Thanks for that check on yours. The black cap is only connecting to the ground lug on the can cap. It's in the bias cct, so only low voltage, not 450V.
zenith PP 6bq5 console amp
- Thread starter JGlenn
- Start date
Thanks for that check on yours. The black cap is only connecting to the ground lug on the can cap. It's in the bias cct, so only low voltage, not 450V.
coman61
Well-Known Member
I don't know if you need them or not. I read on another sight that a cap in the bias circuit like that helps take the noise out caused by heaters on the tubes.
If this is true then why not leave them in, however, I would relocate them. There is no need to have them anywhere near the Power supply. Look at the way its wired and I think you will find that those caps are in parallel with the 125 ohm resistors. Why not just move them down there in the first place? Use one for each pot (new ones). In the mean time just cut them out and see how it does. The way it is now you have a wire 3/4s the way across the amp that you don't need and a different ground that is also 3/4s across the amp that could be grounded at the same point as the 125 ohm resistor. Just cleanning up that mess may help.:scratch2:
If this is true then why not leave them in, however, I would relocate them. There is no need to have them anywhere near the Power supply. Look at the way its wired and I think you will find that those caps are in parallel with the 125 ohm resistors. Why not just move them down there in the first place? Use one for each pot (new ones). In the mean time just cut them out and see how it does. The way it is now you have a wire 3/4s the way across the amp that you don't need and a different ground that is also 3/4s across the amp that could be grounded at the same point as the 125 ohm resistor. Just cleanning up that mess may help.:scratch2:
I don't know if you need them or not. I read on another sight that a cap in the bias circuit like that helps take the noise out caused by heaters on the tubes.
Sounds good
I picked up some new ones at radio shack on my way home So im about to put those in. Thats a good idea to move those down with the resistors I think I will do that as well. I also picked up some better rca jacks so I can move the inputs as far away from the power tx as possible. I was able to get rid of 95% of the hum by lifting the ground on the preamp which common sense tells me to avoid if possible. I think I might have something fishy going on there with a ground loop or something, I never had any hum problems using this preamp with my magnavox amp so Im thinking its something in the amp itself. I have a cheap ground loop isolator I could try but I dont know if it would take away from the sound or anything.
Ive been listening to this amp in my rig for a few days now and have noticed that one channel isnt as loud as the other. All the bottom end kick like bass and drums seems to come from the louder channel as well. Ive tryed swapping inputs and speakers and it doesnt change sides so it has to be the amp. I've already tryed swapping tubes around with no change either. Should I start looked for an out of spec resistor in one side or something ?
Thespeakerdude8
Super Member
IT might be your OPT wiring, if you look at the schematic, Zenith did some weird stuff with those OPT's. I have no idea what its supposed to do, but I am going to replace my OPT's with some similar sized Woodward Shumacher 8k:4/8 from an old Conn Organ. How do the originals fare in the bass dept, and what is your approximate power output? Mine is on the back shelf right now, the chassis has been painted but I have a few other projects to get done first.
well the channel thats getting full bass output sounds pretty good to my ears. I think it has more bottom end then my maggie 175. I did notice the outputs trannies were wired together which I though was wierd but I didnt change any of it. Im guessing its around 8-10 WPC but with the right speakers its plenty loud. Its not so much that one channel is quieter its like only the lower freqs on that side are missing. There is a second set of taps on the tranny that had the terminal strip on top maybe I should try those.
Sounds a bit like phasing.
The left & right OPTs are not identical on that Zenith; the primary and secondary impedance values and ratios are different, and the right channel has the extra winding for the other set of outputs. The left OPT output is added to the right OPT via a center tap on its secondary. Is the speaker from the red wire the one with more bass?
BTW, those other outputs are on a 2x 1.4 ohm part of the right channel OPT secondary (with again a center feed from the left OPT) - possibly for remote/external speakers or 16 ohm speaker taps?
Some consoles mounted the bass speakers in the center part of the console, so it may be that if the original console had the L & R 12" bass speakers mounted like that, fairly close together inside, the L/R separation might not have been noticeable.
The left & right OPTs are not identical on that Zenith; the primary and secondary impedance values and ratios are different, and the right channel has the extra winding for the other set of outputs. The left OPT output is added to the right OPT via a center tap on its secondary. Is the speaker from the red wire the one with more bass?
BTW, those other outputs are on a 2x 1.4 ohm part of the right channel OPT secondary (with again a center feed from the left OPT) - possibly for remote/external speakers or 16 ohm speaker taps?
Some consoles mounted the bass speakers in the center part of the console, so it may be that if the original console had the L & R 12" bass speakers mounted like that, fairly close together inside, the L/R separation might not have been noticeable.
Thespeakerdude8 above said he was going to try doing that, so could be helpful to see what he planned, what success he has, and work with him. New transformers are available from Edcor, Hammond and others and, of course, used transformers sometimes come up on the 'bay. Otherwise, it might be good to start a new thread based on that modification (with a link to the existing schematic) rather than have people wade through this thread.
BTW, looking again at that schematic, the OPTs also run feedback to pins 4 & 6 on that 12-pin socket, so that was probably to some kind of surround/presence/ambiance/balance or whatever control feature on the preamp; also note that the R/C on the cathodes of the L & R channel ECC83s are slightly different: 56mmF/27Kohm vs. 47mmF/22Kohm, so the matching preamp may have been key to getting the right balance with the existing trannies...
coman61
Well-Known Member
I kinda came across the same thing. To me it seemed like the vocals always came from the same side. There is something weird about the way those transformers are tied together. I never did figure it out. Probally not to hard to change them out. I never persued it. If I ever come across the full schematic, the one with the tuner\preamp too maybe we could see whats up. I will keep looking. Whats strange is that it was on the Techpreservation site at one time. I don't know why it is not anymore.
Thespeakerdude8
Super Member
Replacing the OPT's is not hard at all. Gets rid of that silly set up they had and improves seperation. Yes, you will have to figure out how to set up your NFB though.
The NFB did go to a weird presence AND seperation control on the tuner. It gives a weird out of phase thing to the high frequencies and leaves the LF alone so the bass isnt cancelled out. I have no idea how it works, I have yet to trace the tuner schematic. I already gutted my chassis so it is waiting on new resistors/caps. I'll probably use the original schematic, which was a paraphase splitter. The amp with the 12AX7 and the 12AU7 has two cathodynes instead. Not sure which is best, but a paraphase doesn't make much sence to me in hifi. This is because the output of the first driver tube is sampled and then sent into the second tube, which has an inverted output as all tubes do, so it is "in phase" with the INPUT of the first driver tube, but out of phase with the first driver tubes OUTPUT. This is how it achieves phase inverting. Why does this seem like a bad idea? Well any distortion that happens in the first driver stage is going to be sampled and made worse in the second half stage. Doesn't seem to make much sence to me, I mean I'm sure our ears probably wouldn't notice. :scratch2: Just some though.
The NFB did go to a weird presence AND seperation control on the tuner. It gives a weird out of phase thing to the high frequencies and leaves the LF alone so the bass isnt cancelled out. I have no idea how it works, I have yet to trace the tuner schematic. I already gutted my chassis so it is waiting on new resistors/caps. I'll probably use the original schematic, which was a paraphase splitter. The amp with the 12AX7 and the 12AU7 has two cathodynes instead. Not sure which is best, but a paraphase doesn't make much sence to me in hifi. This is because the output of the first driver tube is sampled and then sent into the second tube, which has an inverted output as all tubes do, so it is "in phase" with the INPUT of the first driver tube, but out of phase with the first driver tubes OUTPUT. This is how it achieves phase inverting. Why does this seem like a bad idea? Well any distortion that happens in the first driver stage is going to be sampled and made worse in the second half stage. Doesn't seem to make much sence to me, I mean I'm sure our ears probably wouldn't notice. :scratch2: Just some though.
ltr317
Active Member
I don't remember. A friend borrow the amp for the holidays because his amp crapped out. When I get it back I'll take a look.