Fisher 500c Channel Imbalance?

I forgot... Is it OK to use wirewound resistors for the 10 ohm cathode connections? I meant to order metal oxide and inadvertently ordered wirewound.
 
larryderouin said:
Cut the lead from pin 3 and insert the 10ohm resistor in SERIES btwn the tube and the lead. DO NOT GROUND THE Circuit. The 10ohm resistors will NOT affect the circuit. Testing is accomplished by metering ACROSS THE RESISTOR, not to ground like on a 7591/7868 fixed bias.
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Larry - What do I do on the pins where the TWO wires are connected? Do I insert the resistor between BOTH wires tied together and the pin? OR insert the resistor between just ONE of the wires and leave the other wire connected to the pin? For example V4 pin3 has wires from V5 pin3 (which already has a resistor on it) and V105 pin3 that has no resistor. I'm inclined to think I insert the resistor only between the V105 wire and the V4 pin3.

And what about the wire coming from the cap can to the 1st pin3 in the 7189 series? Do I insert a resistor there too? I would think yes...so that I can measure the voltage coming to that tube as well.
 
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I don't want to get anyone mad for asking about a Scott unit on the Fisher forum, but it's a Scott 299 version 2. I figured it can be viewed as a general question not specific to any manufacturer. I asked here because of the expertise.
 
No problem. Let me look up the scat for it. Sounds quite similar to the KX200 scat in that area. But I'll take a look at the scat to make sure.
 
I found the scat from VTV12 and marked up for the cathode resistors. Basically 1 side(channel...repeat for other channel). Post a picture of the can cap to 1st tube setup. And the V104 to V105 setup. Are you absolutely sure you have Pin 3 and not confusing it with pin 7. Pin3 should go to ground eventually according to the scat. Pin 7 on V105 and V5 have 2 leads. 1 lead goes to the Primary side of the output (plate voltage) with the 2nd going to the secondary side thru a .001uf/2500v cap. This is why I say you probably are confused by Pin 3 vs. pin 7. Find the GAP between pin 1 and pin 9 (use a BIG magnifier!!!!) Orient the GAP to the bottom. Pin 1 is approx. at 7:00 and pin 9 is at approx 5:00. GO CLOCKWISE! This is looking at the socket from the BOTTOM, NOT the top.
 

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Thanks, Larry. Am I understanding that your drawing shows 2 resistors in series with each other? That's not how I inserted. I inserted so that there is only one 10 ohmer in line between each pin 3 lead and its pin. V104's pin 3 comes from a can cap and I put a 10 ohmer between the can terminal from which the lead emanates and V104's pin 3 which is my question if that's correct. I'm wondering if that resistor will affect the cap operations to V104.

As for pin 3 versus 7, the pin numbers are engraved on the bottom of the sockets so I'm not confusing the pin numbers. I put the 10 ohmers in line of the pin 3 leads. Should they be on the pin 7 leads?

By the way I have this 3-LED helmet magnifier that is fabulous for working on these components. It is head-adjustable, the light can be positioned as desired and there are around 6 different magnifiers that can be mounted on it, 2 at a time plus an 8X "pull-down". There's no need for any other external light.
https://www.amazon.com/gp/product/B07DW6FJ6S/ref=ppx_yo_dt_b_asin_title_o04_s00?ie=UTF8&th=1

PS - While I'm at it, I should probably add screen grid resistors. IIRC, 100 ohm 1/2w on pin 9 on each 7189? But resistor at pin 9 attached to which other pin? Do I move the pin 3 leads to an unused pin, join them together again and add the resistor from the unused pin to pin 3? Ah, but if I do this each tube won't get the same bias current anymore, right? Which means IBAM. I should leave it as is then.
 
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According to the schematic there is a grounding lead original equipment, (or should be) between the 10 ohm resistors, going off to the other side of the unit (I'd follow the lead to ground.) Have you got the correct schematic for your unit? If so send a copy to me so I can see what the heck is going on. Your lead back to the can cap could be a ground (if the lead terminates on the outer ring tabs and the cap is grounded direct to the chassis. NO RESISTOR on that lead. If all the pin 3 leads daisy chain back to V105 or V5 then you need to remove the 2nd lead from the pin #3, set aside as we'll come back to it.

Remove the leads from Pin # 3 on each tube, and install a 10 ohm resistor on each tube pin #3. The leads you removed from pin 3 go on the other side of the 10 ohm resistor. If V4 daisy chains to V5, the daisy chain goes on the outer end (lead end ) of the resistor. It seems to me that you are getting confused with the cathode grounding vs. the 10 ohm resistor setup. Remove everything from V4-V5-V104-V105 PIN 3 AFTER doing a HAND OVER HAND TRACE BACK of the Wire to the Can cap. If it's on one of the OUTER TABS, and another lead from the outer Tab continues on to a chassis Ground or the OUTER TAB is actually thru a CHASSIS(METAL hole)slot, then it's a ground.

Original Setup.
ScreenHunter 90.jpg

New Setup with 10 ohm resistors installed. It's literally the same construct as the screen daisy chain setup as on the 400-500c-800c only you're doing the cathode to ground vs. feeding the screen with B+. I got the tube #'s backwards on this one. The 1st pic is correct for #'s.
ScreenHunter 89.jpg
 
dcgillespie said:
Keith -- The difference is rather small so ultimately it's nothing to really stay awake at night over. Regarding your data:

1. Yes, R38. My schematic is small enough that sometimes the numbers look similar.

2. I assume your R40 and R47 are 2.8 M (not K)?

3. I assume your using a frequency of 1 kHz?

4. Forgot to measure R41 and R42 (not 40/41)?

One other test to try is again measuring at the Reverb jumpers, but turn the tape monitor switch on -- that will remove most of the following stage load to remove it as a variable.

When trying to make channels identical (it's tedious to make all the stages cross-identical between the channels), there's nothing wrong with bridging (in this case) the appropriate R38 or R39 with a high value resistance to make the gain equal between the two channels for those stage. Differences in tube sections can cause slight differences as well, so a different tube may help. Ultimately, the reason that there is a difference in level after C31 and C33 is because that is the point at which the NFB around these stages is introduced. That's why the balance of the various resistors mentioned between the two channels will have a definite effect of the signal level observed at this point.

Dave
Click to expand...

If there is an audible channel imbalance, is it attributed to drifting components? Can the section be measured without a scope and if so, is there some value that would indicate things are way out?
 
Linehand said:
If there is an audible channel imbalance, is it attributed to drifting components? Can the section be measured without a scope and if so, is there some value that would indicate things are way out?
Click to expand...

An audible channel difference could be attributed to a number of causes: a dirty pot, bad tube, loose solder joint, bad cable, or out of spec components.
If you don't have a scope, you can still rule things out by mechanical means. I would first go though and clean/tighten all your tube sockets, and also try to verify that all your tubes are actually good.
Then you pinpoint things down by seeing if the imbalance happens on every input. Then you can flip preamp or output tubes and see if the problem "follows the tube" or if it stays the same no matter where the tubes are.
Then you might see where the issue is happening in the circuit by seeing if certain settings make a difference, but you'd have to give specifics as to when and how the problem is happening to eliminate entire sections.
So no, you don't need a scope, but a DMM would be handy for eliminating bad components later.
-k
 
If you are a subscriber you can do it yourself. On the right side above the ORIGINAL POST is a link that reads "THREAD TOOLS". Click on that and then click on EDIT TITLE. Do the Edit and click OK. Sometimes there is hope for "Old dog's & New Tricks". :biggrin::biggrin:
 
larryderouin said:
If you are a subscriber you can do it yourself. On the right side above the ORIGINAL POST is a link that reads "THREAD TOOLS". Click on that and then click on EDIT TITLE. Do the Edit and click OK. Sometimes there is hope for "Old dog's & New Tricks". :biggrin::biggrin:
Click to expand...

Thanks, Lar!
 
Regarding channel imbalance in general, I have one on one of my devices. Per my thread in another forum, I put a 1k 5v signal through the TUNER input on both channels, hooked up a dummy 8 ohm load, turned up the volume to a normal listening level (all other controls centered or off) and ran a scope. I attached the leads to the cathodes on the 7189 output tubes and to the cathodes on the preamp 12AX7 tubes and the channel imbalance showed up in both places. Although the curves clearly showed a difference in amplitude, I measured the amplitude and Vpp of both channels. Are those the stats one should measure to diagnose a channel imbalance?

I continued to back up per the schematic and finally found that when I measured on the input and output sides of a resistor, the imbalance existed on the output side but not on the input side. I concluded that the resistor was the problem although it was in tolerance. It's a 470k resistor and it measured 412k on one channel and 428k on the other. The amplitude on the input side of the resistor in question showed 35v and 37v for Vpp. On the output side amplitude was 5.4v and 7.6v for Vpp while the same resistor on the other channel showed 12v for amplitude and 16v for Vpp.

However, when I replaced the resistor, the imbalance was not fixed. Can someone please comment on what I might be doing wrong to diagnose the imbalance? Thanks, Thorne

PS - Forgot to say that the way I measured was to place the black lead on chassis ground and the red lead on the component being measured.
 
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thornev said:
Regarding channel imbalance in general, I have one on one of my devices. Per my thread in another forum, I put a 1k 5v signal through the TUNER input on both channels, hooked up a dummy 8 ohm load, turned up the volume to a normal listening level (all other controls centered or off) and ran a scope. I attached the leads to the cathodes on the 7189 output tubes and to the cathodes on the preamp 12AX7 tubes and the channel imbalance showed up in both places. Although the curves clearly showed a difference in amplitude, I measured the amplitude and Vpp of both channels. Are those the stats one should measure to diagnose a channel imbalance?

I continued to back up per the schematic and finally found that when I measured on the input and output sides of a resistor, the imbalance existed on the output side but not on the input side. I concluded that the resistor was the problem although it was in tolerance. It's a 470k resistor and it measured 412k on one channel and 428k on the other. The amplitude on the input side of the resistor in question showed 35v and 37v for Vpp. On the output side amplitude was 5.4v and 7.6v for Vpp while the same resistor on the other channel showed 12v for amplitude and 16v for Vpp.

However, when I replaced the resistor, the imbalance was not fixed. Can someone please comment on what I might be doing wrong to diagnose the imbalance? Thanks, Thorne

PS - Forgot to say that the way I measured was to place the black lead on chassis ground and the red lead on the component being measured.
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Hey Thorne, I’d need to know what unit you’re talking about and see a schematic to offer any advice. 5v seems like a lot to be putting into the receiver. Is the imbalance only audible on the tuner section? Plus, I’m a little confused about which measurements apply to where in the signal path.
-k
 
Thanks, bacon. It's a Scott 299 version 2. Schematic is here on page 24: https://www.scribd.com/document/197730181/Vacuum-Tube-Valley-magazine-VTV12

The imbalance is audible on the TUNER and TAPE inputs (haven't tried PHONO inputs). The resistor I referenced that I thought was the cause of the imbalance is R16 (or R116 - I can't tell from the schematic which one it is as they are the same on each channel and tracing the myriad wires is very challenging) which I believe is in the PREAMP section before 12AX7 tubes V2 and V102. The measurements I referenced, amplitude and Vpp (peak to peak voltage), are measurements on the scope I have. I had both channels' sine waves displayed on the scope and besides the measurements, one wave was clearly smaller vertically (peak to peak) than the other. Does that help?
 
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Put a scope on again last night, still trying to track down channel imbalance. Surprise last night was at pin1 of 6BL8 tubes. Amplitude on channel A was 16.1 vdc and on channel B was 18.7 vdc. Back at R16/R116 the imbalance was not present. Replacing R16 two nights ago moved the imbalance further toward the speaker outputs. I conclude that there must be more than one issue in this chassis. Time to start pecking again. Comments anyone?
 
thornev said:
Put a scope on again last night, still trying to track down channel imbalance. Surprise last night was at pin1 of 6BL8 tubes. Amplitude on channel A was 16.1 vdc and on channel B was 18.7 vdc. Back at R16/R116 the imbalance was not present. Replacing R16 two nights ago moved the imbalance further toward the speaker outputs. I conclude that there must be more than one issue in this chassis. Time to start pecking again. Comments anyone?
Click to expand...

Thorne, I would start a new thread in the tube forums to get some more input. You mentioned 16.1 & 18.7 vDC, but the 1k sine wave on your scope is an AC signal, so I'm not sure exactly what you're measuring. Sorry I can't be more helpful.
 
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