St-70 upgrades

FYI- Next item up for restore

I bought an fm-3 today to have a matching tuner from BC, been told the tuning eye even works.

Kinda assumed the extra rca's were inputs. All excellent ideas.
You might be able to use the 5R4 temporarily, but expect to get a 5AR4 or a solid state rectifier for better performance.

BTW, don't get rid of the old tubes (EL34's) since you can always sell them if they have any life left.

Tony

For sure, the tubes will either be sold or stay with the amp. I will look at getting the 5ar4 as suggested, they are cheap, a matching JJ would do nicely.

Should I be doing this mod to the socket? I have the diode's required.


https://dynacotubeaudio.forumotion.com/t1006-tube-rectifier-diode-mod

Absolutely!

Even if you use a new rectifier ($12-25) I suggest it just to avoid the agrivation of having to check the amp out, then replace the rectifier. They will drop less than a volt, and have no effect on the sound of your amp. Well worth the time to install them.

The only reason I suggest using the 5R4 for testing and initial start up is it doesn't handle the same amount of current as a 5AR4 and probably won't last long.

Tony

Actually a weber copper cap rectifier may fit the bill.

Do not use the 5R4 because the input cap in the dynaco exceeds 5mfd and it could ark over. The 5U4 is a good substitute tube but will provide lower B+ voltage.

I bought the board assembled and the seller forgot to send the cover otherwise it would be in the pic. It is on route and should be here monday.

So now the big question, I have the old driver board with 2 7199 tubes, they arent't the same manufacturer but they are 7199's which are getting hard to find. What's it all worth?

Thanks

I sold two dynaco driver boards a while back. I got a grand sum of $15 for one board and two 7199's and I got $12 for the other board so draw your own conclusion from that. If you get the right person at the right time then you might get something out of them if not you might as well head to the garbage with them.

I disagree!

If you don't want the original board and tubes, then do what you have to find someone who wants them. If your 7199's are good, they should be kept. They are hard enough to find that good used ones are worth considering. And there are those who would want to restore an original ST70 with original parts. The driver board may not be worth much but should still stay out of a landfill.

M$2/100.

Here it is- finally all the goods showed up today and I happened to take the day off of work, why not finish it! Dc board installed, fired up on the dim bulb with different wattage bulbs to make sure it wasn't gonna fry, biased all 4 tubes spot on .4V. I didn't fire it up when I got it , I didn't want to be disappointed by the old tech sound plus I didn't trust it. MAN THIS SOUNDS NICE!!!! This is the best amp in my collection for sound. 1/4 volume and it's rocking the house and talk about clear clean sound!!!! The singles tube bias is awesome and so simple to adjust, those pots are really sensitive. Zero hum at idle, i'm impressed.


Here's a shot of the DC board installed-



Just waiting for the driver board clear cover to show up, gonna install some feet and it's done!




It ran fine off the rectifier that's in it, I will still replace it with the suggested one

anbitet66 wrote:I disagree!

If you don't want the original board and tubes, then do what you have to find someone who wants them. If your 7199's are good, they should be kept. They are hard enough to find that good used ones are worth considering. And there are those who would want to restore an original ST70 with original parts. The driver board may not be worth much but should still stay out of a landfill.

M$2/100.

IMHO there seems to be three options for restoring older Dynaco amplifiers and/or preamplifiers, each of which will depend on the preferences of the restorer. 1) Keep it as original as possible doing only necessary cleaning and minimal repairs to restore operation. Basically, ending up with a functional collectible. 2) Thorough cleaning and repairs using new "replica" circuit boards with new and more reliable parts. This would be for those wishing to retain the original performance characteristics, while improving reliability. 3) Thorough cleaning, repairs, and modifications using modern aftermarket circuit boards designed to improve both performance and reliability. This would be for those wishing to raise the level above that of the original product. In any case, still functional 7199's can be either reused or sold (they do have value).

Best Regards,
Charlie

The only downside of using RCA jacks for the bias measurement is that
it's not idiot proof (speaking only about myself, of course).

I suppose if you had a original driver board with good traces and without the typical heat damage that you usually see then its worth keeping and or selling to someone who wants to keep their Dynaco original. We could debate the reality of the Dynaco board being special on one fact only and that is there were mega dynaco produced so they are not rare by any means. They were crude at best and should be replaced by either a reproduction PC board that there are a number of producers for or replacement of the board with an aftermarket board such as the VTA, Mapletree, and or other designs out there. We could also discuss the replacement of the stock 7199 with a 6GH8 tube that will deliver the same if not better performance. This modification requires some PC board trace cutting or the purchase of tube socket adapters to use this tube.Have made this modification in the past and can attest to the fact that the harmonic distortion actually goes down and the amplifier does sound more open and musical.

RCA female jacks for bias adjustment..... not a good idea because some helpful soul could accidentally plug a male rca jack going to another piece of audio gear such as a preamp and the result wouldn't be good. Herman H. Smith manufactures a minature jack for the purpose of testing. CAT # 256 103 terret terminal bananna jack.

That choke looks like its on its last legs and about to release some of that famous brown goo.

I have an old Bogen and an old Scott.....both that use 7199 tubes in the driver circuit. If you want to part with yours, let me know! I need a couple to finish restoring these old amps. As to your board, I agree with the sentiment that if you don't want it.....better to sell it for a buck than to scrap it. I've seen some pretty creative things made up of old Dynaco driver boards. Myself, I've been thinking about making up a ST-70 driver board version of a 6V6 amp using old console transformer pulls...... just to see how it turns out. I'm sure there are others who would make good use of that board as well. Tom D.

The choke is rusty, I may order a replacement. The 7199 tubes are going tomorrow, i'm trading them to a local guy who has a 12au7 and my rectifier tube, I want to lower the gain a bit, on the third notch of my preamp this thing gets too loud, i'll save the other tube in case I want to run my ipod into it. I'd trade the board for something if someone is interested in it, I know it works as the seller fired it up before he sold it, as to how it sounds, I never listened to it in its stock form.


EDIT- replacement choke ordered.

Went and visited a buddy of mine who owns a shop here in town, as luck had it he had a gz34 mullard made dynaco tube with about 5 hours on it. I traded him the board, 7199's and some cash for it. It's a 1964 tube with 7 notch plates, pretty decent trade IMO.

quad wrote:The only downside of using RCA jacks for the bias measurement is that
it's not idiot proof (speaking only about myself, of course).

Explain? each RCA jack goes to an individual tube, stick in positive of meter, ground to ground and measure, it's the opposite of idiot proof IMO.

bktheking

It is common practice to use the correct connector for the application. The Herman H.Smith company and others manufacture connectors and jacks for a specific purpose. You want something that cannot be confused with the RCA input connector. Some idiot could mistakenly plug in say a preamp for example into your RCA female bias jacks thinking it was an input thus causing possible damage to the preamp. Since I deal with commercial and pro audio I would not dream of using an XLR connector for anything other than a mic or line connector just to make sure that something doesn't get plugged in where it shouldn't.

Some idiot would be me, this thing is going to the grave with me. I agree there are specific connectors, this wasn't a mass production unit, this was a one off just for me so i'm not worried about it. Specifically I was talking about the choice by the builder by his/her sole owned equipment, I have the understanding of how it works and therefore will only plug the rca's into the correct spot.

Just trying to point out the fact that if its worth doing its worth doing correctly. Your not necessarily the person that is going to mistakenly plug in another RCA jack into your bias test points. My experience is this... its always the other or next person that you have to worry about. The reason I am making a point about this is because I have used a common connector on a job specifically because a customer wanted it that way. Sure enough when that person retired his replacement did the unexpected and plugged the wrong item into it and took it out.

I would be more than happy to send you some connectors to make your project perfect.

Have you got a manufacturer part # for me and I'll take a look?

Thanks for the input.

What you have done is fundamentally dangerous and should be corrected. It's analogous to storing bleach in a mouthwash bottle. You may know what's in it, but that won't prevent a tragedy if you're not around. Do you remember what happened a few years ago when people tragically mistook citronella oil for apple juice because of the similarities in appearance and bottle? With your configuration, it's not a question of if, it's a question of when.
What I did with my ST-70 rebuild was to utilize the front sockets nearly as suggested and labeled. However, I made a couple of changes I that I thought made better sense. From the front of the amp, the bias set arrow points to pin 8, I kept that. In Bob's diagram that would refer to the front tube. Also in Bob's diagram, pin 4 is used for the rear tube. I thought it made better sense to bring the rear tube wire to pin 5. I also ran a ground wire from each corresponding socket pair to pins 6 and 7.
So-looking from the front of the amp the bias readings are taken from the top halves of the sockets. Pins 6 and 7 at the top are grounds, pin 5 to the right is the rear tube (Right is Rear), pin 8 is the front socket. I would suggest not connecting the heater supplies to pins 1 and 2 to prevent any unexpected problems.
I also purchased an empty octal tube base from Tube depot to construct a more friendly dongle for taking biases. My plan is to wire them to a couple of meters mounted in a hobby box for quick and easy bias checks.
Just my 2 cents.

bktheking wrote:Have you got a manufacturer part # for me and I'll take a look?

Thanks for the input.

I used "tip" jacks (see link below). Red for positive, black for ground. They can be mounted in place of the octal sockets, just as you did with the RCA jacks. Your meter probes will plug directly into these.

http://www.tubesandmore.com/ (on the left click "jacks & plugs", then click "tip jacks")

Best Regards,
Charlie