• The move to the new server is done. There are some software and database maintenance updates in process. This has us passing the hat around to help out. We appreciate any donations. Seriously, even a dollar helps. The payment page may be found here - https://www.audiokarma.org/support.html

Dynaco ST-70 - Upgrade Time?

David_NC

David_NC

Super Member
I bought this ST-70 about 2 years from a local GW. It came with good T-S 7199's and Philips EL34 / 6CA7's. It's in great shape and worked well, but I replaced the selenium rectifier, multi can cap, caps and resistors underneath, and the six caps on the driver board before putting it into use. It's sounded great most of this time, but I noticed the right channel was a bit softer. I figured it was something in the speakers, as I'd just worked on them or, even worse, my hearing.
Having recently gotten a Hickok tube tester, I decided to test all the tubes in the equipment I have. The EL34's test good and are pretty closely matched side to side and very closely matched in each channel. The 7199's are another story. One reads just barely into the bad zone, and the other is in the lower end of the bad zone. The worse one is from the weaker channel.
So, that's easy enough to remedy; socket adapters and with a pair of 6GH8A's. That got me thinking if I'd be better off replacing the driver board with a new and improved version. I've been satisfied with the ST-70 other than the channel imbalance, but my newly restored HK A300 integrated has detail and what I'll call sparkle that the Dynaco doesn't. Would a pair new driver tubes, 6GH8A with adapters, help with this, or would a new driver board really improve it all that much? FWIW, my current driver board is in good shape.
Neither option will break the bank, but the driver board will obviously be a bit more work. I'm curious as to which board others have had good experiences with and how much improvement I'll notice from such a replacement, or if I should just stick with the adapters and 6GH8A's.
 
Register to hide this ad
Nyjblbob said:
I would go for the socket adapters and NOS 6gh8a's first. Easiest, cheapest and should bring it back up to spec. and its original glory. It will Probably have better mids than the HK
Click to expand...
I kinda like that suggestion; it's def the quickest and cheapest option!
 
I found a socket adapter at tubedepot.com, but it's specified for being used with a 6U8A. From what I'm seeing, that should be comparable with 6GH8A, along with 6EA8, 6AN8. If this is correct, is there a reason to choose one over the other?
 
David_NC said:
I bought this ST-70 about 2 years from a local GW. It came with good T-S 7199's and Philips EL34 / 6CA7's. It's in great shape and worked well, but I replaced the selenium rectifier, multi can cap, caps and resistors underneath, and the six caps on the driver board before putting it into use. It's sounded great most of this time, but I noticed the right channel was a bit softer. I figured it was something in the speakers, as I'd just worked on them or, even worse, my hearing.
Having recently gotten a Hickok tube tester, I decided to test all the tubes in the equipment I have. The EL34's test good and are pretty closely matched side to side and very closely matched in each channel. The 7199's are another story. One reads just barely into the bad zone, and the other is in the lower end of the bad zone. The worse one is from the weaker channel.
So, that's easy enough to remedy; socket adapters and with a pair of 6GH8A's. That got me thinking if I'd be better off replacing the driver board with a new and improved version. I've been satisfied with the ST-70 other than the channel imbalance, but my newly restored HK A300 integrated has detail and what I'll call sparkle that the Dynaco doesn't. Would a pair new driver tubes, 6GH8A with adapters, help with this, or would a new driver board really improve it all that much? FWIW, my current driver board is in good shape.
Neither option will break the bank, but the driver board will obviously be a bit more work. I'm curious as to which board others have had good experiences with and how much improvement I'll notice from such a replacement, or if I should just stick with the adapters and 6GH8A's.
Click to expand...
If the "softness" follows the 7199 when swapping them , then you have the answer.
The easiest solution is a replica board that allows 6GH8/6U8/ECF82/ECF80 ( they all have the same pinout)
just i tiny bit more expensive then 2 socket adepters. See dynakitpart.com for the board.
 
peterh said:
If the "softness" follows the 7199 when swapping them , then you have the answer.
The easiest solution is a replica board that allows 6GH8/6U8/ECF82/ECF80 ( they all have the same pinout)
just i tiny bit more expensive then 2 socket adepters. See dynakitpart.com for the board.
Click to expand...
I used the Vintage Electron 6GH8 board, with one change. Good quality overall that renews most critical parts outside of power supply.
 
Adapters would be the easiest and simplest fix, if you have a good operational board already. Many can’t get past the mental piece though, that they are using adapters and will change the board anyhow. Ask me how I know.

Tubes that test bad are bad and will affect the performance of the amp.

One other thing to consider for channel imbalance, is the room. In my case I have a rectangular room for the most part except for the right wall rear third is open to a different room. This sidewall difference throughs the channel balance off quite a lot on every stereo I have tried. May also apply, may not.

Bill
 
7199's have reputation for unreliability. My ST-70 came with marginal 7199's. I used the 6U8 adapter from tube depot. Installed a couple JAN 6U8's. Sounded quite a bit better and have worked with no problem for near a year.
 
peterh said:
If the "softness" follows the 7199 when swapping them , then you have the answer.
The easiest solution is a replica board that allows 6GH8/6U8/ECF82/ECF80 ( they all have the same pinout)
just i tiny bit more expensive then 2 socket adepters. See dynakitpart.com for the board.
Click to expand...
Yep, it follows. I'll grab a couple adapters and some 6GH8A's or 6U8's and go from there.

Thanks all for the replies.
 
You must log in or register to reply here.
Back
Top Bottom