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Tube newb Dynaco pt. 2

bored184

bored184

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I recently got the following parts in to start in the fixer up Dynaco ST-70.
1) Orange drop caps for the board
2) New bias kit
3) Can cap
4) UF 4007 diodes
5) New choke

How ever I picked up a second Dynaco to give me something to listen to while I am working on the "fixer" upper. The problem is the "working" one has a weak left channel. During power up the smaller left 7199 tube lights up faster and somewhat brighter then the right 7199 tube. After the initial warm up, while looking at the amp while running I swear the hole left tube side is somewhat dimmer then the right.

EDIT: On closer inspection the two power tubes that appear to be glowing dimmer are made slightly different then their counter part. It just happened out of coincident that I paired them together. I swapped 7199 tubes and the initial "flash" follows that one tube.
 
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bored184 said:
How ever I picked up a second Dynaco to give me something to listen to while I am working on the "fixer" upper.
Click to expand...

See that!?!! I'm not alone!! :beatnik:

Help me out here. I'm trying to justify the move to the mono blocks now. :drool:
Should I buy the vacation home? It would need some audio!
 
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vinylkid58 said:
Is it possible they're just getting tired. Have you tested them?
A Mullard prehaps?

jeff
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I have not tested them. I have a Mighty Might tester but its not been used in years so its not help at the moment.


Windup said:
See that!?!! I'm not alone!! :beatnik:

Help me out here. I'm trying to justify the move to the mono blocks now. :drool:
Should I buy the vacation home? It would need some audio!
Click to expand...

Same here! I want a pair of MK IIIs bad!
 
Some tubes are flashers ... some not. More of a brand and build thing, nothing to do with the quality of the tube or sound. If the problem follows the tube, looks like an easy fix.

You should also plan to replace all the carbon resistors in the box ... those go way out of spec over time. Also if it's a kit amp, I'd advise going over every solder connection carefully with a magnifying glass to check for cracks or just slop. Might work new, but get progressively more problematic with age.
 
bored184 said:
I recently got the following parts in to start in the fixer up Dynaco ST-70.
1) Orange drop caps for the board
2) New bias kit
3) Can cap
4) UF 4007 diodes
5) New choke

How ever I picked up a second Dynaco to give me something to listen to while I am working on the "fixer" upper. The problem is the "working" one has a weak left channel. During power up the smaller left 7199 tube lights up faster and somewhat brighter then the right 7199 tube. After the initial warm up, while looking at the amp while running I swear the hole left tube side is somewhat dimmer then the right.

EDIT: On closer inspection the two power tubes that appear to be glowing dimmer are made slightly different then their counter part. It just happened out of coincident that I paired them together. I swapped 7199 tubes and the initial "flash" follows that one tube.
Click to expand...
To comment the last item first, flashing filament: Don't bother. SOme tubes does this, it's of no concern for function.

About rebuild in general : I would do different, i would replace driver board with something that does not use unobtainium tubes ( 7199) . At the same time you get a more recent design and you might end up with new bias cirquit and individual bias adjustment pots.
There is at least 2 vendors tubes4hifi and triode electronics that market such boards.
 
What solder would your recommend for repairing/modding Dynaco amps. On the list of things I would like to get done today is to replace the selenium rectifier and the diode rectifier mod.
 
bored184 said:
What solder would your recommend for repairing/modding Dynaco amps. On the list of things I would like to get done today is to replace the selenium rectifier and the diode rectifier mod.
Click to expand...
if you read my recommendation, the selenium is replaced when you install a tubes4hifo board ( or triode board)

As solder, use 60/40 or simular. Avoid leadfree as plague!
 
I second the tubes4 hifi board. Honestly I really like radio shack solder. Kester is very good too. Make sure stuff isn't oozing out of the choke too. Very user friendly. On the Dynaco Forum website is a troubleshooting guide t the ST 70. It might help.
 
Oh , on the driver board, I would skip the octal version. Nothing wrong with it. It'll just cost more in the end for the same results
 
cardas-quad.jpg


Does this put me in the running for "audio snob" of the week? <G>

I truly do love the stuff though. Expensive, but a little goes a long way.
 
Many like the sound of the Octal tubes better, and classicvalvedesign sells nice affordable PCB's of their own design for another Octal choice along with 2 other popular choices with the 9 pin tubes. . The 6SN7 & 6SL7 are well regarded tubes sonically that many builders use today. Lot's of affordable new production also along with NOS.
 
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Opened her up tonight someone has already replaced the selenium rectifier with a diode. The choke does not appear to be oozing but it is discolored with age. This may be a taboo idea but at the moment I don't plan on modding it to much. I want to keep her pretty well stock, I never grew up with tubes so I want to hear the amp as it was intended. If I do end up replacing the board it will probably be one that resembles the original from Dynakit. Below are some inside shots of the amp. I apologize for the poor quality, they were taken with my phone.

The "fixer" upper will get modded.


 
That actually doesn't look too bad.
Enjoy! :banana:

Half the enjoyment, while deciding how to proceed, is listening to the "before". :scratch2:
 
Shadow I take note of the info on the octal version maybe I was misinformed. Not looking to change mine (very happy with it) but will do some research and some reading. I never heard stock ST 70 but have heard good things about it. Those who have heard both say the upgraded driver board is a big improvement. If this is your first tube amp I'm sure you'll be happy when you get it going right and you can upgrade at your leisure or leave it alone if you like the way it sounds when you get your rebuild done.
 
I just read if your going to change the PCB to a different circuit the Octal was worth considering as some say those tubes can sound great. Some say a fuller more robust sound. Anyway they are just a bigger version of the small tubes that came later.

The 12AU7 doesn't have that many fans as those that try other tubes that can sub for it like the 12BH7. I haven't heard the Octal tubes either and plan to try them someday as I've never read anything neg. about them. Maybe it depends on the circuit and other parts like everything.

I kind of like this idea as it's simple a low cost $. It is the Mullard circuit (like on Eico HF-87/89) just about everyone uses on the affordable after market PCB boards. You could always add CCS later with diy or after market small PCB 's later if desired.

http://www.diytube.com/phpBB2/viewtopic.php?f=6&t=4672&sid=fb0120bf88705d19e1658042f1f071a4

Anyway it shouldn't sound worse and if you have some of those tubes kicking around , maybe worth a try.
 
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I'm deciding towards getting a new board, board only, and supplying my own parts. I have the correct set of Sprague Orange Drops for fixer upper but I am thinking about using those for this "stock" rebuild. What manufacturer would you recommend for resistors?
 
I like Ohmite, and you're probably best sticking with ceramic resistors as they hold up to the heat and don't have the drift problems of carbon that come with age. More importantly, they look purtier on an exposed board like what you're talking. Gotta have priorities, right? <G>

Big fan of Mouser here ... they're my go to source for electronics. Used to be DigiKey, but their S&H is ridiculous nowadays and a lot of the stuff they used to stock for immediate shipment ... ain't anymore. Not sure what all's going on there.
 
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