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Dynaco Tubes - HELP

V

valco

Active Member
I have an early Dynaco ST-70. I rebuilt it with new caps, resistors, PCB board, filter cap, etc. All stock values. No mods. It all works great.

I'm considering getting a matched quad of KT77 tubes for the amp.

Each channel of the amp has different brands of tubes.
Original Mullard EL34s in the left.
Matched pair of Winged C EL34s in the right.

The other pair of mullard EL34s have sadly failed.

So I really have a two part question, I guess.

1. Is it a bad idea to have different brands of tubes for each channel? When set to mono, jumping back and forth between the channels, I can barely tell the difference. So they seem pretty well matched.

2. I find the overall sound to be a bit dark. The treble seems pretty rolled off to me. Its wonderful on older recordings, but sounds a too muffled on modern stuff.
I've read the KT77 tube is brighter than the EL34. Anyone have experience with these tubes? I'm considering getting a matched quad of these. But I'm not looking for a subtle bump in treble reach. I'm hoping for something more pronounced. Would the KT77 deliver more treble over the EL34s?

Thank you all for your input.
 
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wow, thanks for the response. I checked all voltages when I rebuilt it. It all checks out perfectly.
So a .1 cap on each leg of the quad cap? And the other ends of the cap to ground? And what exactly is this doing?

Yeah, leaning towards the JJ since the GL are a bit more than I want to spend. I dont remember the brands of 7199. They are two different brands. Might be worth mentioning that when poking around reading voltages on the driver board, one channel is alway about 10vdc more than the other channel. And it follows the 7199s. If the left has the higher vdc, and i swap the 7199s, the right will then have the 7199. So its seems its just unmatched 7199s. Not sure if this is a big deal or not not.
 
Distortion sometimes is experienced as "bright".
Matched tubes will reduce distortion. If you live in US i recommend Eurotubes and
a quad of matched KT-77 for your ST-70. Or EL34 if you like, both fits well.

If you feel the sound to dull, turn the treble control up a bit !
 
Thanks for all of the responses.
I'll start by changing the tubes to KT77s and see where that leads me.
Is there a way to tell if the 7199s need to be replaced? The amp is VERY quiet. No noise at all. Kind of amazing actually.
 
valco said:
Thanks for all of the responses.
I'll start by changing the tubes to KT77s and see where that leads me.
Is there a way to tell if the 7199s need to be replaced? The amp is VERY quiet. No noise at all. Kind of amazing actually.
Click to expand...
Your 7199 is fine !
When they start "hum" you can start planning for exchange.
 
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