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Beginning my foray into tube gear. Looking for some advice

GeneParmesan

GeneParmesan

New Member
Fisher29T-1.jpg

This past weekend I picked up a Fisher Philharmonic V (Contemporary) console at an estate sale. This unit has the Fisher 29T amplifier. I've been wanting a console project for a while now, and came upon this unit. The cabinet was in great shape, and was kept indoors (although in a basement) so it wasn't abused, just forgotten. This will be my first time working on tube gear, so I am pretty excited and really cautious at the same time.

The person running the estate sale said they had tried to turn the unit on (sigh), but it wouldn't power on, but the changer did turn. Once I got it home, I took off the back panel, put it on a DBT and Variac just to confirm what he said, and sure enough the unit didnt power on, but the changer did turn. I realized there was a fuse on the amp, and checked it. As expected, it was blown. Not wanting to do any further damage, I decided to remove the amp from the unit and take a look underneath since I was planning on recapping it (at a minimum) anyway.

The first think I noticed was the resistor circled in red, which I believe is R85 (330 Ohm). Well, well, looks like we got a party now! Takes a lot of voltage to fry and crack a resistor clean in half like that. A quick inventory and limited check of the tubes (using a DMM, I dont have a tube tester) I was able to verify they all seem OK. However, I did notice that the Getter on one of the ELL80's is gone (V7), and the glass looks like it is melted a bit on the side. I can post a picture of that. Of course, from my research, the ELL80's are the most expensive tubes in this amplifier. Great.

So the initial plan is to do a shotgun recap of all the electrolytics, (including the multi-cap can), and the fried resistor. I plan on bumping up the voltage levels on all the electrolytics, but keeping the capacitance the same. From there, I plan on removing the ELL80's and seeing if the unit will power up. (Of course I still plan on using the Variac and DBT).

So before I bite off more than I can chew I have a few questions to get me started.

1. The two caps pointed to with the purple arrows. Are these the "death capacitors" everyone talks about? I'm pretty sure R80 is because it is connected to a leg of the power cord and then to the chassis, but is R78 as well? Google tells me they are and I can safely remove them, but as a result it may introduce some noise. Just want to make sure removal is the proper way to go about this.

2. I also plan on replacing the 2 Diodes to the right of the failed resistor during the cap replacement. No relevant part numbers are given in the schematic, but since they are in the high voltage circuit, I plan on replacing those with a pair of 1N5408 which can handle 1000V/3A. I wanted to confirm that these are acceptable replacements.

3. I would like to replace the current power cord with something safer; either a polarized 2-prong plug, or even better a 3 prong grounded one. If so, what is the proper wiring method? The fuse is the red object in the picture. From what I can tell, the top post wire goes to the power switch, and the post on the side goes to a transformer winding? Again, I did ask Google about the proper method, but I got 3 different answers when I pressed, so I have no confidence in any of those answers now.

3. Is it worth adding a grounded 3-prong plug instead, and grounding it to the chassis? I have heard this would cause more noise.

If you read this far, thank you. I'm sure I will have more questions as things progress (and hopefully, they will). Also, if you can offer any advice, I would welcome it.
 
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Caps from line to chassis would be "death caps". If you aren't putting a 3 wire cord on this, I'd just sub these with modern safety caps. X1Y2 type would do it. I don't know for sure on the resistor without seeing the schematic but it looks like it is. I own one and possibly ought to know this but its been a long time since I messed with it. No cabinet for mine, its just the bare chassis and i use it as a shop stereo.

Stock diodes are probably not more than 500v at 0.5 amp, anything beyond that is fine. 1N4007 would do it, 1N5408 will too, its just even more overkill. If you've already got one in your parts bin, use it. I believe the ones in my 29T are still original though, silicon is pretty reliable.

Stock the wiring probably has the fuse on one side of the cord and the switch on the other. Without a polarized cord you're either fusing the neutral or switching it. Not uncommon, but not ideal. Personal opinion here, and consider those are always worth what you paid, but if I were going to some other type of cord, hot would go to the fuse, then to the switch. Switch to the transformer, other side of the transformer to neutral. That would ensure the switch or the fuse always breaks the hot. You'll have to map out the finer points of the record changer switching, I don't recall if its in the loop to shut the amp down at the end of a record stack. That wiring is gone on mine but I don't remember if I jumped it or just disconnected it.

If you do a grounded cord, same wiring, just plus a ground of course, and get rid of the line to chassis caps.

Noise isn't a given, but its more likely (IME anyway) in things with multiple ground connections. If this lives inside it's original box and isn't connected to an external grounded source, you're probably going to be fine. If you cable it to your TV with a grounded coax connection, then maybe it will have a ground loop hum. I've done tube amps with grounded cords and added them, the only ones that got bitchy were mono amps used in a stereo system. Lift one ground or unplug one of the input cables and they behaved, but a grounded cord plus a connection to each other through the RCA cables going to the preamp was a problem.
 
this is a fixed bias amp, but its not adjustable and has no means of monitoring current flow in stock form. If a coupling cap went leaky it could certainly do this too.

I modified mine to have adjustments and a bias test point but you can't really adjust it per-section easily since its got a shared cathode. It could be done if you were up for poking the meter into the high voltage stuff, but I'm generally not. I just set it per-channel and hope the sections are close enough to behave themselves with no fuss.

ELL80 tubes tend to run fairly hard too so it may just be significant run time. Its a decent amount of dissipation jammed into a small space.
 
gadget73 said:
Caps from line to chassis would be "death caps". If you aren't putting a 3 wire cord on this, I'd just sub these with modern safety caps. X1Y2 type would do it. I don't know for sure on the resistor without seeing the schematic but it looks like it is. I own one and possibly ought to know this but its been a long time since I messed with it. No cabinet for mine, its just the bare chassis and i use it as a shop stereo.

Stock diodes are probably not more than 500v at 0.5 amp, anything beyond that is fine. 1N4007 would do it, 1N5408 will too, its just even more overkill. If you've already got one in your parts bin, use it. I believe the ones in my 29T are still original though, silicon is pretty reliable.

Stock the wiring probably has the fuse on one side of the cord and the switch on the other. Without a polarized cord you're either fusing the neutral or switching it. Not uncommon, but not ideal. Personal opinion here, and consider those are always worth what you paid, but if I were going to some other type of cord, hot would go to the fuse, then to the switch. Switch to the transformer, other side of the transformer to neutral. That would ensure the switch or the fuse always breaks the hot. You'll have to map out the finer points of the record changer switching, I don't recall if its in the loop to shut the amp down at the end of a record stack. That wiring is gone on mine but I don't remember if I jumped it or just disconnected it.

If you do a grounded cord, same wiring, just plus a ground of course, and get rid of the line to chassis caps.

Noise isn't a given, but its more likely (IME anyway) in things with multiple ground connections. If this lives ide it's original box and isn't connected to an external grounded source, you're probably going to be fine. If you cable it to your TV with a grounded coax connection, then maybe it will have a ground loop hum. I've done tube amps with grounded cords and added them, the only ones that got bitchy were mono amps used in a stereo system. Lift one ground or unplug one of the input cables and they behaved, but a grounded cord plus a connection to each other through the RCA cables going to the preamp was a problem.
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Thank you so much for the detailed repsonse!

I was searching YouTube the other night and saw someone has a 120T sitting alone on a table for their setup. It looked really nice, but Im not sure I would leave it open like that. Here is the video

=======================================

I have no idea if this unit went south when the estate sale guy plugged it in or prior to that.

I tested all of the caps in circuit the other night, most read OK, a little low capacitance, and high ESR on most, but nothing appeared shorted. The only thing I found bad so far was that crispy resistor and one of the death caps was open.

I will remove the death caps, install the new caps and diodes when they get here and see what happens. Hopefully I will find some time to trace out the power cord thing and figure out how to do that, but at least I can use the existing power cord to test in the mean time.
I don't think the schematic has any color code info on the transformer wiring. I have been spoiled using modern schematics and printed circuit boards, so this is all new to me and it makes me it an incredibly slow process.

As for the tubes, I did see they make ELL80 -> dual EL95 adapters but I don't think there is enough clearance on the 29T to install them. I will bite the bullet and get a couple of new (or used) ELL80's if I can at least get this thing powered on without catching on fire.

Stay tuned, and again, thank you!
 
GeneParmesan said:
The first think I noticed was the resistor circled in red, which I believe is R85 (330 Ohm).
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Any chance that you posted the wrong photo? The thing you circled seems to be a fuse holder. Fisher used Red plastic fuse holders in the console amps.
 
grindfix said:
Any chance that you posted the wrong photo? The thing you circled seems to be a fuse holder. Fisher used Red plastic fuse holders in the console amps.
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The burnt resistor is circled in red.
The fuse holder (which is red) is circled in green
The death caps have purple arrows pointing to them.
 
I missed it when viewing on the phone. The blown resistor indeed appears to be R85, which is the dropping resistor for the output tubes' screen supply. Without screen (control grid) voltage, output tubes will not conduct, so no sound. Why it's blown and what else smoked in the excitement is something you will need to investigate before investing in parts and labor.
 
Forgot to mention, and you most likely know this, all circuits that are powered downstream of that burned resistor will not get power either.
 
@grindfix

Thanks so much for confirming the resistor, and suggesting checking the transformer.

Well, I'm kind of in a chicken/egg situation. Can really do much investigating without buying some parts, and can't see what's wrong without replacing some components.

If I am going to pull components anyway, might as well replace them. Everything I have tested in circuit so far has not been shorted, but I know that is not a valid test. So I will at least pull/replace the resistor and the big caps including the can cap, testing the old parts along the way and go from there.

I'm guessing either the 2-100uf Caps and/or the caps in the can are bad, or that ELL80 shorted.

Hopefully testing with the DBT and the ELL80's removed will protect everything new I put in.
 
You can Ohm-test each winding of all 3 transformers for opens and shorts. If all checks out good, replace the blown resistor and power up on DBT to monitor all vitals. You will need to install a 10-ohm resistor between each EL80 cathode and ground to monitor current flowing through the tube. Measure the voltage in mVDC across the resistor. Dividing the reading by 10 will give you the current draw in mA. Be safe - lethal voltages there. Use mini grabber leads for multimeters; don't use needle probes.
 
Thank you!

I would be lying of I said I'm not a even a little bit intimidated by this thing. It's kind of scaring the sh!t out me!
 
GeneParmesan said:
Here is a few pics of the ELL80 I mentioned in my first post.
View attachment 3821256View attachment 3821257
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Mullard tubes, dont know if they are new builds but I remember calling them muddy tops because of the oily looking tube I see some small acid etched numbers simular to the old mullards I have here at home those numbers can tell you the plant they were built at. something that i cant figure is did Mullard ever have a NOS plant in germany. mine say great Britain
 
You should respect voltages that can kill you - when measuring voltages, clip the minus lead to the chassis and mesure with one hand in your pocket. Or just clip in both meter leads with power off, then turn it on. Use the dim bulb tester until you're sure that any shorts are cleared.

The screens of the ELL80s and a filter cap are the only parts connected to that circuit node, and there are resistors in series with any downstream components. So the short is a tube or that cap - I'd say no other possibilities but a wire could get grounded somewhere. So measure (ohmmeter) across that filter cap - short? If so, pull the ELL80s one at a time. Still short? Shorted cap. If no short in the first place, it could be a breakdown that only occurs with high voltage.

No reason you couldn't power it up (with the DBT) with the halves of the fried resistor separated. You won't learn much but a gassy tube often won't even light, and the first B+ node will have voltage. Up to you whether to replace any parts first - check the diodes and filter caps at least.

I have a cap tester that can apply current-limited high voltage - connect it to the B+ (unit not powered) and measure voltages - shorts, leaky caps will be obvious. Of course, this is just as dangerous as turning it on, as the caps will fully charge - the cap tester has a discharge switch for that. A person could do the same by pulling the tubes and connecting a high value resistor (>100k) in place of the blown 330 Ohm resistor - there's no load on B+ so all filter caps should charge to full B+.
 
Success (so far)!

The parts finally arrived late last week. I removed the death caps, replaced every capacitor, diode and the burnt resistor and fired it up on the DBT first without the ELL80's installed. Everything looked good, so I moved over to mains power. I let it run for about 2 minutes and then powered it off.

I replaced the dial bulbs to get a little more voltage flowing through it and powered it back on (on mains with no ELL80's installed) .

I let it run for a good 5 minutes, everything appears to be work so far. All the bulbs glow, the display bulbs all light up, as well as the Magic Eye. I even got the stereo indicator light to work!

I'm still on the fence about replacing the power cord. Since it interfaces with the power switch and TT, I might just leave it alone now that the death caps have been removed. And honestly, the way its wired confuses me. Love to hear some thoughts on this.

Now, the hunt for an ELL80 begins!

Thanks!
 
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