Dynaco st-70 mod identification help

[Joebob1]

Update!

Not that interesting but here are some photos of the bad capacitors and resistors I removed today.

And a bag full of caps that didn't belong =].

 

[gadget73]

CC resistors are prone to drift. It happens. Did Dynaco use deposited carbon film or any other sort of high accuracy resistors in the spots where they must be matched? Usually those types are more stable long-term than the usual carbon comp type.

 

[Joebob1]

No update, but another question.

Found some el34's locally on Craigslist. They are 1 matched pair of mullard xf1 and 1 matched pair of GE xf2.

Do you guys see any red flags from the photo? I was planning to check them out tomorrow. He also said they test as NOS.

Thanks.

 

[Joebob1]

That's good to know. I'll look into getting some gold lions too (would like to eventually have 2 sets).

Do you think the difference in the channel due the different tubes will be really noticeable? I could always just buy one pair.

 

[Joebob1]

It was just a thought since they look to be good tubes. I would have to pick up another matching pair later or may be off ebay.

I will try on some mismatched shoes and see what I think.

 

[gadget73]

Two pairs of Mullard tubes with different date codes aren't going to bother anything and I very much doubt you will ever be able to tell the difference as long as the tubes are in good condition. The trick is knowing if they are actually matched, in good condition, and if the price is not stupid. Unless all that lines up, go for the KT-77 tubes from a reputable vendor and call it good.

 

[Joebob1]

Isn't the xf# the type code? So they are different type and not date?

 

[gadget73]

I still seriously doubt you'd ever notice the difference. I wouldn't hesitate to run them if they appeared on my doorstep. I don't have an ST-70, but I'm sure my Pilot SA-260 would be fine with it.

 

[Joebob1]

Update!

Finished restoring the amp back to stock this weekend, except for the modded rca's input. Tested the transformers and ran through post startup checks. Temporarily set the Bais to 1.24v and tested with pas 3 preamp and ads l400 speakers. Everything seems to be working. Not getting any distortions or hum.

Previous owner replaced the bias resisters with 2 resistors in parallel measuring 15.9 ohms. Guess this means I would have to set the Bais to 1.59v or replace it. Any issues with setting the Bais to 1.24v until I decide to replace it?

Also, anything else I should check or keep an eye before using it daily? I will eventually use it to run my thorens table.

Thanks!

 

[s-petersen]

I would keep an eye on the can cap if original, to make sure it doesn't get hot from internal leakage

 

[Joebob1]

I would keep an eye on the can cap if original, to make sure it doesn't get hot from internal leakage

It is the original can. Any idea what would be considered hot? I could borrow a temperature gun and check later this week.

 

[s-petersen]

If it is more than luke warm in the first hour, or seems warmer than it's surroundings, I would be concerned.

 

[Joebob1]

If it is more than luke warm in the first hour, or seems warmer than it's surroundings, I would be concerned.

Thanks, I will check and replace it if it feels warm.

 

[Joebob1]

Thanks for the suggestion. Not being an expert, I will do some more research this week before attempting this check.

Something else I just realized. Do I need to rewire the speaker 2 terminal binding post, if I'm using 4 ohm speakers?

 

[s-petersen]

Which color wire is connected to the red terminal? brown is 4 ohm, and the yellow stays connected to the feedback loop

 

[Joebob1]

Photo 2 in my original post shows orange wire connected to the red terminal.

Edit....

It hard to tell from the photo, will check when I get home.

 

[Joebob1]

So the orange wire is connected to the red speaker terminal. If I plan to use 4 ohm speakers, I should replace the orange with the brown wire?

Also, the yellow wire is not connected. Is this correct?

 

[s-petersen]

Yes, on the brown wire, but the yellow wire is supposed to connect to the feedback loop
eyelets 12 and 13 on the PC board
It looks in your pic the black wire to the red terminal goes to the PC board eyelet.
Currently it is connected to the wrong tap on the transformer

 

[Joebob1]

Yes, on the brown wire, but the yellow wire is supposed to connect to the feedback loop
eyelets 12 and 13 on the PC board
It looks in your pic the black wire to the red terminal goes to the PC board eyelet.
Currently it is connected to the wrong tap on the transformer

You are correct. The orange wire (8 ohm) and the BLK wire that runs to the PC board are currently connected to the red speaker terminal.

So to run 4 ohm speakers I should:

1) Disconnect the orange wire from the red terminal and replace it with the brown wire.
2) Disconnect and connect the BLK wire that runs to the PC board to the yellow wire.

Its interesting the amp sounded fine when I tested it.

 

[s-petersen]

Yes, it will be somewhat quieter now, and a little "tighter"