Dad's old ST-70

Hey everyone!  new here and just wanted to say hi to everyone and talk about my new dynakit project.  My dad gave me his old ST70 a while back, i decided to resurrect it and try and use it as a guitar amp.  

the amp has problems though.  i run out of physical range on the bias pot on one of the channels to get the voltage right.  still sounds good though at first, but after the amp warms up and you start to really drive it it sounds as if its like phasing in and out or something.  now as you can see in the pics, there was some modification done to this, my dad cant remember what it was for.  the first one i see is the two (actually three) resistors going from the quad cap to the left take off socked on the front.  the other is a secondary set of outputs that are wired from the quad cap (thru a resistor) and the output tubes (thru a cap).  anyone know what these outputs may have been for?

its not in the greatest shape cosmetically either, so ive decided to go ahead and rebuild/restore it.  the transformers all checked out, and i think i can get the chassis back to a respectable shape.  Im not overly concerned about keeping it all original as much as i am about making it the most suited for guitar as possible.  so im not against getting whatever kit/board i need to, new tube sockets, tubes, etc.  not sure what would be the best way to go to use this as a guitar amp so any suggestions would be greatly appreciated.

idealy, what i would like to do is this.  there is an accompanying PAS-2 he gave me as well.  i would love to use that as well and have the whole setup for guitar.  maybe wire the "special" input to accept guitar and then maybe get a reverb tank and patch it in somehow as a kind of effects loop using the tape out and then back into the spare?  again, id love any input on this.

here are some photos to check out of what i have.  

Thanks for reading my post everyone, any input would be greatly appreciated!

-jim

I hate to say this but your best bet would be to sell it and use the money to buy a guitar amp.  Or better still, clean it up and use it for audio.  It seems a shame to use it for guitar.

Having said all that, I have seen an ST70 used for guitar, and it sounded great.  I don't know all of the details, but I do remember that the guy used only one channel, and the most (and maybe only) significant mod was to put a 1 meg pot on the feedback loop.  He would use the pot to go from clean to pretty heavy distortion.

Your conflict is that audio amps work very hard to minimize or eliminate distortion, where guitar amps work to create it in all the right spots.

That setup is worth quite a bit.  I'm sure it'd get you pretty close to the cost of a 5E3 Tweed Deluxe DIY kit.  The 5E3 is probably the sweetest, most versatile guitar amp ever made.  I should disclose that I have built and own an ST 70 and a 5E3.

Keep us posted!

The extra stuff including the extra speaker terminals on the back is to bypass the output transformers and should all be removed. Not sure why anyone would add this. Maybe to use it with transformer coupled speakers in a PA system? All of the electrolytic caps need to be replaced including the can cap and you need to remove as much of the dust off the top as possible. If the caps are not replaced one day you will get a big and expensive surprise. Get some deoxit and spray the tube sockets. It would be cheaper to sell it and buy a new kit. If you buy a kit pull the tubes and keep them for the new unit.
The best thing to do with this amp and the pas 2 preamp is to remove the extra parts in the amp and sell both separately on ebay and buy what you actually want. The tubes and transformers are where the value is in these 2 units so even in this poor condition they should bring a reasonable price. I sold a 70 in similar condition for $235 and kept the power tubes.

wow, thanks for all the quick replies, this is exactly why i joined this forum!  sounds like i would definitely be better off keeping it as a hi-fi set up in my living room.    totally down with that as i am an old school kind of guy anyway.  as far as selling it, that's not gonna happen, wildiowa you are right, there is some family value there.  sailor that is what i thought about those outputs, like some 70V systems where the transformer is attached to the basket of the speaker itself.  very high voltage though, found that out the hard way by reaching back to turn it off without looking!

All that being said, i think im gonna go ahead and restore it as close to original as i can.  my question to anybody is should i use the original driver board and just replace caps or do you think it would be more prudent to just get an upgrade?  also, is it worth it to go to ceramic sockets or just clean and keep the old ones?

Thanks everyone,
-jim

In this particular case, you'd WANT to go with a tube rectifier for the sag, right?

Yes, definitely want the sag!

You can try to use the old board.  You'll need to be careful to not lift the leads off of the board.  It's certainly worth trying.  I would gut it, clean it, keep the chassis, transformers, board and the choke (although it looks like it may have leaked a bit).  New caps and resistors, also put in new RCA's and speaker jacks and you will have an amp to be proud of.  I personally would replace the board with the VTA board that Bob & Roy sell, but I understand the desire to keep it original.

By the way,

Since no one else mentioned it, please remove and replace the cover on your PAS2 so it is behind the faceplate!  It is driving me crazy;  If some nut rings your bell, charges in, then corrects it for you..... it won't be me, I live too far from you, but you've been warned!!!

kaner, thanks for the advice, i think ill do what you suggested, gut it and clean it all up, keep all that stuff you mentioned.  im pretty handy with the iron so i think ill try my hand at keeping the original board.  maybe get a new choke too as you said this one looks like it leaked, they're inexpensive enough.


anbite66, this ones for you

travelnjim,

I know what you mean about family values.  I am eventually getting my dad's console he bought to house a turntable and amp he had from the late 50's.  It is a monster in size:


but I grew up with it, and I feel that it should be saved and enjoyed for future use.  Not discarded and forgotten.

Tony

If you are going to do a quality rebuild  you need to replace everything but the transformers and if you are OK with a painted chassis and no silk screening, you can use the chassis. Everything else including the driver board should be scrapped. If I was doing it I would look and read about your choices in driver boards and once you pick the driver board contact the kit seller for that board and buy a full kit minus the transformers and chassis. It will be a lot easier than buying all of the parts from several sources.