SC 120/ Hiwatt Conversion Pics

[gldtp99]

My latest Sound City 120-to-Hiwatt conversion (the 4th, and last, one for me).
Amp is up and running-- passed initial function tests--- i played it for about an hour with different pedals--- pulled the set of cheapo "assembly" preamp tubes and slapped in a Mullard I63 in V1, GE 12ax7 in V2, Tungsram 12ax7 in V3, and a Brimar ecc81 in V4 (PI)--- quick gut pics:
[IMG:800:600]http://i46.photobucket.com/albums/f112/gldtp99/001.jpg[/img]
[IMG:800:600]http://i46.photobucket.com/albums/f112/gldtp99/002.jpg[/img]
[IMG:800:600]http://i46.photobucket.com/albums/f112/gldtp99/003.jpg[/img]

Amp sounds as expected, freaking HUGE !!!!----- also very quiet until guitar strings are operated----- then a massive blast of sound is possible if the amp is turned up very much at all---takes pedals as well or better than any amp i've ever played.
I know the build quality isn't up to Harry Joyce standards---but this is a conversion meant for sale locally at a reasonable price--- the Xicon carbon film resistors, Alpha pots, and Mallory 150 caps make the amp sound terrific but aren't cork sniffer approved ---several people/bands are interested in it now so this isn't a "for sale" post.
I wasn't planing to do another one of these SC 120/Hiwatt conversions but this amp came to me in a trade----- it had been butchered with a Stancor 100 watt OT running EL34x4-----the real butchery was in how somebody tried to build a Marshall-type preamp into it----one of the biggest messes i've ever seen---the amp was inop.
I used a spare SC 120 Partridge OT (to run the spec EL34 x 6), i pulled the coveted 3-prong Voltage and Speaker impedance switches from a parts SC 120R---the heat shield from the same parts amp.
I used new turret boards, all new tube sockets (NOS orig-type output sockets), Teflon Mil-spec wire, etc, etc.
The parts re-used from the orig amp are: Chassis, PT, orig Cliff speaker output jacks (cleaned up)---that's all.
With the local interest in this re-build so far--- i might end up having to build another Hiwatt DR103 clone or two for some of them---but it will only run EL34 x 4 for only 100 watts-- ----- these SC 120's, i feel, have always been under rated in terms of power output-----i have a '72 DR103 and have built DR103 clones---and these SC120/Hiwatt conversions are quite a bit more powerful than 100 watt DR103's.
Not that all that extra power is a needed thing in today's world---but it's there- .............................gldtp99

[cbass]

Nice Job.I've always wanted a Hiwatt Like I need an amp that loud

[xtian]

Awesome! I love Hiwatt layouts.

[Colossal]

Looks great!

[jelle]

Awesome!

[Structo]

Cool!

I was always amazed at what Pete Townshend could get out of his P90 Gibson SG and the Hiwatts.
The P90 guitars I have played sounded nothing like that.

That 1970 Isle of Wight concert is a good example.

Not sure, did he plug straight in or use a pedal?

[Reeltarded]

Rockin'!

[eddie25]

What's the screen voltage on this thing?

I know no one will probably crank this one you have here, but I tried a 50 watt hiwatt circuit build (with what I gathered to be correct voltages) and it eats any tubes I have tried for lunch, except JJ 6CA7's... Screens are in the 500 volt range! Am I the only one who can't get away with this?

[bb5000]

Nice build !

[bb5000]

What's the screen voltage on this thing?

I know no one will probably crank this one you have here, but I tried a 50 watt hiwatt circuit build (with what I gathered to be correct voltages) and it eats any tubes I have tried for lunch, except JJ 6CA7's... Screens are in the 500 volt range! Am I the only one who can't get away with this?

http://www.hiwatt.org/Schematics/DR_PS0506.gif

The screens should probably be around 410 V or so...I built a DR201 clone and have 400-410V screen voltage, 670-680V at the plates.. (6xEL34)

[Gibsonman63]

You guys go through lots of butter! Paula Dean school of cooking?

Great looking work. I have repaired a couple of vintage amps built on tag strips. I would think it is easier to build with those than to modify or repair.

[gldtp99]

What's the screen voltage on this thing?

I know no one will probably crank this one you have here, but I tried a 50 watt hiwatt circuit build (with what I gathered to be correct voltages) and it eats any tubes I have tried for lunch, except JJ 6CA7's... Screens are in the 500 volt range! Am I the only one who can't get away with this?

Screen voltage is 468V--- plates are 480V------ this is a Sound City 120-to-Hiwatt conversion so I had to go with the SC 120 PT and live with what it gives me---the voltage levels are similar to the other three SC120/Hiwatt conversions I've done. This amp isn't going to give the 200 watts that a Hiwatt DR 201 will give----- I do have an inop Sound City 200 Plus that i might convert to a 200 watt Hiwatt circuit one day-----this would be the KT88 x 4 version instead of the EL34 x 6 Hiwatt. But this SC 120 based amp does provide more than enough power than most people will ever need.
The 500V screen voltage sounds a little high on that 50 watter----- I've run into similar problems with vintage 70's Orange amps---one way to solve the problem is to use a Variac to drop the input voltage to what the amp was designed for---but i modded the screen supply circuit to drop voltage on my '74 OR80 and also on another that somebody brought me.
The OR80 that the guy brought me would ruin JJ E34L's in a few minutes when he first brought it in------i watched a new pair toast before my eyes even with a low, cold bias setting.
I don't know what effects P. Townsend used at shows--- but i know that his stage Hiwatts were modded versions that are quite different than the Mid 70's 4-Input circuit that i like to build.
Different versions of Hiwatt circuits here: http://hiwatt.org/tech2.html

[eddie25]

What did you do to the Orange?

http://www.hiwatt.org/Schematics/DR_PS0506.gif

The screens should probably be around 410 V or so...I built a DR201 clone and have 400-410V screen voltage, 670-680V at the plates.. (6xEL34)

If I use a 350V transformer like in that schematic you posted, I get a little over 500V b+, and it doesn't drop all that much for the screens. I used a different power supply than that one though (http://www.hiwatt.org/Schematics/DR504.gif). My voltages pretty much mirror the ones in that schematic.

Anyways, sorry to hijack, just wondering if I was the only one who can't seem to push the limits on screen voltage. Gldtp seems to have a lot of experience with Hiwatts and Orange so I thought he would have some good input. Thanks!

[gldtp99]

I added resistance to the screen grid supply line in the Oranges---- i did it differently in the two amps but it worked out pretty much the same.
I looked at the DR504 schematic you linked to--- change the screen grid resistors to 1k/5W instead of the 100 ohm listed---you can also try upping the value of the 1k/10 watt resistor feeding the "B" screen grid supply line---try a 2k or a 3k.
You should always use 1k screen grid resistors with modern EL34's--- they just aren't as robust as many old production EL34's-- orig Mullards, etc---- i ended up using 2k/5W screen grid resistors in my OR80 and installing a 25W chassis mount power resistor (don't remember exact resistance value) in the screen grid supply.
Dropping screen voltage will effect the power level and the tone of the amp somewhat--- but when the guy heard my OR80 when he brought his to me--- he said, "Make mine sound like yours !!!"
I put Shuguang EL34B's in both of these amps and mine is still running fine--- i haven't seen the other amp since he picked it up so I can't be sure about that one but i told him to bring it back if there were any further problems.................................gldtp99

PS--- typical USA wall supply voltages are higher than the 100V-117V that some vintage amps are designed for----- if they are designed to run high plate/screen voltages (like vintage Oranges, etc) then you can run into output tube problems---that's why some people use a Variac to drop the supply voltage to what the amp is designed for

[cyclecamper]

I picked up a sound city 120 chassis for the transformers. Its pretty rough but might be salvageable as a restored 120; I haven't decided whether to use a Hiwatt or Sound City type preamp circuit yet. I'm likely to put it away for a future project. What I really want is a big-sounding amp with adequate gain that's SILENT without signal; no hum from the power amp and no hiss from the preamp. And usable as a bass guitar amp too. I'd really like a longer chassis so that I could fit a huge heavy main choke and larger filter caps for bass.

I've been toying with the idea of trying grid chokes and maybe even plate chokes on a guitar amp. On hi-fi amps a grid choke gives a nice stable bias, and a plate choke a nice stable B+, yet they are easy to drive with the AC signal. Done right (correct inductances) it gives really nice treble and really powerful bass, better efficiency and a bit more output, easier on the tubes. But with a huge main choke for the main B+, another good-sized choke on the filter ladder to the preamp B++, DC heaters on the preamp tubes (another choke), grid chokes for bias, and plate chokes, it gets to be a really large and heavy amp with that much iron and those big Partidge transformers too. I'm the only guy crazy enough to use a filter choke on the main B+ for such a big amp, but I've got the choke to do the job without much drop in voltage. It weighs more than the power transformer. It is cool though to have a big loud tube amp with absolutely no audible hum at all in a silent room. I'll have to beef up the main rectifier bridge, and add a thermistor to give a soft-start with a big B+ filter. I don't realy know when I'll get around to it anyway, I can't even afford 6 decent EL34's at the moment. The truth is that with grid chokes I probalby don't need the huge B+ choke. I obviously don't have the circuit thougth out, more like just thinking about the parts I have. That doesn't mean I have to use them all in one amp LOL.