| Mike K2 |
I am a complete neophyte when it comes to electronics. Ohms law is about the limit for me, but for some time I have been intriqued by tube sound.
I recently dug out a Scott 299B from storage that someone gave me a long time ago. A few years ago I had tried to set the bias and the rectifier tube failed. Since the only rectifiers I found were over $40 I gave up and put it away. I just recently found a $15 Russian made tube so I put it in.
The sound was very nice! I was encouraged and decided to try the bias adjustment again. It was very high, and even with the trimmer turned all the way toward the minimum, it was still way to high. If memory serves me, it was well over 40 volts across the bias resistor. I had read somewhere that the selenium rectifiers can go bad and cause this to happen.
I got the bright idea of replacing it with a silicon bridge rectifier. That must have been a mistake. The bias resistor cracked and got so hot that the ends of my meter leads melted! The voltage drop was well over 50 and climbing fast. One of the EL84s started getting a purple glow that didn't look good at all.
I though I might have wired the rectifier wrong so I double checked. It was right.
What am I missing in my ignorance? If anyone has suggestions, I would really appreciate them. |
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| EC8010 |
| The voltage dropped across a selenium rectifier is much higher than that dropped across a silicon rectifier, so when you replaced your rectifier you increased the voltage applied to the output valves. You may have damaged some other components as well. Can you post some photographs of the insides of the amplifier? |
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| Frank Berry |
Check the electrolytic capacitors in the bias supply.
I'll bet one section is shorted. Please beware .... this electrolytic "can" has a common positive.
You will probably need to replace the output tubes as well.
If you have not replaced the coupling capacitors on the grids of the output tubes, please do it.
Visit www.hhscott.com for more information and schematics. |
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| Mike K2 |
Darn.
It sounds like I am going to be spending some time and money on this.
Why does education have to be so expensive and hard?! I thought the silicon rectifier was a good suggestion. Looking back on it, is bringing the amp up to voltage slowly with a Variac while monitoring the bias voltage the way to try new components?
EC8010- I will see if I can manage a photo. What area in particular are you interested in?
Frank- Is there a convenient diagram showing the location of the caps that you mentioned, or do I need to find them by reading the schematic and tracing the wires?
Thanks for the responses! |
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| JandG |
| My 299B had the silicon rect. allready installed when I got the amp, but I am sure there is a resistor change that goes with the swap out, go to the Scott web site & down load the schematic, also I am sure someone over there could answer questions on this specific amp, be carefull when messin with caps man...I am sure there are caps that need to be replaced.. they don't need to be expensive. just take your time, ...mine is gone thru & is a outstanding sounding amp, especially the phono stage..Well worth spending the time & a liitle money on.. |
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| EC8010 |
| Well, a few good general shots of the insides may show damaged components. As others have said, you're probably going to have to replace all the capacitors. The electrolytics will have dried out and gone low capacitance and high ESR, whilst the coupling capacitors may have gone leaky. It's not a big deal, modern low ESR electrolytics are far better than the old ones, and polypropylene coupling capacitors will also be far better. Yes, you need to add a series resistor to the output of your silicon rectifier - someone with more experience of selenium should be able to give you a suitable resistor value and rating. And yes, a variac would have been a good idea. |
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| Mike K2 |
OK, I attempted to include some photos of the amp. The resistor closeup is an attempt to show the cracks in the bias resistor that are now present.
Oops.
Uhmm, is it not possible to include multiple images in the same post?
If it is, would someone please explain how?
For now I will limit it to one shot showing the entire bottom and the face panel. |
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| Mike K2 |
| Sorry, the photo is actually one of the power supply area. I don't think at this resolution there is much chance of seeing any problems short of something actually being on fire. |
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| EC8010 |
| It's not so much resolution that's the problem, it's the lighting (or lack of it). Try to position the amplifier in front of a window and at window height. I also use a scrap of sheet aluminium as a reflector to get extra light where I want it. |
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| kevinkr |
As someone else pointed out in his post the bias supply electrolytic can is common positive.. Note that this supply also provides power to a series string of 12AX7 in the front end. (Phono stage as memory serves.)
Those cans are now unobtainium because of the inverted polarity. To retain the original look and because of the limited amount of under chassis space you might want to consider gutting this can and installing modern replacement caps inside it - there should be more than enough room to do so if you choose appropriately sized caps.
There may well be a bad cap section in the filter circuit, it is also possible that you swapped connections on the bridge. IIRC the original was a siemens selenium bridge..
I've had several of these and imo it is definitely worth fixing. Hopefully the output tubes weren't really damaged as they are actually 7189A and almost no current production 6BQ5 is going to survive in the output section of this amplifier for more than a couple of minutes. IMO The Russian made NOS 6P14P-EV is the only tube that might stand up long term in this amplifier.
I have a heavily modified 299B, and a Realistic Stereolyne 40 and these both seem to work ok with either 7189A or 6P14P-EV.. JJ 6BQ5 did not work in either of these amplifiers. Both have plate voltage on the order of 380V - 400V depending on line voltage.. |
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| Mike K2 |
EC8010,
See if this image is better. The light does help a lot.
kevinkr,
The output tubes are 6BQ5s in my amp. The 7189As are really hard to find in my experience. I was under the impression that most people running 299Bs were using 6BQ5s as a sub. I have heard of certain brands being able to handle the extra voltage. Is this a myth?
The 6P14P-EV that you mention; Where are they obtained? I saw a listing on E-bay, but the seller was a little vague about them. His feedback wasn't all that great, either.
I think that I am going to go for it and try rebuilding this thing. It will be interesting and with a little luck, maybe I'll end up with a nice amp!
I just hope I don't wear out my welcome here with questions.
Thanks. |
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| KBK |
E84L tube is a high voltage EL84, the Sovtek's are supposedly of this type.THere are other companies, like the 'Electro Harmonix' brands, as well. Basically out of Russian and similar factories.
Go to 'the parts connexion', or 'The Tube Store' for the skinny on the required output tubes, and reasonably priced matched sets. (ie, $50 or thereabouts for a quad set)
Ebay also has many Russian tubes you can buy which handle this B+ voltage level, but you'd have to know exactly what you are buying. |
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| EC8010 |
That amplifier looks remarkably like an EMI555 (or something, the one with the CRT) that I used to have. I wonder who copied whom?
Your photography is improving, but aren't those curtains I see in the background? Natural light is your friend. |
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| kevinkr |
HH Scott developed that styling idiom back in the very, very early 1950's when they were based in Cambridge. Scott gear used aluminum chassis and was one of the very few American companies to do so..
Is that an A or a B? My B has different neon lamps in the front panel, so if a B it must have been an early one.
EC8010 I know the EMI amplifier you make reference to, it's extremely rare and expensive here, and because of the type of resistors (low noise cracked carbon?) used I have never seen one in proper working order - so I haven't heard one unfortunately. Scope was definitely a cool and unusual feature.. Looked like a rather nice bit of kit..
The 6P14P-EV is available from multiple vendors on eBay, I got mine from a vendor based in the Ukraine Alexer1 who I now buy things from frequently.. Shipping is fast, items well packed, and the seller is extremely reliable. Look in his eBay store. |
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| Mike K2 |
I just ordered 8 6P14Ps from Alexer1. Thanks for the tip, kevikr. Incedently, I had noticed the front panel isn't what was shown as a 299B. It has the oval logo, which is shown on the A series on the HH scott.com site. What makes me think it is a B is the device layout. The 4 power tubes are in a line left to right, which is very different from the A's front to back layout. Assuming that their pictures are right, of coarse.
EC8010- The picture was taken at night. Opening the curtains would have just added some window glare.
KBK- Thanks for the E84L suggestion. I hadn't heard of it. I guess the scarcity of true 7189As isn't such a concern after all. |
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| EC8010 |
| quote: | Originally posted by kevinkr
I know the EMI amplifier you make reference to, it's extremely rare and expensive here, |
Bah! I was happy to get £80 for it at a vintage fair - and that after I'd done some work on it.
| quote: | Originally posted by Mike K2
The picture was taken at night. Opening the curtains would have just added some window glare. |
Fair enough... |
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| kevinkr |
| The last one I knew anything about went for about $750 a couple of years ago. That's one of the reasons why I don't own one. Guess that's the difference between something from your own back yard and something comparatively rare and exotic.. I've seen them go on eBay for obscene money as well. |
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| frank754 |
I picked up an 1955 HH Scott 311-A FM tuner off of Ebay a week or so ago for around $50. Since I already built a few hifi tube amps I figured why use solid state for the tuner? I like the brass and copper look, and it works fine, I see that a few caps had been replaced over the years, only 1 old one left, plus the can cap has not been changed.
There's a great shrine site: www.hhscott.com
with a lot of history & photos of all the models. I spent quite a bit of time there the other day. |
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| JandG |
| The far front left CC resistor is the one that needs changed I think , I can look if you want..I have been running Ei el84M elite's without problem..They sound excellent as most Ei does, but long term reliablilty is hit & miss on late production runs. I think it is a Tele copy. The russian versions seem like a good deal. I would ditch all coupling caps while your at it,, nothing fancy is needed...it will still sound excellent..I really like the phono stage in it. I definatelly would keep that unit, it is many, many peoples favorite of all Scott's.............. |
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| Mike K2 |
I went through the 299B last night and tried to list all of the capacitors in the thing.
5 multiple value electrolytics on top of the chassis. These I definately plan to replace.
Other caps that I found were:
10 Sprague orange drop film (?) caps that don't look original.
25 caps similar to the Spragues in appearance except for being dark brown.
10 ceramic tubes with wax ends, some marked "Ceracap" that are rated at 400V and .001 to .0068 mfd.
2 ceramic tubes with wax ends, 2500V and .001 mfd.
4 black plastic cylinders, 400V and .022 to .047 mfd.
I didn't count the small ceramic disc caps.
Would someone be so kind as to advise me on which should be replaced and what they should be replaced with? The electrolytics are a given for replacement, but is there any particular type that is best?
JandG,
What front left CC resistor? Maybe I'm showing my ignorance, but what is a CC resistor? Perhaps you were addressing someone else. |
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| JandG |
| looks like someone been in there before, you might well have 35 new caps in there. Check them all..get rid of all ceracaps & check the cans well.. The CC is a Carbon Comp resistor in the front far left, that is the one that needs changed, but go to the Scott board for a value & correct rectifier..That one you have there is not even close to what o got,, might be o.k., but I would ask them..or post a schematic here for one of these tube fanatics that hang here. I am not qualified to get technical with ya. If the 35 film caps are new, then you are way further on the re-build, What tube rect. are you using..? My coupling caps are nothing special, no-name film caps,..& still this amp is excellent , especially with vinyl. That is all I use it for. I might change all coupling to Vitamin Q, simply cause I like them in my other gear. Gutting the cans is a good idea mentioned, that is what I will do with the remaining in mine that need changed. I am not modding mine.. I have enough modded gear.. actually all my gear is modded, so a stock vinatge unit of this type suits me well. & I had a quad of Tele's in there, but yanked them & put them in some Klangfilm SE mono's.. so I can get the most use out of them..way to expensive to risk burn up..All the ceramic & plastic & what ever else old caps in there should go to dumpster..just use proper rating & research what type..I think PP films would be just fine of proper voltage rating..& could be done very cheaply. Go read the Scott bourd & post for some info also. |
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| Mike K2 |
Unfortunately, many of the replacement caps are in bad shape. One seperated from the leads as I was trying to read the value on the side.
Just have to replace them all, it would seem.
I've found several replacements (MWR series) at the Illinois Capacitor website, but I'm a little confused about something. There are several small brown dipped caps that are about .60" by .40" by .15" with values on the side ranging from 47 to 560. Am I right to think that these units are picofarads? They seem way too small for it to be microfads. I don't find any caps of that value on th IC website but I could be missing something.
Any advice would be much appreciated. |
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| Mike K2 |
After digging around the HH Scott website, I've figured out that this amp is an A type rather than a B type as I had thought. It turns out that the late A's had the tubes lined up left to right just like the B's.
I also think that it has one output transformer from an A and one from a B. I'm guessing that one of the original A transformers failed and was replaced with a B (larger in size) transformer. So JandG is right, someone has been there before. What effect will this have on the sound when I finish restoring it? Should I give up? The thing sounded nice and I'm inclined to go ahead with it. Perhaps another transformer could be found?
I'm also still hoping (please, please) for an answer to my earlier question about the small dipped capacitors. Picofarads? I don't find any film capacitors with that small a value. Do I have to use ceramics? I have heard that they suck for audio, so I would rather use film. |
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| Eli Duttman |
| quote: | | Do I have to use ceramics? |
Definitely not. You can use mica or polystyrene dielectric parts. |
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| JandG |
| No, I would not bag the project because of the OT's. Do the restore & then if the OT's give you trouble down the road Heyboer Transformer's does these trannie sets I think, ..I would e-mail them & see what they can do...I use there trannies in extremely nice guitar amps & they are good. Mnay people use the company in hi-fi also.. |
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| Mike K2 |
Hello all,
Thanks for the encouragement (I think) on my 299A project. Here is the latest.
I just finished replacing all the caps and many of the resistors. I powered it up slowly this time using a Variac. When it got to 90 volts, it made a light popping sound and I noticed a few small flashes in the rectifier tube near the bottom. I dialed it down to 70 or 80 volts and it stopped for a while but then started to do it again. I shut it off then, but the fuse had already blown.
Next, I tried powering up with the output tubes removed. It went right up to 120 volts and stayed there for some time without a problem.
I then tried it with only the left channel tubes in place. The problem returned.
I tried again, but with only the right channel tubes in place. The problem was still there.
I should also add that this is the same rectifier tube that I was using when I had the earlier incident with excess bias voltage. Do any of you tube experts have an idea as to what the problem is? Should I try another rectifier or would I be jeopordizing it because of some stupid mistake I could have made when I installed all of those caps and resistors? |
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| kevinkr |
| Replace the rectifier and see where you are at that point. Anytime that a 5AR4 arcs over it is highly likely that it has been damaged and may arc again under load. (Usually when this happens to me they are forever a dead short.) |
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| Mike K2 |
OK, I have ordered 2 new rectifiers.
When I try this again, will it just destroy a brand new rectifier or is the old rectifier the problem? |
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| kevinkr |
Before installing the new rectifier double check that the bias voltage is present at the pins of your output tubes which should be 7189A or equivalent. Something on the order of -15V is what I would expect to see.
Inserting a lamp in series with the ac line (extension cord with 100 - 250W lamp connected in series) will limit the current in the event of a fault to a safe value and in most cases prevent the destruction of components like the rectifier.
I suspect the rectifier itself was damaged and this is showing up under load, however it is also possible that your output tubes are not biased correctly and look something like a short circuit to the power supply. |
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| Mike K2 |
| Thanks, Kevin. I'll try that when I get the new rectifiers. |
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