first tubes pioneer sm 801

Sorry about the way those voltages came out in the last post. I'll try this.


283
104
115
0
0
104
33.4
signal ac
1.12

That's pin 1-9 of the right channel.

200
177
177
0
0
177
5.02
ac
0.36

1-9 of weak channel.
 
Hi Greg,

That's your driver/splitter tube before the output tubes, and so they can't be doing anything if indeed the heater voltage for each is 0 between pins 4 and 5 . as it should be 6.3V or very close to that V. If they hopefully light up you should see in maybe dim light a glow in the very center of the tube in the thin hollow tube at the top and bottom. But since your getting good volume out of the right channel you must be measuring the heater voltage wrong.

I don't know about the other voltages except the high ones aren't over spec if you look at the tube data sheets, but if all the resistors and caps are in spec they should end up okay if the tubes are half decent I would imagine. Sansui 500A/1000A receivers and Dynaco Mark III amps use that tube for the same purpose. The Fisher 500C uses 12AX7's in that position and a number of others in the amp section.

Google will get tube data sheets.
 
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Or download TDSL Personal Edition. http://www.duncanamps.com/tdslpe/ It'll give you all the info you need including pinout's with the nomenclature. You can look up almost any tube made.


a 6an8 is a triode/pentode (two tubes in 1). Each side has a different function. pins 1 thru 3 are the triode, 6 thru 9 are the pentode and 4-5 are the heater.
On the 6AN8 pinout is as follows:
Legend: a=anode (plate), g=grid, k=kathode european or Cathode for US, h=heater or filament, g2= 2nd grid (or screen)

1 = a''
2 = g''
3 = k''
4 = h
5 = h
6 = a
7 = g2
8 = g
9 = k+g3
taken from TDSL
attachment.php


All the HEATERS / FILAMENTS on the pioneer are AC Voltage. Either you measured them in DC or you have no voltage to them. Check them again in AC.

Learn how to read the pin #'s on the sockets. Generally #1 is to the left of the space or locating pin(on an octal) and they go around clockwise to 7, 8, or 9 depending on the type of tube(couterclockwise on the top of the socket). Look at the sockets really close. There are usually numbers right next to the pin on the bottom.

Swap V5 and V6. Check all the voltages again. If the Bum voltages follow the tube, get a pair of new 6AN8's. If it doesn't then you've got problems in the circuit.

The pots you are replacing are Tube BALANCE POTS. They balance the tube load in the pair. On a push pull output you want the tubes to be as close as possible when making power. One tube that isn't pulling it's weight throws all the load on the other tube. The pots will even this out. These are not BIAS pots and there are no bias pots on the 801. Which is why I put that blurb in there about the bias pot replacement.

Take a picture of these "HUM BALANCE" pots, and trace them out exactly to which pin on which tubes. I believe you're getting VR8 and VR9 (hum balance) confused with the output balance control. VR8 and VR9 are 2 pots in the Heater circuits that are best left alone unless there is a hum with the volume down, and then they get adjusted for minimum or no hum in the heater as they are AC.

ARE YOU READING THE SCHEMATICS? Doesn't seem to me that you are at times. If you are I apologize, but some of the questions and statements you are making lead me to believe either you aren't or you are getting confused by it, and don't understand it. If you are having problems let us know.

SLOW DOWN! Everyone gets the 1st time jitters, but in the end 99% of the time all it does is cause more problems due to mistakes, and getting ahead of the help. The closer you get to the end the more you want to rush. This is the time to SLOW DOWN, Step back, take a few deep breaths, take a walk to clear your head. Slow, methodical work will get you a fully working unit faster than trying to rush it....(think the turtle and the rabbit. Who won the race?) Quite a few of us are "retired" but all that means is SWMBO is now the boss and she don't pay too good. Plus you're never done. So hold on, we'll get to you within 24 hours or so.

Swap the 6an8's and recheck the voltages. If they move, replace the tubes to get a matched set. If not start checking resistors (lift one leg), and check all the electrolytic cap polarities in the area. Also check grounds.

BTW Pin 8 on the 6an8 is the input lead. You won't get a votlage on it unless a signal is going thru it. Ohm out from pin 8 to the center tap of the BALANCE CONTROL, and you see close to zero.zero ohms.
 

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Yes, my schematic reading skills are pretty bad, although way better than when I started this project.

And good advice on not getting too excited and rushing into more mistakes.

I'll add another, no late nights after a couple beers.

So in going over the circuits for v-5, v-6, I found some issues with c-21, c-45.
They are polar caps and should be 3uf @ 150v and the one on the left (weak) is correct however, the other one is 3.3uf @160v. That doesn't seem like enough off to be my problem but I'll get the right one.

The other thing was c-45 was not clearly marked for polarity but I noticed that the electrolytics seem to have a rubber end and a metal end. Anyway I pulled it checked on the cap checker on my vow it seemed O.K. so I flipped it and put it back, and my voltages are more close too each other now.

The " hum balancers" on the back panel seem to adjust the heater voltages . They are : v5 pin4= 3.2v pin5= 2.8v V6 pin4=3.0 pin5= 2.8v. Are these low? the tubes do glow.

Swapping the tubes did not change the problem. Flipping the cap did seem to make the channels a little closer in out put.

The following are my voltages across the 10 ohm resistor on the cathodes of the output tubes: V7= 250mV V8= 263mV V9= 284mV v10= 293mV.
 
A shot of the hum balancers.

I should add that at this point it sounds good, no distortion. The left channel is lower than the right but not by much. I think overall output is still low, this thing has power meters and I only get about 1/4 of the range.
 

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Congrats!

How does it compare to the Fisher volume wise?

I don't think being slightly off on values of those caps should matter as long as they are good new ones, and as you probably know going to higher in voltage rated ones doesn't hurt and is probably a good thing with today's higher line voltages. The higher voltage rated ones may last longer also.

Maybe your lower power is from what looks like to be lower bias settings that you can't adjust, but probably great for very long output tube life. If you want to add adjustment capability one could maybe copy the schematic for the 4 pot Sansui 1000A found on the AK database. The amp part of that receiver looks to be very much the same in the PS area and driver splitter tube area.

Actually looking at your bias readings for one pair of tubes for the the one channel (V7 & V8) probably accounts for that channel being a bit lower in volume. I think that should be the case.

Maybe a fresh set of 6AN8A'S may help also. I don't know if you tried swapping 12AX7'S from your Fisher as that may help also.

I think adding those heater voltages from pins 4 and 5 may be the way to get the correct V which is close to 6V as maybe your measuring each one to ground? What do you get measuring between pins 4 & 5?
 
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The volume is much lower than the Fisher. At low, like backround sort of levels it sounds O.K. At higher levels it sounds thin, no power on the bass the highs get kinda shrill and its got no punch.

I tried swapping in some 12ax7's and not much difference in sound, the heaters did seem to be brighter.

Are there any things to check that might confirm problems in the 6AN8's?

Were they subject to the same abuse that the power tubes received when they were run with no bias and low grid voltage?

Measuring between pins 4 & 5 gets the correct 6.3 Vac.
 
Great! Perfect heater V. I don't think the 6AN8A's would have suffered from the bias problems, but they could be getting tired. Good to have spares around anyway.

I could be wrong. but your bias readings look low especially in the one channel.

Do the low ones in the one channel correspond to the lower volume in one of the channels?

Here's a good mod from Larry in post #9 and info on Bias settings.

http://www.audiokarma.org/forums/showthread.php?t=584041

Maybe try different control settings as this fellow found:

http://www.audiokarma.org/forums/showthread.php?t=303253
 
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Yes, the lower bias readings are from the channel with less output.

I have some 6an8's on the way, so we'll see what that does.
 
DO NOT SWAP in a 12AX7 for a 6an8. They are two totally different tubes. The 12ax7 is a HI mu TWIN TRIODE. The 6an8 is a Med mu Triode/Sharp Cutoff pentode. You can do serious damage to the unit, or blow the tube.

C21 and C45. 3.3uf is the modern replacement value for 3uf. remember most caps back then had a variance tolerance of upwards of +50% to -20%. Using that a 3uf cap could be as high as 4.5uf to a low of 2.4uf. the 3.3 is fine and what's is needed there. Don't waste the $$ unless it's bad. Most all caps today have a 20%+/- tolerance. Unless they are classified as 10% or even 5%. I've seen 1% but they get expensive.

The rubber end is the positive side. So that side would go to the screen side on V5-V6. Positive normally goes to the side with the higher voltage. In this case that holds true.



The hum balance pots just balance the AC between the heater pins. When it's off too much (for example the total may be 6.3, but pin 4 is getting 4.2v and pin 5 2.1, the AC isn't balanced and it'll hum. So you adjust out the hum, by basically listening to the speaker.

I had the same problem in my 1000A. Low volume, thin sounding over a certain point on volume, and lower output on one side. Swapping the 6an8's didn't change it from side to side. But a new set of 6an8's solved the problem. They will break down even tho they tested ok.

The bias is a bit low. Install 2 10K .5w pots between the junction of R96/C55 (point "K" at power supply) and replace the current wire from point "K" to the balance pots with a separate wire to the same place for each pair. Now you have paired bias instead of no adjustment for the tubes(see post 222 Dave's comments) This way you can either bias UP the lower pair to match the higher pair or vice versa. See drawing (excuse my lack of computer drawing skills, I do better with pencil and paper)
attachment.php

Input to the wiper (Pin 2) and output to pin 3. Adjust for ZERO RESISTANCE. Connect your DMM Leads to pin 6 on the WORST output tube(showing the least ma....V7), and chassis ground. Turn on, with the function set to AUX, VOLUME OFF, all other controls centered or off, and warm it up for a minute or two. Take initial pin voltages pins 3-4-5-6 The pin 6 voltage will be more negative than it was (which is good as you don't want to turn them on HARD and blow them). switch the probe to pin 5 and slowly and with very small movements bring the pot up toward center keeping an eye on the meter. you'll be reading mv. bring it up to approx 30ma(.300v). Then check the pin 3 and 4 voltages again.

Now with the meter leads on pin 5 of each tube of the pair (meter to DCV) adjust the BALANCE POT until you get a NULL or 0.00v reading. Once that is done, retake the bias readings for that pair. They should be equal.

REPEAT for the other side. Adjust the lower or higher pair to match the other pair.

You're procedure for the future with the new EH tubes would be.
1.) Take pin 3 thru 6 voltages for all output tubes.
2.) probes to pin 5 on each pair and balance using BAL POT.
3.) Adjust bias on one tube of the pair(as they are paired and balanced, what you adjust to one also takes place on the other of the pair). Verify comparable readings on both tubes.
4.) repeat steps 2-3 for the other pair.
 

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Hey Larry,

Don't worry, I wasn't going swap 6an8's for 12ax7's, I'm just going replace the existing 6an8's with new ones incase they are responsible for my low output.

Check my understanding here.

The bias circuit you drew allows me to both balance bias between unmatched tubes, as well as changing the resistance of the pairs to increase the current draw to achieve ~30mA.

Also just making sure you are aware that the figures I gave for the current draw was with the new matched Quad of EH7591's.

Is it the case that a "matched Quad" should mean these tubes were selected from a whole bunch and matched for the current they draw? If so does the fact that a matched set drawing different currents means that there is still something wrong in the circuit?

Also should I first try the new 6AN8's before building the bias circuit?
 
For the bias circuit your recommending 10k 1/2 watt pots.

Would the 5k 1/2 watt trim pots that I used for the balance trimmers work here?

There is not much room in there, and I have a couple extra, so it would be great if I could use these little guys.
 
5k pots aren't going to give you the voltage range needed to bias them sufficiently. It's like putting a governor on a car throttle and expecting to get full range. Not gonna happen.

V7= 250mV V8= 263mV V9= 284mV v10= 293mV
If AES matches their tubes to within 20% these are fine. But ideally they should all be within 10%. There's about 14% between the highest and lowest. You won't notice much if any difference between channels when it's running right, but when someone says matched, I expect a bit tighter tolerances. This makes the case for adding bias/balance adjustment circuit.

What was the plate and bias voltage when you took these readings?

Yes you'll be able to balance the pair to a happy medium with the Balance pot, then after calculating 75% of max dissipation using ohms law, you bias the tubes to that point or just below. The bias voltage becomes moot at this point because you aren't using it to set the bias of the tubes. You see the results currently.

75% is 14.25W @ 400Vplate voltage 356mv is 14.24w. 35.0ma to 35.5ma (350 to 355mv)is close enough. If you can't getthe lower set to bias that high, set for the highest (not to exceed 35.5ma) and then adjust the other pair to match. OBVIOUSLY your mileage will vary as your plate voltage most certainly will be different and the numbers will end up different.

Put the 6AN8's in now.

Larry
 
The 6AN8's should be getting here today and I want to get this bias circuit built.

Can I use a 10k ohm, 15 turn, cermet potentiometer for this. Or do I need to use the full size linear pot?
 
cermet pot is fine. Actually better as it's not so twitchy. Make sure it's mounted securely.
 
O.K., my Bias / Balance circuit is installed voltages are:


Bias is -21.5 on all tubes.

Pin 3 : V7 = 500v , V8 = 496v , V9 = 492v , V10 = 492v


Pin 4 : V7 = 431v , V8 = 431v , V9 = 426v , V10 = 426v


Pin 5 : V7 = 300mv, V8 = 300mv , V9 = 298mv , V10 = 301mv


Pin 6 : V7 =-21.6 , V8 =-21.5 , V9 =-21.1 , V10 =-21.1


Those are all @ 120V at the wall. With N.O.S 6an8's installed.


So my plate voltage seems high?

Still sounding week, a little worse than before.

Not that I would know, but these numbers don't seem too out of line for the amp section. So, may it be the case that the remaining problem is in the preamp section?

I have used my I pod with RCA cables into a SS amplifier, using the I-pod volume controll the I pod is basically a "preamp"? Anyway as a means to diagnose if the problem is in the pre-amp can I use the I pod to put a signal to pin 8 of the 6an8 tubes?

Also, when the amp is on and the lights off all the tubes of the amp section glow bright but little to no glow from the 12AX7's in the pre amp section.

I replaced 2 of them (all I had) they are brighter but it sounds the same.

Getting closer. Thanks for sticking with me.
 
1.)Your screens are a bit low compared to the plates (in 4 to pin 3), but that may be how PIONEER set it up. Also the plate voltages are a little high, but again, pioneer may have set it up like that. It's not that far off from what my Sansui 1000A runs the plates at.

Take voltages at the can caps and at the junction of R95/D4/C52, and R95/C51. Post them.

IPOD's have high output voltages that can overload the inputs causing distortion. Better if you used a worst a CD Player, better a cassette deck or R2R that have Line level voltages under 300mv. Or a function generator with an adjustable output level. If you must use the ipod just use it as normal, ie: plugged in to the AUX jack, with the volume turned way down.

If your preamp section has not been worked on yet, go ahead and recap it all, and check each and every resistor (it's gets tedious, I know but without a scope and such, about the only way to do it.)

Depending on how the tube is constructed with regard to how exposed the filament is, the tube will glow bright or dim. 12ax7's filaments are pretty well covered up and you won't see a lot of glow. DON'T go by this. Go by the actual filament voltage at the heater(filament) pins.

Have you got another integrated amp or receiver? If so make a signal tracer from an RCA CABLE and a .01uf 600V film cap. Solder the lead on the cap to the center lead on the RCA. The free end of the cap is now your probe. Plug in to the AUX jack on the other amp/receiver. Turn down volume. Turn on the pioneer with a cassette tape or normal line level output in aux and work back from the coupling cap on the 7591's listening to the results. The sound will clear up before the bad part when working back from the output's. and sound like crap after the part(toward the output). Compare both sides L to R.

If you have Audacity or one of those programs, make a file of a 1kHz tone(sine wave). 0db, and time for 10 minutes. Save the file then play it thru the computer, out thru an output cable to the pioneer. Turn DOWN the volume on the soundcard and the pioneer to a LOW listenable level. This will be a lot better than the IPOD as far as voltage match at the inputs and won't tend to overload it causing distortion. Then use the cap probe you made above to probe the circuits back from the outputs.
 
Hi Larry,

I have already recapped the pre-amp section. When I first started this project I didn't know the amp from the preamp or even that right and left channel are symmetrical. I just went through and replaced everything that looked like a cap with the shotgun approach. If I ever do something like this again I think I will be more systematic and go section by section testing in between.

We are going out of town for the week so I'll get you those voltages when we get back.

Perhaps I should have mention this before, but a while ago, passing through the thrift store that got me started down this whole vintage audio road, they had a scope for $5 so I grabbed it. I just have no idea how to use it. So are you rolling your eyes and thinking," Oh geez now I gotta teach this guy how to use a scope?"

Anyway, have a good thanksgiving and thanks again for all your help, I'll try to find a book about scope basics.

Cheers,

Greg
 
Hey Greg, top marks for enthusiasm! This Pioneer repair may seem endless, but you are learning as you go along, and this will help you with your next amp repair... there will be others. :) Have a good thanksgiving break.
 
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