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Q & A / Re: 1970 1200s restoration
« Last post by _peter on April 20, 2017, 09:05:31 am »

you aren't talking about the 600V 20 filter caps, right?

Q & A / Sunn Coliseum Bass Prototype?
« Last post by landonverse on April 20, 2017, 08:56:34 am »
Grabbed this in a trade recently. Was told it was a 60's Coliseum Bass prototype. Serial etched into the back, 0101. Was also told there are only two known to exist, the other being in Ohio. Can any of you experts shed some insight on this amp.
Q & A / Model T problem
« Last post by Isaac on April 20, 2017, 07:40:03 am »
My Model T popped its circuit breaker at rehearsal last night. Reset it and it popped again. So something is wrong.

This amp hasn't had any issues at all for about 40 years. Back around 1975 or 76, the canned multicap was replaced at the Sunn factory. About ten years ago, I did a couple of mods to make it closer to a 1G Model T. Since then, I've used it nearly every week with no problems. So maybe it was due. It did take a tumble off the top of a stack a couple of years ago, but that didn't appear to cause any trouble other than breaking one of the tubes.

The amp hasn't had any caps replaced since the incident in the 70s. Forty-year-old amp, forty-year-old electrolytic caps, that's an obvious place to look. A new set of tubes is never a bad thing to do, whether it solves the problem or not. After the stage dive a couple of years ago, something might have been knocked loose that's only now creating a problem. Guess I'll find out.

When will I get to it? Who knows? I tend to procrastinate pretty badly. I have other, higher priorities this weekend, and once something slips down my list, it can be hard to get it back on top, but we'll see. I'll keep you posted.
Q & A / Re: Unwired Rectifier Tube Socket Stock From Factory??
« Last post by Tdlunsfo on April 20, 2017, 06:28:56 am »
Just so happens I am working on this very same amp. Mine had the diode and spare socket. Diode was also using pin 1..for some reason. My diode was original and tested bad. I wired the other socket (using pin 2) and threw in a couple of 5AR4's. Sounds great but has lots of hum and a weak tremolo. I was questioning if I had the right model trem bug being a -12 model and schematic says 6 volts. Your pics seems to answer that question. Mine has a sticker on the side that states passed inspection December 1970. Curious what date yours is? Pretty cool amps, mine still has the original tung sol 6550's. I am no stranger to working on tube amps but the voltages in this thing are pretty scary, 775 VAC on the secondary! This hum is giving me fits at the moment. Would you believe I bought this thing and an empty bassman cab for $40!
Q & A / Re: 1970 1200s restoration
« Last post by Tdlunsfo on April 20, 2017, 06:20:54 am »
Forgot to mention that the death cap has been removed and the polarity switch bypassed. I did pull some tubes a minute ago and the PI tube had an effect, the others had no affect. PI tube tests good. Also tried grounding rivets here and there with no effect. I did remove and leave out the big 20uf orange Mallory caps. These appeared to be added but I cant verify it. The are not on my version of the schematic...but then again the cap doesn't match the schematic 100% either.
Q & A / 1970 1200s restoration
« Last post by Tdlunsfo on April 20, 2017, 05:24:46 am »
I recently acquired a 1970 1200s for a song and trying to restore it. Here is where I am at currently.

Fuse holder replaced
Power cord replaced and grounded to chassis
Power supply cap can replaced.
50uf caps replaced
1000uf cap replaced.
2w resistors replaced on cap can
1w resistors replaced in bias circuit
Removed diode recitifier and wired up second 5AR4 socket. (This is the model with two sockets but sold with the diode)

Fired up the amp and it sounds nice and loud. The problems start with the hum and a weak tremolo. I checked the riveted grounds as they are known to be loose but they are tight. Will try some solder on them later today. My tremolo has the Sigma Datacell LDR module. I can hear it oscillate but its weak. I noticed mine is the -12 model LDR, but the schematic denotes 6 volts across the light. Should mine be the -6 model that I see being sold as NOS? All the old caps in the trem test ok as well as the transistors.

So I still need to check bias and reverb but wanted to get your opinions on the hum and tremolo first. Also, would like to know which LDR you guys have in your 1200s amps as information on this is limited.

Q & A / Re: Line in to 200S power amp
« Last post by _peter on April 20, 2017, 04:37:45 am »

of course, going to pin 2 of the 6AN8 (i.e. the grid of the triode section) is NOT the way to go.

For one thing, your mixer's output doesn't deliver enough voltage to drive the power amp. The triode phase inverter only has a gain of ~1. So you need another gain stage.

But above all, you will mess with the phase inverter's bias point when connecting something to its direct coupled grid - and possibly even destroy your mixer due to the high DC voltage.

Just go from the input jack to the grid of the 6AN8's pentode section, which is pin 8, if I'm correct. Shielded cable and a grid stopper wouldn't do no harm.


Q & A / Re: Unwired Rectifier Tube Socket Stock From Factory??
« Last post by Derrick on April 19, 2017, 06:00:22 pm »
Uhhh... I just got a notification that this topic was replied to but the replies were in mid November?!?!  OK, strange.  Well anyhow, what was the original wiring configuration... both 5AR4s wired in parallel?  I'd rather use two 5AR4s than a single solid state module.  Maybe I will get a notification to the next reply 6 months from now.
Q & A / Re: Line in to 200S power amp
« Last post by Cartoonist1 on April 19, 2017, 03:48:28 pm »
Thanks Isaac

I'll check this out. 
Q & A / Re: Cone Excursion
« Last post by eddiemac on April 14, 2017, 08:09:12 am »
I don't actually have a D140F anymore.  I didn't make that clear.  I was speaking about back in the day, and I know what you mean about getting "soft."  Mine did that  -  I even sent it back to JBL under warranty.  Naturally, they sent it back unchanged, but I did get a very nice letter asking me what I was talking about.
I appreciate your insights and wealth of experience, Ed.  I play blues in small clubs, quite often without PA support.  Your rig sounds really cool.  I just need one or two setups that can be stand-alone units, like we all used to play through (I'm 66).  The Fearfuls are useful for very large venues and outdoor events, and my Fearful 12/6 with 12 Sub gets rave reviews from audience and other musicians.  I think it sounds pretty darn good.  But the Sorado is apparently not the amp for it.
I have a K140, but it's been reconed.  So far, the Sorado has worked best with an EV 15B in an EV design box, but I plan to experiment with other cabs on the JBL.
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