SCA-35 squeals when powering up

I recently obtained a CSA-35 that was someone had tried to get working. Wired for 220 volt, wrong driver tubes, open cathode resistor etc. As it was missing both 7199's and the PC-10 boards were bad shape i decided to replace the PC-10 boards with new boards which use the 6GH8A. All went well until i powered it up and was greeted with a howling noise from both channels that gets quite loud.

Now, here is where it gets interesting.If i connect this amp to a variac and raise the AC input over several minutes it will perform perfectly. Tried different sets of tubes for either channel made no difference. The 6GH8A's are Dynaco. I would like to do the EFB mod to this amp but i need to get the power up problem figured out first.  

Anyone ever had this problem?

dockbuster wrote:I recently obtained a CSA-35 that was someone had tried to get working. Wired for 220 volt, wrong driver tubes, open cathode resistor etc. As it was missing both 7199's and the PC-10 boards were bad shape i decided to replace the PC-10 boards with new boards which use the 6GH8A. All went well until i powered it up and was greeted with a howling noise from both channels that gets quite loud.

Now, here is where it gets interesting.If i connect this amp to a variac and raise the AC input over several minutes it will perform perfectly. Tried different sets of tubes for either channel made no difference. The 6GH8A's are Dynaco. I would like to do the EFB mod to this amp but i need to get the power up problem figured out first.  

Anyone ever had this problem?

I will assume you have double/triple checked your connections and wiring? Give it 24 hours and check again!

First, try shorting the input jacks - all of them.
Then, examine all the input jacks for close/touching wires.
Then examine (and clean) the selector switch. These are notorious for jumps, opens and shorts. Get in there with a dental pick and tighten connections as well. Some positions are to short on the switch to send stray signal to ground. And age may loosen some of the wipers.
Then, ohm out all the controls. Make sure there are no shorts or crosses between channels - and pay special attention to the hum controls. Always start with them centered.Why this would not necessarily be an issue when powering up slowly could be from several causes - most will lead back to the selector switch. Or the slide-switches.

Having "been there, done that", I am going to go out on a limb and suggest, in rank order, a loose connection, solder bridge, partial short.

dockbuster wrote:I recently obtained a CSA-35 that was someone had tried to get working. Wired for 220 volt, wrong driver tubes, open cathode resistor etc. As it was missing both 7199's and the PC-10 boards were bad shape i decided to replace the PC-10 boards with new boards which use the 6GH8A. All went well until i powered it up and was greeted with a howling noise from both channels that gets quite loud.

Now, here is where it gets interesting.If i connect this amp to a variac and raise the AC input over several minutes it will perform perfectly. Tried different sets of tubes for either channel made no difference. The 6GH8A's are Dynaco. I would like to do the EFB mod to this amp but i need to get the power up problem figured out first.  

Anyone ever had this problem?

6GH8A is no dynaco thing. Replace them with ecf82 or 6U8 from a reputable source.
The problem occurs since there is positive feedback involved in the 7199 circuit, and
using a pentode with too high amplification will cause it to go "over the edge"
What boards did you get ? They might be flawed too. You might have a better chance
to use the original boards and use adapter sockets + ecf82/6u8 .

When using the sovtek 7199 the same squeel occured, and the cure was to junk them and
get real 7199

peterh wrote: 6GH8A is no dynaco thing. Replace them with ecf82 or 6U8 from a reputable source.

peterh. This is the same to replace the 7199 of the ST70?.

Peterh, you are right on when you recommend using 6U8A's. Found a pair in a box of old tv tubes and they looked ok on my tube tester so i installed them and no more squeal on power up. I currently have the amp connected to a pair of Tannoy C-8's and a Sansui turntable. SWEET!!!

Also, you are also correct, the 6GH8's are labelled Dynascan, not Dynaco. Should wear my glasses more often.

Thanks for your help and on to the EFB mod.

T.

Pillo69 wrote:

peterh wrote: 6GH8A is no dynaco thing. Replace them with ecf82 or 6U8 from a reputable source.

peterh. This is the same to replace the 7199 of the ST70?.

Yes