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Scott 299b trouble

dmag

dmag

AK Subscriber
Subscriber
Some years back I bought a 299b that a guy came across a the dump recycle area. Iirc it had a gassed 7189. I rebuilt it only to find it had a bad OPT. I guess I should have checked it prior. Then before covid i bought another that the previous owner had reworked but was a very sloppy job. I used its OPTs in the first and was happy. A few months ago i bought replacement OPTs from Heyboer for those i had pilfered. Sometime prior i had cleaned up the messy rebuild.
The transformers were tacked in. The bias was very high and squirrely. Checking things i found a wiring error in the c1 and 2. That corrected, the bias is more stable, but I have a very loud hum, i believe to be 120hz. So i havent worried about it. Also my dummy loads get hot enought to burn fingers. They are those gold 100w 8 ohm wire wound jobs. Eventually the 3a slo blow fuse will pop, so i have been running it for very short periods. Removing the 6bl8 tubes stops the hum and hot dummys. Replacing them makes no difference. The voltages on them dont match the schematic. Ill go through and get them posted.
Ive been tracing the power supply and bias supply, and it seems to be in order so far. Most of my voltages are within reason. The 33 ohm dropping resistor is installed, and im using a variac at 115v. Ive been checking resistors against opposite channels, and nothing has jumped out. Ive been ringing the grounds and nothing there as yet either.

Im hoping for a little insight where i might look for issues. I havent touched an iron since covid, so im pretty rusty.
 
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might be out of phase, try unhooking the negative feedback connection and see if it also doesn't do goofy stuff. if it behaves, flip the plate leads and restore the feedback.
 
Is that the connection between off r47 and c38 to pin 8? And disconnect both channels?
And if that helps do I switch a single pair of the plate leads on only one channels OPT leads ?
 

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Sounds like it's oscillating. That could be caused by several things, including the primaries of the OPTs being wired backwards.

Jack
 
dmag said:
Is that the connection between off r47 and c38 to pin 8? And disconnect both channels?
And if that helps do I switch a single pair of the plate leads on only one channels OPT leads ?
Click to expand...
Its the connection to the 16 ohm speaker terminal.

If both channels are humming and making heat, unhook the feedback on both and verify it stops doing that. If its only on one channel, then just unhook the one that doesn't behave.

As for fixing it, if both channels are doing this you'll have to flip the leads on both. If its only one channel, flip the leads only on the one thats acting up.
 
Thanks. Ill go see if i can track that down. Its pretty tight back there.
 
dmag said:
Thanks. Ill go see if i can track that down. Its pretty tight back there.
Click to expand...
If I am not mistaken, the theory is that the primaries (from the output tubes) to the OPTs are reversed. If this is the case, feedback will be positive instead than negative. This can be verified by temporarily unsoldering the wires on pin 7 from the 6BL8 on the channel(s) where the loud hum is present.
 
Unfortunately disconnecting those leads at the 16 ohm terminal didn't help. It also disconnects the center channel, which shouldnt matter. Ill try disconnecting pin 7 next, and get back. reconnect feedback first?
And Mitch, I looked at a few of your posts, and came across your TT1a info. I have the same I rehabbed a few years ago. I dont remember where the info came from. Nice unit. I like being able to test both sides of some tubes without changing settings.
 
Shouldn't matter which end of the feedback is unhooked, different end of the same circuit. Unhooking either opens it.

the black lead is grounded yes?
 
It's a pretty sure bet that the output transformer primary leads are reversed. Don't trust Heyboer's color-coding, they often get it wrong. Swap the primary plate leads on the 7189's and the problem will likely go away.
 
Progress!
With the feedback reconnected to the 16 ohm terminals, and the lead to pins 7 of the 6bl8's disconnected (at the speaker terminal strip), the hum is gone, I can bias the tubes, although I didnt set the dc balance.
It is playing music now. I'm happy so far.
What does this tell us? And how would I proceed?
 
Swap the leads at pins 7 of the output tubes and reconnect the feedback to the 6BL8s. The oscillation should stop. You definitely want to have the feedback connected for the amp to perform the way it should.

To be clear, if you look at the schematic you posted, the brown and blue leads to the plates of the 7189s should be reversed.

What's happened is that the phase of the Heyboer replacement trannies is not the same as the originals. This isn't a big deal, it just means you have to swap the two plate leads in each channel.
 
Zach, If I swap the plate leads, why would that affect the problem?
Re Heyboer, I had a bit of trouble with the order. the leads were run out of both bells. Existing was one bell only. My old OPT was sent to them as an example when I returned them. Also ordered was a PT for a Triode specific version Dynaco ST70. In this case the leads were all out of one bell where the original had them coming out both bells. It wasnt the end of the world, just not right. I made it work. The takeaway... send an original or at least photos.
 
Success.
The thing appears to be working properly sans some channel imbalance.
So what I heard was not hum, but oscillation. I'll have to read up on that.
I'm so glad I followed my gut (not trusting myself) and only tacked the OPT's rather than cutting the leads to length.
Many thanks for all the expert assistance, and a very happy new year.
 
There is one other thing Im confused about. The schematic and the plate on the bottom of the amp give different designations for the tube numbers. Id appreciate if someone could clear this up for me.
Thanks
Steve
 

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It could be a revision. There are a few versions of the H.H.Scott 299.

A, B, C, D.

Except for different pin outs, the 6BL8 is like a 6AN8, 6U8A, 6GH8, 7199.
 
Looks like Sams numbers vs Scott numbers. The Scott schematics I've looked at use VXX for channel 1 and V1XX for channel 2, basically V2 and V102 are doing the same thing, just on opposite channels. Sams numbers just count up from V1.

That voltage chart also looks very Sams, not sure I've seen that on original HH Scott schematics
 
gadget73 said:
Shouldn't matter which end of the feedback is unhooked, different end of the same circuit. Unhooking either opens it.

the black lead is grounded yes?
Click to expand...
I called out the end on pin 7 as access is (or should be) be more straightforward.
 
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