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Help with H H Scott 299B

T

TubeGator

Member
Hi folks, first post here, hoping someone can help me with my 299B. I'm a little rusty in the tube department, my first training was in 1971 and that quickly moved on to transistors and IC's. I have had a wonderful (partially restored) 299B for about 12 years now and recently it got a bit scratchy sounding and one channel is a bit muddled/distorted. I dragged my Hickok 800 out of the attic and all the tubes tested good. I shot the rotary switches and pots with deox and that cleaned up the scratchiness. The B channel is still a bit reduced in volume and muddled. I shook the rust off my VTVM and checked the power supply voltages (all good) and then moved front to back checking plate voltages. On the A side (good side) all plate voltages checked good. On the B side (muddled side) all the plate voltages checked good except for the final push pull side of the 7189's. Bottom 7189 plate (and primary of the output transformer) reads 420V as it should, center tap of output transformer reads 430V as it should. The top 7189 plate and other primary tap of the output transformer reads 0V. So maybe I'm getting push but no pull? I unplugged the unit, removed the tubes. Resistance check of the primary side of the output transformer reads infinate ohms (almost like a cap would read, pops down then goes to infinate ohms) top tap-centertap-bottom tap. My A side output transformer primary reads 200 ohms from top and bottom to center. I'm thinking the output transformer is toast.

Do I have this right?
Any other checks I should do?
Suggestions on where to find a replacement?

thanks for any help!
 
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That is sure what it sounds like. I was working on a Bell 2200C with those same symptoms and the primary was open on that side. Check Barter Town to see if anyone has one. Watch ebay. Mine are otherwise occupied in my 299B. Love that amp so hope you can get yours going.
 
I'm just wondering what might have caused this OPT primary to go open circuit? Might be worth asking this question while you look. Others may have ideas of where to check.
 
That's always a good question and great advice, Dandy. The 2200C I mentioned had some sort of catastrophic failure that took out a couple of heavy duty power supply resistors and, I suspect, the output transformer primary as well. I suspected that a serious output tube failure took all that out, so I'd recommend being leery of the tubes that were in it prior to the failure, if you can identify them. Check and clean the tube socket pins, particularly for the output tubes. If the pins get dirty or corroded, you can lose bias or have other strange things happening around the outputs.

Check also the resistors around the output tubes. Make sure the proper bias voltages are present at control grids of the output tubes. Lack of bias can cause a red/orange-plated run away that can cause a tube to fail and maybe take an output transformer with it. Similarly, look for leaky coupler caps between the phase inverter/driver stage and the output tubes. If they start to leak and put DC on the grids of the output tubes, watch out.

Do some resistance checks of power supply resistors and look for signs of distress from heat. One of the failed ceramic wire wound resistors I found had a circumferential crack all the way around and was completely open. Distrust the electrolytic caps in the power supply string as well. If these go, they can take other things out as well. My experience with the 299B is that it is a hot-running little amp and the power supply components are run hard and stressed.

Look at the bias supply string as well. If there's a selenium rectifier bridge still installed, they will fail or just start to peter out with reduced output voltage and reduced/no bias for the output tubes. Also, the 18 ohm resistors in the bias supply string were often high in the 299B's I've restored, and in other Scott amps that use this type bias supply and that will further reduce the negative bias voltage.

An OPT can be lost due to a 'speaker event' as well. The Scott amps are stable, but if someone mis-treated it by leaving the outputs open (no speaker or load) or mis-connecting the speaker outputs, this can lead to a high voltage transient in the OPT that will cause it to fail. Have a look around the OPT secondary wiring as well for anything odd, particularly around the negative feedback R/C network coming off the output transformer 16 ohm winding.

That's a bunch of stuff (and I may have forgotten some), but Dandy is right. It would be tragic to get a new OPT and lose it to an undiagnosed cause that may have taken out its predecessor. Also, these things are good things to do when restoring these old amps to high-performing, reliable operation and with the 299B it is worth it. Good luck!
 
In addition to what is mentioned above, try taking the covers off of the old one and visually inspect for a break. Its possible that one lead came loose over the years and simply needs to be hooked back up. It cant hurt to check. Best of luck.
 
Re-reading your original post (good PS voltages and no apparent 'event' with speakers, etc), I think I'd focus first on bias supply and output tube couplers with a good cleaning of tube sockets. Also, I take it from your comments that the output tubes all tested good, but that doesn't necessarily mean that one had a transient arc and resumed normal operation, hence the 'not trusting' attitude on the tubes.
 
Dave451 said:
Re-reading your original post (good PS voltages and no apparent 'event' with speakers, etc), I think I'd focus first on bias supply and output tube couplers with a good cleaning of tube sockets. Also, I take it from your comments that the output tubes all tested good, but that doesn't necessarily mean that one had a transient arc and resumed normal operation, hence the 'not trusting' attitude on the tubes.
Click to expand...
Yes, I was wondering about that, while all checked good on tube tester, one 7189 looked very different on the dome, changed color and looked more see through.
 
The change in the look of the 'getter' (especially if disappearing) speaks to a well worn tube and one maybe in different shape that the others. Is that the one that was in the zero voltage position after the failure? Presume your Hickok 800 tests mutual conductance, right? An emission only tester may show a poor performing tube looking better than it is.
 
Dave451 said:
The change in the look of the 'getter' (especially if disappearing) speaks to a well worn tube and one maybe in different shape that the others. Is that the one that was in the zero voltage position after the failure? Presume your Hickok 800 tests mutual conductance, right? An emission only tester may show a poor performing tube looking better than it is.
Click to expand...
yes, the zero voltage position was under the changed look tube. Also, closer inspection of the 1K ohm bias resistor shows leaked substance I think from the .1 cap above it in the signal path. Can I disconnect the primary of the OPT and power up and check bias and balance of the 7189's?

and yes, the Hickok 800 tests mutual conductance, I'll look at that one again
 
Due set aside the tube that was in the socket with the open primary winding, as it is now likely ruined. It may still test as OK, but when the full level of screen grid voltage is present while no plate voltage is present, the screen grid then tries to pass all the current that normally would have flowed on to the plate but now does not because there is no plate voltage to attract it. Therefore, all current flows to the screen grid. The screen however is quite delicate, and cannot handle nearly that level of current flow, and is therefore either likely destroyed so that no current flows, or has been permanently deformed so that the characteristics of the tube are permanently and radically changed.

Once the OPT issue is resolved then, do make sure to address the tube issue as well.

Dave
 
well yes, the tainted 7189 was 50% low on the GM test :(, I missed that on the first try cause my tube tester meter is sticking a bit and requires tapping :(
 
If you're going to work with Heyboer, suggest you call them on the phone. I tried to contact them by email for a rewind and got no response, but a number of folks on this forum have gotten good service from them.
 
And -- if you go with them, make sure they rewind the transformer exactly as the original was built in every detail. The original transformers had significant interleaving of the primary and secondary windings, and this needs to be duplicated in the rewind. Winding (or rewinding) a quality output transformer is very complex compared to (by comparison) a simple power transformer. Whatever rewinder you chose needs to understand the importance of the details involved for a successful outcome.

Dave
 
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