Retubing a Vintage 1969" SUNN 1200S - need info on tone and preamp section!!

[knw]

I got one of these beasts besides few other SUNN heads.

Now I got this head over a year ago from a older fellow who had it for about 4 years or so (before me) and in that time he had it, he had it check out by a tech and replaced 1 of the preamp tubes and 1 power tube and also change the power cord to 3 prong.

Now I'v done 3 tours with this head and it finally gave out on me 

I mean it would turn on and everything but once I fliped the stand by switch for about 30 sec it would start to produce one continuous SQUEEEEEL like a pig almost.

I took the amp to my tech and he just called me back saying that nothing is wrong with it all he did was clean it up and check the power section and everything was working fine he told me he was trying to find something wrong with the beast but there was nothing wrong.

But we started talking more about the amp and he said it might be an issue with the preamp tubes.

Anyways I'm starting this tread because I wanted to FULLY RETUBE the 1200S (power/preamp section).

Now I've been using this amp as a bass amp with a Rick I also play guitar trough it but I manly want it to sound good for bass.

The version I have has the Tremolo, Reverb  and Mid boost Switch.

It has 5 tubes in the back 1 doesn't light up that's because the rectifier was modded to a solid state one so it no longer needs the tube.

The power section consists of 4 mismatched 6550 (some are labeled RCA)

The preamp section is:
V1 = 12AX7
V2 = 12AU7
V3 = 6AN8A

My tech told me that I don't need to change the tubes cause the amp is loud as hell.

But I think the amp could sound louder my 60watt SUNN Sonaro is pretty loud and 1200S is louder but not by much.

Anyways I wanted to change the tubes for quite some time now and I'm mainly concerned about TONE.

I like BIG ORGANIC sounding LOW END and nice mids, nice BOOMY punch.

I've done lots of reading but I couldn't find any info on what preamp tubes to go for and by my understanding the preamp tubes color the tone a lot more then the power section.

Right now I'm looking to go with reissue Tung Sol 6550's matched QUADS for the power section.

V1 = 12AX7
V2 = 12AU7
V3 = 6AN8A

Also 12ax7 tung sol for the preamp also my tech said I should replace the 12au7 with a 12ax7.

Now I can't find anything for 6AN8A tube I don't know what replacement to get or could I found a no name one at the tubestore.com but I don't know how would this preamp tube color the tone or could I just go with 3x12ax7's.

I know the 12ax7 are high gain were the 12au7 are not.

Right now my amp doesn't even break up a 10!  I kind of miss that awesome breakup that sunn heads get when you run them up pass 8.

For the record I don't go past 7 on the volume knob on this head.

I'm gonna be getting all the tubes from www.tubestore.com since it's local to me I can go in there and pick them up no shipping or damage in the mail nightmares.

I'm open to other option and brand names of tube that are availabe at the tubestore.

So please share your experiences.

I'm currently running a BI amp setup with s Sunn Concert Bass into 2x15b SUNN cab and 1200S into a 118M SUNN cab or a 4x12 Acoustic cab.

I play bass against 2 guitar full stacks in a REALLY LOUD HEAVY down tuned Metal/Doom/Down Tempo band.  Very saturated organic warmth driven by the SUNN low end section 

Thanks in advance!!

LONG LIVE SUNN O)))

[loudthud]

The 12AU7 is in the reverb circuit and doesn't change the dry signal. The 6AN8 is not campatable with a 12AX7 so the amp won't work with a 12AX7 in that socket. It would take a substantial modification to change the amp to accept a 12AX7 as the phase inverter.

[xsolarusx]

The Tung Sol 6550's are a winning choice.  I was using a pair in my Solarus, and they kill it. My T had a matched quad of EH 6550's which were really nice.

As far as loudness goes, my Solarus is a 60w model, and compared to my 1st Gen Model T, it definitely holds it's own.  I think after a certain point, the levels stop registering to the ear. I don't quite know the science behind it, but I'm sure someone can lay it all out.

[Isaac]

A comment regarding loudness.

knw, it's true that the 1200S has twice the power of the Sonaro, but that's only a 3dB difference. So you'd expect the 1200S to be only slightly louder. That's the way these things work. If you really want to increase loudness, increase the number of speakers.

[Happy Face]

Would KT88s add a bit more headroom?

[knw]

The 12AU7 is in the reverb circuit and doesn't change the dry signal. The 6AN8 is not campatable with a 12AX7 so the amp won't work with a 12AX7 in that socket. It would take a substantial modification to change the amp to accept a 12AX7 as the phase inverter.

What's the purpose of the 6AN8 tube and how does it color the tone if it does?

Should I stick with the 12AU7 instead of 12AX7 for the reverb circuit?  Or does it matter?

Thanks,

[Isaac]

The 6AN8 is the phase inverter.

I'd stay with the 12AU7 in the reverb circuit. I can't imagine any benefit from going to a higher gain tube for that.

[knw]

Ya I just got all the tubes in the mail.

I went with:

4 x 6550 Tung Sol
1 x 12ax7 Tung Sol
1 x 12au7 JJ
1 x 6NA8 NOS random tube they had

They are all in the package right now.

I will swap out the old tubes on the 1200S on SUNDAY.

I'll let you know how it sounds to me ears.

Thanks for all the help.

Long live SUNN O)))

[knw]

Finally got this beast re-tubed with my tech he biased it in front of me and all.

Now I've been playing it and it sound much cleaner then before and it's super clean at 10 volume no break up what so ever.

I couldn't believe the difference.

I'm not sure if I got extra volume with the tube change maybe a little bit but the amp sounds much cleaner.

I love it I'm running it trough a 2 x SUNN 118M stack.

The old magma speakers just got re-coned and are super clean but they still need of break-in time.  I've only played them once.

I love this fucking amp.

[Mike B.]

You know, I'll bet some vintage Mullards would really sound nice in the pre-amp section....

Mike