7189s in Dynaco ST-35 - Ouch!

Posted by Bryon (A) on July 21, 2002 at 07:13:16

Isn't this fun. One of the great joys of tube amps for me is tube rolling and I have played with a variety of tubes in both my foreplay and my Dynaco ST-35 and love listening to the variation in soundstage, response and tone.

After reading numerous posts here about being able to substitute 7189s for 6BQ5s, I bought a quad of TFK 7189s for the Dynaco. I checked them on my tube tester, then rolled them in and went into the kitchen to test some other tubes. Shouldn't have done that! As the tubes warmed up there was a loud hum. By the time I got to the amp, one of the 7189s was lighting up like a roman candle. Before I could get to the plug there was a loud snap and copious smoke emanating from the ST-35. Three of the tubes survived intact but the fourth is toast. When I looked (after giving it some time - I have a healthy respect for the blue deity) I found that a cap had literally been blown apart. Ouch!

Now for my newbie questions.

1. Could the fact that the 7189 draws higher voltage have caused an old cap to blow or could have the tube itself (even though it tested good) have caused it?

2. Is there likely to be any other collateral damage that I should look for or can I just change the cap and fire her up again.

3. The blown cap was a sprague 100uf 35wvac, which I assume is an electrolytic cap. How can you tell the difference between the different types of caps? Usually I know that you can substitute higher value caps which will often improve the sound, but will putting in a higher quality (like a 220uf 200w) cap cause something else to blow.

4. Does anyone have a lonely TFK 7189 (has to test at 88 with 60 good) which is looking for a home or should I sell the remaining triad and stick to 6BQ5s in my dynaco?

Sorry for all the questions and thanks for your advice.

Bryon

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Topic - 7189s in Dynaco ST-35 - Ouch! - Bryon 07:13:16 07/21/02 (22)

Re: 7189s in Dynaco ST-35 - Ouch! - Ivan303 07:44:16 07/21/02 (21)

Re: 7189s in Dynaco ST-35 - Ouch! - Bryon 13:26:18 07/21/02 (10)

Re: 7189s in Dynaco ST-35 - Ouch! - Steve O 20:35:18 07/21/02 (8)

Non-match power tubes - steven oda 22:48:42 07/21/02 (7)

Re: Non-match power tubes - Steve O 13:14:39 07/22/02 (6)

Re: Non-match power tubes - steven oda 13:29:56 07/22/02 (5)

Re: Non-match power tubes - Steve O 16:20:57 07/22/02 (4)

Thanks... - steven oda 11:25:39 07/23/02 (3)

Re: Thanks... - Steve O 12:04:56 07/23/02 (2)

Electric character of the grids - steven oda 09:24:23 07/24/02 (1)

Re: Electric character of the grids - Steve O 18:46:47 07/24/02 (0)

Ask Steve Oda.... - Ivan303 13:39:07 07/21/02 (0)

Re: 7189s in Dynaco ST-35 - Ouch! - steven oda 12:20:37 07/21/02 (9)

Re: 7189s in Dynaco ST-35 - Ouch! - Bryon 13:30:02 07/21/02 (5)

Re: 7189s in Dynaco ST-35 - Ouch! - steven oda 17:08:49 07/21/02 (4)

Re: 7189s in Dynaco ST-35 - Ouch! - Bryon 17:56:38 07/21/02 (1)

Schematic - steven oda 18:16:55 07/21/02 (0)

Adden-dumb - steven oda 17:12:57 07/21/02 (1)

BTW... - steven oda 17:14:05 07/21/02 (0)

Re: 7189s in Dynaco ST-35 - Ouch! - Ivan303 12:49:18 07/21/02 (2)

Re: 7189s in Dynaco ST-35 - Ouch! - steven oda 13:27:12 07/21/02 (1)

Great - Thanks (nt) - Bryon 19:08:44 07/21/02 (0)

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