Sunn Model T, Bringing it back to life. I need some help

[spin]

Hey guys, my name is Tommy, I am new here. A buddy from over at the BYOC forum sent me this way to get some help on an old Sunn Model T amp I am trying to overhaul. I'm certainly no expert on tube amps, but I have built a few over the years. So, I'm trying to go back to stock with this old Sunn. It looks to me it has had its fair share of tinkering done over the years. On the positive side, it looks like the caps and screen resistors are already taken care of. I do want to add, it works right now, I'm just not happy with the sound. It just sounds overly middy and dull.

Things I'd like your help on:
First, I'm going to need a schematic, the ultimate goal here is to put it fairly close back to stock
It looks to me his amp has the dreaded 'death cap', even though the 3-prong looks factory. I could use an education as to what's going on there and what I should do
Is there any other servicing I should be looking at with amp?
Where can I read up on Sunn amps in general, I'd like to find out exactly which year this is and general information about it.

Thanks in advance for any help you are able to give me. This will be my first experience with a Sunn amp. I've attached a couple pictures, let me know if I should post anything else.

[Jazz P Bass]

Sunn Model T Schematic

Here is the schematic.

[loudthud]

Your amp is the "Red Knob" or 2nd generation with the Mid Shift switch. That amp does not match the schematic that Jazz posted. That schematic is for the 1st generation. Note that there are some 1st generation amps that have red knobs, it's unknown if they were shipped that way from the factory or if a retailer switched the knobs and logo to match a speaker cabinet. But if it has the Mid Shift switch, it's a second generation.

The 2nd gen is sometimes prefered by bass players because it has more of an Ampeg like tone circuit. The 1st gen is more like the 1959 Fender Bassman or Marshall amps that were patterened after the Bassman. YMMV.

[spin]

Thanks for the links guys! I just got done comparing what I've got the the original schematic. Here is a pic of my findings:

I do have one question though, while I can understand most of the tone shaping mods that were done, I don't get why R28 was cut out. How is the phase inverter even working without it (but then again, maybe it's not and that is why this amp sounds like ass)? It's not jumpered or replaced with a different value, it's just plain gone. I've even convinced myself that I see the legs where it used to be.

[52 Bill]

I do have one question though, while I can understand most of the tone shaping mods that were done, I don't get why R28 was cut out. How is the phase inverter even working without it (but then again, maybe it's not and that is why this amp sounds like ass)? It's not jumpered or replaced with a different value, it's just plain gone. I've even convinced myself that I see the legs where it used to be.

Well the amp would sound like ass without R28!

Easy test would be to read the dc voltage at the plates of the phase inverter tube. If R28 is missing so to will be the dc voltage on pin 1, and therefore half of the signal to the power tubes.

[spin]

Exactly what I was thinking, but I love to second guess myself. I'm glad you confirmed my thoughts here. I'll do a voltage test to verify, then I'll see what it sounds like with the other half turned on. I also want to go ahead and get rid of that death cap tonight while the iron is hot. I'll post back my findings. Thanks again guys!

[loudthud]

It won't work very well without R28. Do you get a crazy low voltage on pin 1 of the 12AX7 like 20V? That would confirm that the resistor isn't hidden somewhere.

[spin]

yup, I was even getting negative voltage on pin 1. I'm going to have to order all the 1/2w resistors for this project. My parts stash only consists of 1/4w for pedal building.

[spin]

Thanks for all the help guys. I just finished up the amp a few days ago and gave it a workout. I fixed the cathode resistor on the PI first and the amp got noticeably louder and somewhat better sounding, but not great. So I went back and rebuilt it to stock. I also did a few mods. First was the removal of the death cap, I also took out that volume loop back wire out. I added a cap to the mid knob that will take the mid boost out of the circuit when the knob is at zero (it used to boost even with the knob at zero). The V2 cathode resistor on the schematic is at 68K, install was an 82K, the reference material said go to 100K. I left it at 82K, switching to 100K is supposed to give you more clean headroom and a crunchier sound. I think it sounds good as is. This amp doesn't sound bad at all now, real punchy bass and some real sparkling highs with the treble and presence cranked. It was just muddy and lifeless before.

Side note, I ordered MLCC's by accident for some of the caps. Other than being small as hell for use in a P2P setup, is there any reason why I should not leave them in there?