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Dynakit ST70

Lavane said:
I saw those and wondered about them also. I wondered if the disclaimer was more focused towards cathode biased guitar amps. Being that the bias can't be adjusted.
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Not really. There are very few cathode biased guitar amps using anything other than the 6V6 or El84. The issue was that the normal bias circuit for an EL34 guitar amp doesn't usually have enough bias voltage range to dial in these tubes. You would have to change a resistor or two in the circuit to get the range required. More in the range that a 6L6 amp would use.
 
Alright, sorry for the lack of updates. I finally ordered everything. For tubes, I sucked it up and got a TungSol 5AR4, adapters and 6GH8As for the 7199s, and JJ Apex burned-in matched EL34s.

I got the 80, 40, 30, 20 can, metalized polypropylene caps for the boards, and the diode kit.

When it all gets here I'll get it installed!

PS - my house voltage is between 120 and 122. I've been measuring it for a week or two a few times a day. When the AC is on, it tends to be closer to 120.
 
Tucker_515 said:
Alright, sorry for the lack of updates. I finally ordered everything. For tubes, I sucked it up and got a TungSol 5AR4, adapters and 6GH8As for the 7199s, and JJ Apex burned-in matched EL34s.

I got the 80, 40, 30, 20 can, metalized polypropylene caps for the boards, and the diode kit.

When it all gets here I'll get it installed!

PS - my house voltage is between 120 and 122. I've been measuring it for a week or two a few times a day. When the AC is on, it tends to be closer to 120.
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You can check across heater voltage at the accessory preamp sockets for how close to the 6.3VAC baseline standard. Also, bias will vary with line voltage, go for averaged 1.56 VDC each channel over several readings during the listening day. Then, check bias every few months for substantial changes and adjust if necessary.
 
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Hey everyone. Sorry it's been so long. I got very busy with work and school, and the ST-70 just sat on a shelf! Last weekend I finally sat down and replaced all of the capacitors and swapped the selenium rectifier with a diode kit from dynakitparts.com. The last thing I did was clean and retension the tube sockets and put in a new set of tubes, just in case there was an issue I didn't want to risk the OEM ones. I brought it up on a variac and set the offset to 1.45 - 1.50. I figured I didn't need to crank up the offset right away. One thing, my house current is 120-122, and that puts the tube heaters at 6.5-6.6 volts. I keep the variac at 117 and that puts the heater voltage right at 6.3-6.4. I am thinking about getting a 5 volt bucking transformer to lower my main voltages.

I have a PAS-2 that also needs to be gone through, but to test it out I used the pre amp of my Marantz 2330. It sounded fantastic. The only issue I had is with the left channel. It has quiet random crackling, inaudible at higher volumes, but noticable at low ones. It is annoying. It is not 60 or 120hz, just random crackling. I picked up an oscilloscope and isolation transformer the other day, so if you have any places I should check for static, let me know. I did swap the EL34s around and the 6GH8A's and nothing changed.

Thank you guys for all of your help, going back and reading all of the comments gave me the confidence to get this thing up and running!
 
egellings said:
ST-70 has to be the most modified tube amp in all of tube amp history!
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And probably never went out of production for any length of time since introduced. Even after Dynaco rode into the sunset there was ways someone to bring it back in one form or another. Four companies now, including the ST-70 being sold by the present Dynaco brand owner. I think it just all comes down To bang for the buck.
 
I'm reviving this since I just got a 1 owner ST-70. Seems pretty good. I need to replace the caps and the EL34 tubes and see if she'll fire up. I revived a Mark III, so I'm feeling confident in maybe a few mods.I've read every post and remember a little of it.:)
 
Would there be any benefit to using the higher dollar capacitors like Jensen or Jupiter or just a waste of money in this system?
 
Everyone has a favorite. Russian PIO's sound nice and won't break the bank.
I used them in my ST-70. You see a lot of orange drops used in these amps also.
 
Thanks for comments. The Russian capacitors look interesting. Are they new or NOS?
I have Altec A5's as my tube amp speakers. The horns take over when pushed. Eric's Hi-Fi blog has a bunch of boutique capacitors and gives the characteristics of each in his opinion. Of the reasonably priced capacitors you guys mentioned, is there one that leans toward the bottom end and doesn't push vocals forward?
 
MissionX said:
Thanks for comments. The Russian capacitors look interesting. Are they new or NOS?
I have Altec A5's as my tube amp speakers. The horns take over when pushed. Eric's Hi-Fi blog has a bunch of boutique capacitors and gives the characteristics of each in his opinion. Of the reasonably priced capacitors you guys mentioned, is there one that leans toward the bottom end and doesn't push vocals forward?
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They are old stock. They are known for using a very acid free paper, so pin holes developing in the paper is pretty much a non issue. There are a couple versions.
One with a coating on the case, the other is bare metal. If using the bare metal type it's a good idea to put some shrink tubing over them. I've seen pictures of the bare metal types wrapped in black electrical tape with a small thin zip tie at each end also to keep the tape in place also.
 
MissionX said:
Does that apply to electrolytics as well? Many put higher capacity in the can and in the line. I did on my Mark III.
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Yes. When ripple is below a certain level there is no point in more capacitor, it might load the GZ34 to breakage.
 
I used 40/80/30/20 quad elyticap replacement on mine, having the increased reservoir at the shared outputs can be a good thing. The 40uF is first off the rectifier, the 80uF after the choke.
 
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