effects with an SC120 transformer, it was on an ebay auction, I called him out on it and he got a bit arsey with a loads of made up technical info on why it was fine, which mostly showed total failure to understand how they work. Anyway, he seemed the sort to spout off similarly on forums and such so you might have read it elsewhere that its ok. But basically, don't do it.
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Re: SC120 transformers

Postby gldtp99 on Fri Jul 27, 2012 7:53 pm

burnout wrote:So basically the tubes would be wired like this (adapted from the M. Huss diagram) and the output section wired accordingly?

I'm going to keep going with this diagram (full output section) for my own reference but it seems like it should be this way?

Image


Looks like you have the right idea but i'd go with 1k/5W Screen Grid resistors instead of the 100R's----- today's EL34's are generally less robust than the EL34's of yesterday so limiting the screens isn't a bad idea. The orig big green 1k Screen Grid resistors (7w?) from the 120 might still be usable------- you can mount them as the 120 does, one lead on tube socket and one lead on turret strip------ or get shorter 1k/5W resistors and mount them on the tube sockets as Hiwatt did--- both ways work.
SC 120's use 10k control grid resistors and DR103's use 22k's ------ i've built 120/Hiwatt conversions both ways and i can't say that it makes a difference ..............................gldtp99
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Re: SC120 transformers

Postby burnout on Thu Aug 02, 2012 5:56 pm

Thanks for the advice! I'm a fan of robustness and actually as I was drawing up that diagram I sort of wondered to myself why the SC needed 5W resistors and the hiwatt could get by with seemingly lesser ones. I'm going to finish up my layout sketch first, study it (and probably post here for comments), and then try to wire it up. I'm trying to do lead dress as best I can but it's nowhere near hiwatt specs. I just want a strong amp with modern components (save for a few mustard caps in the tone stack that I salvaged) that will deliver consistently and everything I've read says that this is the amp. I'm getting pretty psyched on it.

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Re: SC120 transformers

Postby gldtp99 on Thu Aug 02, 2012 7:56 pm

Sounds like you're on the right path---- just do the best wiring job you can and don't worry about if it looks as clean and orderly as a Harry Joyce wired classic Hiwatt------ making sure the amp operates properly is the most important consideration--- and your wiring skills will improve as you do more actual wiring work.
None of the amps I've built, so far, are wired as clean as a H. Joyce/Hiwatt---- but that's not exactly what i'm striving for-------proper operation is the 1st concern--- and my wiring skills are improving over time, too.
The amp build i'm finishing up now isn't super orderly inside------ it's a true point-to-point build and these almost never look very neat---- did initial power-up last nite and some minor tweaking is needed but everything operates properly---- it's an old 1940's Knight PA head gutted and rebuilt as a 30-35 watt 6L6GC x 2 Tweed Fender-based circuit--- Vol and Tone pots only---i did a burn-in test with a Strat for @1 hr (swapping different preamp tubes)---- the guy is going to be happy with this one--- :thumbsup: ...........................gldtp99
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Re: SC120 transformers

Postby burnout on Fri Aug 03, 2012 1:54 pm

Thanks for the excellent advice. This has been a fun project, especially since there's not really any time stresses. I have a good sounding working amp right now.

I finished up the sketch of the DR103 power section with 6EL34s. I'd appreciate comments or overview. Looks like I just had to expand the preamp input links to the 22k resistors. Is it OK the way I've wired it in the diagram? Intuitively it seemed to be right when I compared it to the MHuss diagram. I haven't soldered a lick of this yet, I'm just sketching it out first to make sure I have a guide and don't forget anything ;)

(you'll have to click the link to expand it - it didn't quite fit fullsize on this post)

Image
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Re: SC120 transformers

Postby burnout on Fri Aug 03, 2012 6:03 pm

OK this is a better image of my layout sketch for converting the DR103 power section to use 6 EL34s. Let me know what you guys think. This seems right?

larger image: http://farm8.staticflickr.com/7113/7705583972_ba4e8f242e_h.jpg

Image
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Re: SC120 transformers

Postby Papa Dog on Fri Aug 03, 2012 6:46 pm

If you can source the cap values which you are showing for the power supply, please do let us know.
I "settled for 500v 220mfd cap cans when I re-did my amp, and am quiet the happy camper with the results.
I wanted to be closer to "stock" but try as might I could not find the "correct" values and went bigger instead of smaller.
Some folks say that it is better to go one way or the other, but YMMV.
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Re: SC120 transformers

Postby burnout on Mon Aug 06, 2012 12:52 pm

Vintage Hiwatt Restorations has them:

http://shop.hi-tone-amps.com/VHR-1632-Re-Cap-Kit-1632-recap-kit.htm

I haven't placed my order yet but this is probably where I'll get 'em from.

~burnout
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Re: SC120 transformers

Postby Papa Dog on Mon Aug 06, 2012 5:14 pm

burnout wrote:Vintage Hiwatt Restorations has them:

http://shop.hi-tone-amps.com/VHR-1632-Re-Cap-Kit-1632-recap-kit.htm

I haven't placed my order yet but this is probably where I'll get 'em from.

~burnout


Thanks for the information.
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Of shoes--and ships--and sealing-wax--
Of cabbages--and kings--"
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Re: SC120 transformers

Postby pault on Sun Mar 30, 2014 8:04 pm

The sound city 120s were designed for a clean output stage using 6 EL34s. To try and use 4 EL34s is really asking for unnecessary trouble. Are EL34s that expensive?
I started a discussion on Linkedin on this very topic, and the best replies were "not to turn its output up to 12" (one more etc). The output transformer impedance matches correctly with 6 EL34s, you can calculate impedance matching additional loads if you want to add additional loads which may in fact make the transformer hotter by unbalancing the winding loading. One thing that most people do not realize is that removing one pair reduces the loading on the HT/B+ which may have adverse effects on reliability as it may rise. I would suggest, if possible to use all 6 and think about bias matching or adjustment for each so that all 6 are happy.
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