SC200+ with smaller PT?

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SC200+ with smaller PT?

Postby Harvey58 on Fri May 27, 2016 4:48 pm

Hi guys. Question for all techs.
Could be the SC200 output transformer used with hiwatt DR103 clone PT and with 4xEL34s (with the power amp and bias circuit improvement, of course)?
Is there any reason why will it not work (less wattage, but thats not what bother me)?

More and more im in love with EL34s for a bass amp, much more than the KT88. They are more juicy. Im using later Marshall super bass now with el34 and i like it, i play friends Hiwatt DR103 and i love it. :excited:
In the past i tried using the EL34 in my SC200+ (with improved BIAS) > it works, but the anode Voltage above 760V and the screens about 420V today valves cant handle so long > one of JJ EL34L fails after few days. But the sound from the amp was realy cool, so im still thinking about this conversion. I dont want to sell this one and try another.
My SC200 was rebuilded to the HIWATT preamp specs with Mustard caps, its very clear wired and so i dont want to let it go. Its not my main amp now, so im thinking about the moding.
I got this amp hugely moded, so other mods dont bother me, it was never in original specs.

The other issue of the PT is that it have less heater voltage, even with hi anode voltage.
Here we have about 230V in the wall, the amp is set to 225V, but the heaters are about 5.8V (with only 4 preamp tubes and 4 x Kt88EH). If i change the settings to 245V the anode voltage is less, but the heater is also less (5.6V). So im thinking about InMadOut DR103 PT clone, which is wired to the modern wall voltage spec (primary - 220, 230, 240V)

PROS and CONS?
Thanks for opinion.
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Re: SC200+ with smaller PT?

Postby gldtp99 on Fri May 27, 2016 5:55 pm

I think it would work well------ but it would require drastic changes to the 200+ power supply since the 200+ uses different B+ windings for the Plates vs Screens voltage supply and the DR103 PT runs all B+ from a single winding with the Plate and Screen supply being separated by a Power Resistor---- It might make more sense to build a new amp with the DR103 PT and a DR103 type OT or another oversized OT (rather than pull the one from the 200+).
The other night I tried out some tube heads for the bass rig I'm putting together for a future band project ------ I first ran my stock, black Sound City L100 Mk3 (works well, I liked it)----- then I tried my 1972 Hiwatt DR103 (also sounded good)----- one of my BF Fender Showman heads (liked better than the British heads)----- and one of my '70's Ampeg V4B heads (The Winner !!)------- running thru an Ampeg ported 4x10 cab.
Actually all of the heads sounded very good and I'd be happy using any of them----- I was playing an old Yamaha mid level P-type 4 string which I plan to use as a back up------ My choice of amp heads may change once I start using my '70's Fender Jazz Bass that I pulled out of storage and still haven't done the minor TLC it needs to be back in action.
A friend of mine uses the last SC 120 I converted to Hiwatt preamp for a bass head----- He says it is the best Bass amp on the planet, so the EL34's are working for him....................... gldtp99
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Re: SC200+ with smaller PT?

Postby gldtp99 on Fri May 27, 2016 6:15 pm

Also---- I've got an inop SC 120R in storage that I could rebuild with a Hiwatt preamp to use as a head for my bass rig----- every time I do one of these SC120 to Hiwatt rebuilds I swear that I'll never do another because it is a lot of work, much more than building a similar amp from scratch---- but this one would be for me and I swear it will be the last one !!!! :lol: :lol: :roll: :roll:
I remember A/B'ing the last SC 120/Hiwatt rebuild against my DR103 and SC L100 Mk3 with my friend playing bass------ the six EL34's and larger SC 120 iron did make a noticeable difference vs the 100 waters......................... gldtp99
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Re: SC200+ with smaller PT?

Postby Harvey58 on Fri May 27, 2016 7:28 pm

Nice to hear that its possible, thanks man, that helps me and encouraged me.
I know what you talking about, its always too much effort, but its always "the last one i will do" :fingersx:

Im glad, that someone else has similar experience with EL34 for bass. My Masrahll Super Bass is realy cool.
But the Hiwatt preamp plays differently and i also like this kind of sound and its good to have backup or other amp to choose.

After few years with the KT88 in the SC i feel that this kind of sound calls for "bigger stages" (which i have not) and also that this sound is too linear and (i feel that) hard edged and little cold.
Too much clear headroom. Maybe its good for some, but im looking for a little more saturation (juice) and softer tuch of the EL34s.

The Ampeg may have sound in between both worlds (KT88 vs EL34) isnt it true?

BTW im playing thru ampeg SVT412HE box. It has Eminences in it (something like Alfa or Beta12). I give a chance to guitar Celestions G12K100, because of using vintage amp and not todays SS subbass amp. Great change IMO, i can recommend this for any vintage tube amp using with a bass guitar. They have only one minus, that they have resonant frequency about 85Hz. So the E in octave is little boosted, but who cares. I prefer the 12" against 10". But i must say that ampeg BSE410HLF was one of the nice sounded and lively box (unfortunately he had to go, before years :dumass: )

Good luck with the SC120. :wink:
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Re: SC200+ with smaller PT?

Postby Papa Dog on Sat May 28, 2016 11:20 pm

here I am debating on if I want to join this discussion at all.
Harvey58 wrote:Hi guys. Question for all techs.
Could be the SC200 output transformer used with hiwatt DR103 clone PT and with 4xEL34s (with the power amp and bias circuit improvement, of course)?
Is there any reason why will it not work (less wattage, but thats not what bother me)?

what "bias circuit improvement?" I understand changing the one up in the 120+ but what needs to be done to the 200+?

In the past i tried using the EL34 in my SC200+ (with improved BIAS) > it works, but the anode Voltage above 760V and the screens about 420V today valves cant handle so long > one of JJ EL34L fails after few days. But the sound from the amp was realy cool, so im still thinking about this conversion. I dont want to sell this one and try another.
My SC200 was rebuilded to the HIWATT preamp specs with Mustard caps, its very clear wired and so i dont want to let it go. Its not my main amp now, so im thinking about the moding.
The other issue of the PT is that it have less heater voltage, even with hi anode voltage.
Here we have about 230V in the wall, the amp is set to 225V, but the heaters are about 5.8V (with only 4 preamp tubes and 4 x Kt88EH). If i change the settings to 245V the anode voltage is less, but the heater is also less (5.6V). So im thinking about InMadOut DR103 PT clone, which is wired to the modern wall voltage spec (primary - 220, 230, 240V)

PROS and CONS?
Thanks for opinion.


I would recommend a "bucking transformer" or variac to get the voltages right on the input, but you seem to have some other issues. The heater voltages are too low for good tube life.
that you are getting 760v to the plates in an issue. voltage at the standby switch to the plates should be ca 620v.

gldtp99 wrote:I think it would work well------ but it would require drastic changes to the 200+ power supply since the 200+ uses different B+ windings for the Plates vs Screens voltage supply and the DR103 PT runs all B+ from a single winding with the Plate and Screen supply being separated by a Power Resistor---- It might make more sense to build a new amp with the DR103 PT and a DR103 type OT or another oversized OT (rather than pull the one from the 200+).
....................... gldtp99


not exactly...the pate winding output is actually "stacked" on the normal B+ line. ca 345v to the screen droppers. The output of the "plate winding" should be ca. 280v at the "hot side" of the rectifier(s).


Member Racing has described rebuilding SC 200s and some mods he did to make the power supply less "stiff" and has also described using zener diodes to drop voltages. I know little of how that might be done, as the process has never been of interest or need for me.
I DO know that you can tame those voltages with "dropper" resistors, although I do not have the values in front of me or the links to the calculators in my favs on this machine.

Were it my amp and I got it already modded, I would find a way (resistors or zeners) to "tame the voltages" so that I could run whatever tubes I wanted. But first you need to find out why your heater voltages are so low.
Last edited by Papa Dog on Fri Dec 23, 2016 1:20 am, edited 1 time in total.
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Re: SC200+ with smaller PT?

Postby Harvey58 on Sun May 29, 2016 11:38 am

"bias circuit improvement?" I understand changing the one up in the 120+ but what needs to be done to the 200+?

I know, I say improvement, but i mean rebuilding bias circuit and power amp to the DR103 PT specs, for using it with EL34.
As i say, i have KT88 (for over 10 years) in it, so i know how they play and i decide to change it to EL34, because they plays better (for my needs).

So i dont need to swap different valves, i will use it only with EL34s.
For that reason, the changing the bigger PT with a smaller DR103 PT will also improve the overall weight of amp, which is good :D

Im only asking, if the bigger SC 200 OT iron will work with EL34s with a less voltages from DR103 PT.
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Re: SC200+ with smaller PT?

Postby Papa Dog on Sun May 29, 2016 1:47 pm

Harvey58 wrote:
Im only asking, if the bigger SC 200 OT iron will work with EL34s with a less voltages from DR103 PT.


yes, yes it can be made to. The issue will be dropping the voltages from the plate (anode) supply and the screen supply.
As I mentioned, Racing made some posts on this section about modding a 200+ and "softening" the power supply. He also has several posts detailing how he lowers "excessive" voltages. He uses Zener diodes to do so. I would be more inclined to use a wirewound dropping resistor.
You would neede to find a chart or calculator or else know the formulas to calculate how bid it needs to be, but you could drop the voltages sufficient to allow the running of el34s, or 6v6s even.
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Re: SC200+ with smaller PT?

Postby Harvey58 on Sun Dec 18, 2016 5:54 pm

So, after a while, the work is done. I changed the power transformer and power output of the SC200+ for a HIWATT DR103 like and it works.
And also plays as im expected, so im satisfied (the preamp was converted to Hiwatt earlier).

Im using the InMadOut DR103 power transformer http://www.inmadout.com/T-power.asp.
Its made in Italy, Hiwatt specs. Its robust, looks kinda hiwattish and have right votages for EL34.
Interesting about this transformer (the one i got) - it has HT winding with CT (can be used with lower voltage filter caps, said the builder).
But i dont use the CT, so i leave the CT unconnected.

Second thing about this transformer, it has the wires only on one side. I put the secondary to the hole of the unmounted CHOKE, so i dont care.
In the 230V condition (with old TESLA EL34 @ 38mA) gaves 452V on anode, 440V on screen, ca 48V BIAS taps, 6.3V Heaters.

Last thing with the tranny, its smaller than the SC Output tranny and smaller also than the hum sheet, so i put sume nuts on the upper side, to make it higher, for the mounting purposes. Quick and easy :)

I can recommend this tranny, i pay 130 EUR (with transport) for it, looks robust and stable.

I want to do as minor changes to the power amp as possible, so some time eats the thinking of wiring :nutjob:
Here is the original and the new wiring (only for the important things) and some shots after the mod.

http://drive.google.com/open?id=0B-tbxu ... 25xbm40aG8
http://drive.google.com/open?id=0B-tbxu ... kVKMFNpWWc

http://drive.google.com/open?id=0B-tbxu ... 2tNb21IQUE
http://drive.google.com/open?id=0B-tbxu ... EozVTBweUk
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