zenith PP 6bq5 console amp
- Thread starter JGlenn
- Start date
that did it ! thanks for all your help. I've still got some hum so I'll pick up a new death cap and some other caps to replace whats in tha cap can. should probably try to get a variac too huh, I'm still not understanding what made it spark like that when I went to plug it in. Is there a big in rush current when the circuit is powered on and that arcing just drew enough to pop the breaker ?
Hey, good work Coman. I missed all the fun. Glad you got it back running again.:tresbon:
coman61
Well-Known Member
We sure could have used your help.:yes:
Nah, I would have only slowed you down...you did great.:thmbsp:
This amp has a nice sound to it but not quite the power output I had expected. Its got about half the power or less than my pp 6v6 amp which doesnt seem right. I put in some new 12ax7s I had and that cleaned up the sound a bit so maybe the 6bq5s are just on there last legs. I wish I had or had access to a good tester.
The 6BQ5 should give the 6V6 a good run for the money; I like both those tubes. Are you comparing similar amps with same pre-amps & source?
If the OP tubes are getting tired, the Russian 6P14P (or better yet 6P14P-EV) are a bargain at ~$5 each and IMHO very good sounding.
If the OP tubes are getting tired, the Russian 6P14P (or better yet 6P14P-EV) are a bargain at ~$5 each and IMHO very good sounding.
Thats what I was thinking. I am using the same preamp but I had a different tt hooked up just because it was already in my workshop so I guess I should try it with my regular tt first before making any assumtions. my 6v6 amp is just a magnavox 175 so I figured it should be comparable but I could be wrong. The cap can is a 40 70 80 450v and I was just going to replace it with individual caps under the chasis but im having trouble finding all three from the same company , does it really make much difference? I would rather keep them the same if possible id there a better place to buy stuff like this online ?
There are many good places to buy caps online: AES (tubesandmore.com), Mouser, RadioDaze, justradios.com, Digikey etc. etc. Individual caps underneath are probably the least expensive and easiest way to go. You should be able to get those values (or close) from most of those online vendors.
If you wanted to do a new multi-section electrolytic can, you probably wouldn't find an exact match anyway, for example there's a 40/40/40uF 500V available but it runs ~$28 and you'd be adding 22-33uF or more to two of the sections to bring them up to spec. There's a JJ 100/100uF 500v can that runs ~$10, which you could possibly use to replace the 70uF & 80uF sections, then just add a 40uF underneath for the first cap section after the 5U4.
Whichever way you go, don't exceed 40uF for that first cap following the 5U4 rectifier.
Oh, and of course, make sure the existing caps are discharged before getting in there.
ltr317
Active Member
Wow, looks like I missed all the action since yesterday. I'm glad you got it working again, thanks to coman. I had a hum problem when I got my amp, until I changed the electrolytics. I used individual caps of 40 68 and 80, which I wired underneath the chassis. Got them from Just Radios. You can use higher values for the 70 and 80, might even give you better bass. You should also change the 0.1 coupling caps, as they are probably leaking after 50 years. I would also check the resistor values as specified on your schematic.
Also, after you recap and get better tubes, adjust the bias to zero or as close to zero as possible, as the bias pot is probably wonky by now. But be careful, there's not much room as the hole is pretty small. I don't want you to get shocked.
The output should be about the same as your Magnavox 175 or 93 series. I had a 175 and currently have two 93 series, so I've compared the volume among them.
Good luck.
cademan
Lunatic Member
Your original tubes may still be good. Re-cap and then see how it sounds. I re-capped my maggie 175 and am still using the original 50 year old magnavox tubes. That sucker really cranks! Different pre-amplification schemes makes a big difference on some of these tube amps as some don't have a pre-amp section built on the chassis. Some have better pre-amplification when using the original tuner that came with some of the maggies. The 175-67 came with a tuner/pre-amp where as the 185 did not, but had the extra 7025 tube on the chassis for the phono stage. I know your's is a zenith, but they basically work the same way.
Now heres something interesting, I brought the amp back out to where it popped the breaker the other night checked that the outlet was wired correctly first (it was) and plugged it in with the power switch off and the tubes lit and the amp came on. I flipped the switch and got a click but the amp stayyed on. I then checked polarity and realized that the chasis was hot and switched off the switch and unplugged the amp to reverse polarity. I plugged it in and it was off but when i turned on the power switch the breaker popped again. what could be causing this ? the amp plays when the chasis is hot and the power switch has no effect yet it pops the breaker every time when chasis is grounded. Its wierd that it doesnt do this in my workroom. The fuse in the amp never blows either. I'm thinking I have a bad capacitor but shouldnt it do the same thing regardless of where its plugged in if that were the case ? I have all new power supply caps on the way for it but still I want to know whats casuing this. I have a few things plugged in down there that I really dont want blowing up. I wouldnt be suprised if the house wiring was at fault but still this doesnt happen with my 6v6 amp or any other equipment I have fo that matter.
ltr317
Active Member
Yes, DC voltage. Look at your schematic notes for instructions. It should be similiar to my 7B31 chassis. All you need is a multimeter and small screwdriver. Alligator clips would be helpful for attaching the test leads on the chassis clips.
Sounds like something in the switch, power transformer, or primary wiring (between the wall cord and the primary of the transformer, anywhere in there) is shorted to the chassis. You might not be noticing it on the test bench, if the other piece you're using (as a source) does not have a ground...
I'd run a continuity test between the chassis and the power cord pins... if there's noticeable continuity (any lower impedance than mega-ohms) then you probably have a short.
Is there a "death cap" in there (a cap wired between any of the 117V power wiring and chassis ground)? If so, temporarily try disconnecting it and see what happens...
Regards,
Gordon.
Is the value of the death cap particularly important ? I have another .01 on order but can I sub something else close in for the time being for testing ? I used it with the same preamp on the bench and in the other room, it does have a ground. With the power switch on I do get some continuity between the blades on the power plug, maybe a new cord should be installed, I had planned on intalling a polarized plug anyways to make sure the chassis isnt ever hot.
You would expect a reading across the power plug blades when the power switch is on because that should be the transformer primary reading. I think Gordon was suggesting measuring from each blade (separately) to chassis ground.