When you remove the cathode resistors you render the output stage completely nonfunctional which is somewhat pointless, no path for current to flow hence no noise..
Think carefully about the electrical fundamentals at each step along the way and things will get much easier.
Think carefully about the electrical fundamentals at each step along the way and things will get much easier.
I removed the bias pot and soldered it back in - it has three legs, it only goes in one way - i cant see the top of the circuit board due to the construction of the amp - could i have unwittingly unsoldered something? or shorted something..? I can see how i could have reversed the plate lead connections - but i will re trace my steps..
Ah but the amp never came with cathode resistors - i only put them in to measure / set bias. Its a well publicised mod for this amp.. i need to find which channel is screeching..
The quickest way to check the pot is just to verify that you can adjust the bias balance grid to grid and that it looks to be approximately the same as on the good channel. (It did previously based on what you posted.)
Anything is possible. I concluded if I were to modify mine beyond the obvious things I already talked about it would be a complete gut job. When I complete my current list of obligations I might tackle a redesign of this amp, that's one of the reasons I bought one. (No board!)
Anything is possible. I concluded if I were to modify mine beyond the obvious things I already talked about it would be a complete gut job. When I complete my current list of obligations I might tackle a redesign of this amp, that's one of the reasons I bought one. (No board!)
Refer back to post 23 and make sure the plate leads are all still connected as shown in your pre-postmortem photograph.. 😉
One other comment is that the later amps have a pair of resistors in the plate leads that can be used to set both the balance and overall operating current without the need for an additional resistor - this is how the manufacturer set up the amplifier in the first place; however two meters are required and this is in fact how I will do it on mine which has the resistors. (On mine they are 1 ohm resistors, yours might be 0.2 - 0.24 ohms if present, you might want to look for them and make sure they are OK)
FIRE!
FIRE!
Ok , I've got to the bottom of the channel not working problem. I figured out the pre amp amp section wasn't working - put in some new small tubes and voila - back to normal. This was with the original 829bs that came withe the amp.
However, having these NOS 829B (US Navy) and with the amp and speakers on the bench, rather than lug it all back into the house, id thought id try the NOS 829bs as it would be easy to bias out here in the outside office.
So insert one Tube - then pop - smouldering smell and FIRE. The two resistors connecting pin 4 Cathode (Red black black - 20 ohm?) to ground smoldered and burst into flames. See photos. So there is something wrong with both these NOS tubes. I remember now that i lost the other channel when i tried the other NOS 829b.
So i need to get two new 20 ohm resistors (what wattage? they look chunky) - and *hope* nothing else has blown. The bias pot measures ok, all the other components i could get my DVM on check out ok, i can put in new pre amp tubes (have many of those)..
Question 1 - what in the valve would cause the cathode to dump a lot of current through those two resistors and then to ground? - what else could i have damaged..?
Queston 2 - any idea of the power rating of the replacement resistors.?
SO i have now bought 2 pairs of duff 829bs! The first flouresced like crazy, this pair takes down channels! Any advice on getting some 829bs that might actually work?!
cheers
FIRE!
Ok , I've got to the bottom of the channel not working problem. I figured out the pre amp amp section wasn't working - put in some new small tubes and voila - back to normal. This was with the original 829bs that came withe the amp.
However, having these NOS 829B (US Navy) and with the amp and speakers on the bench, rather than lug it all back into the house, id thought id try the NOS 829bs as it would be easy to bias out here in the outside office.
So insert one Tube - then pop - smouldering smell and FIRE. The two resistors connecting pin 4 Cathode (Red black black - 20 ohm?) to ground smoldered and burst into flames. See photos. So there is something wrong with both these NOS tubes. I remember now that i lost the other channel when i tried the other NOS 829b.
So i need to get two new 20 ohm resistors (what wattage? they look chunky) - and *hope* nothing else has blown. The bias pot measures ok, all the other components i could get my DVM on check out ok, i can put in new pre amp tubes (have many of those)..
Question 1 - what in the valve would cause the cathode to dump a lot of current through those two resistors and then to ground? - what else could i have damaged..?
Queston 2 - any idea of the power rating of the replacement resistors.?
SO i have now bought 2 pairs of duff 829bs! The first flouresced like crazy, this pair takes down channels! Any advice on getting some 829bs that might actually work?!
cheers
Attachments
Just to add, i had replaced all the small pre amp tubes before, but with no luck. As i have about 20 of these, i replaced them again with other NOS and this time it worked. If only there was an easy way to test a tube without owning a tube testing machine..
Itgoestoeleven, I have a pair of nice NOS USA 829b with Italian air force stickers on them. I think they have 1959 on the labels. I also have some Russian GU-29.
You can have them for the price of postage from the UK to Australia.
I'm away for just over a week so can't test them to be sure they are good.
Drop me a PM if your interested.
Cheers
Matt.
You can have them for the price of postage from the UK to Australia.
I'm away for just over a week so can't test them to be sure they are good.
Drop me a PM if your interested.
Cheers
Matt.
Also, looking at your last photo. Those contacts on the ceramic sockets look very dirty. Give them a clean with IPA and a tooth brush and pipe cleaner. Also clean the valve pins. I use a method that someone posted on here a while back. Get a used .22 bullet casing and stuff it full with Scotchbrite pad, push the valve pin into it gently and give it spin a few times.
Cheers
Matt.
Cheers
Matt.
Hi Matt - i am in the UK over Xmas, in Bristol , I'll drop you a PM and get the valves sent to an address in Bristol. Also, any ideas where to get a .22 bullet casing? - i'll ask around..
cheers
James
Ok that sounds perfect. I only live 40 mins from Bristol so I could drop them to you.
A piece of brass tubing will do fine maybe even a pen lid. I just happened to have a .22 casing in a box of brass sheet and stuff I inherited.
Cheers
Matt.
A piece of brass tubing will do fine maybe even a pen lid. I just happened to have a .22 casing in a box of brass sheet and stuff I inherited.
Cheers
Matt.
Hi itgoestoeleven
I have looked through my boxes of valves to find the 829's I thought I had and have found they were actually GU32 and QQE04-20's. My apologies, bad memory.
I do have a nice pair of NOS QQV06-40 that you are welcome to though. I think they should work fine in your amp and are quite beautiful valves.
Cheers
Matt.
I have looked through my boxes of valves to find the 829's I thought I had and have found they were actually GU32 and QQE04-20's. My apologies, bad memory.
I do have a nice pair of NOS QQV06-40 that you are welcome to though. I think they should work fine in your amp and are quite beautiful valves.
Cheers
Matt.
Attachments
Hi Matt
Greatings from Old Sodbury - sorry for long delay in replying - we have made it back to the westcountry via ireland and france despite plane cancellations due to fog.
I will be in Cheltenham sometime on Saturday seeing friends - i could pop in and buy these from you..? not sure what time yet - but a time to suit you for sure. Does that work? Would these work with my amp..?
Thanks, James
Greatings from Old Sodbury - sorry for long delay in replying - we have made it back to the westcountry via ireland and france despite plane cancellations due to fog.
I will be in Cheltenham sometime on Saturday seeing friends - i could pop in and buy these from you..? not sure what time yet - but a time to suit you for sure. Does that work? Would these work with my amp..?
Thanks, James
Hi James, if you are around in Cheltenham Saturday afternoon I could meet you. We are out during the day I'm afraid.
You can have them for free I wont be using them any time soon.
Cheers
Matt
You can have them for free I wont be using them any time soon.
Cheers
Matt
They should work just fine in your amp. The pin out is the same, the heaters draw slightly less current but otherwise they should be fine.
That is the QQV06-40's
Cheers
Matt
That is the QQV06-40's
Cheers
Matt
Hi Matt
Sorry to mess you around. My father took ill this afternoon (was the reason we came over) and i didn't get a chance to go up to Cheltenham. Tomorrow we leave to go back to Adelaide, (but that may not happen now) - if we stay on, let me see if i can get up to see you one evening this week.
cheers
James
Sorry to mess you around. My father took ill this afternoon (was the reason we came over) and i didn't get a chance to go up to Cheltenham. Tomorrow we leave to go back to Adelaide, (but that may not happen now) - if we stay on, let me see if i can get up to see you one evening this week.
cheers
James
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