Weird.... Thoughts?

[Jbeaudin]

Which Deoxit are y'all using? I've seen a few different ones, not sure which one works the best in this scenario...

[Jondog]

For tube sockets use the D series. If you plan to do the potentiometers use the Deoxit Fader F series . Is the "popping" sound prolem an issue thats been resolved by repair, or still a problem?

[Jbeaudin]

Nothing has been repaired yet... have some NOS 12at7's on order. When they get here I'll clean everything up and throw them in hopefully it will fix any issues I've had. I do find the tone to be very much on the treble spectrum. It could need new power tubes. It I am hoping these two 12af7's will solve some tonal problems as well.

[Jondog]

I'd suspect power tubes for the bass missing, and would look into changing all the tubes if they haven't been changed. The MKl has been out of production for a while now, so if they are the original tubes I'd say they've had a good life. Also, 12AT7's are going to be different then the 12AX7's, have about 60% of the gain and maybe not as bright.

[Jbeaudin]

For sure, the amp has had several tube changes in the last 10 years I've had it now. But the time between now and the last tube change is very few hours. I'd say probably less then 150hrs on the amp... shouldn't tubes last much longer then that? Has anyone else tried KT66's in this head? Are they a lot like KT88's but with less power?

Thanks!

[Jbeaudin]

Alright, so in a few weeks I hope to have this all resolved. I ordered up some JJ KT77's and some NOS ECC81's. Hopefully I get my amp back after the retube is complete. Anyone have opinions on what percentage to bias these tubes at?

[Jondog]

I had an RV50 and thought it sounded best around 60%. But, different amp, different tubes.

[pedecamp]

Which Deoxit are y'all using? I've seen a few different ones, not sure which one works the best in this scenario...

I did a lot of research on this awhile ago so I don't remember but I settled on the red spray can of D100. Says safe on plastics, improves metal to metal connections, unlimited uses on all electronic equipment. Also says wait 2 minutes before turning on equipment after application.

[pedecamp]

Alright, so in a few weeks I hope to have this all resolved. I ordered up some JJ KT77's and some NOS ECC81's. Hopefully I get my amp back after the retube is complete. Anyone have opinions on what percentage to bias these tubes at?

A popular bias is 38 mA for a 50 watt or double that for a 100 watt. I forget what percentage this is somewhere between 60-70%. 77s are good tubes, I had them in a Marshall I used to have.

[Jbeaudin]

This evening I replaced the power tubes with KT77's and biased to 70%. I still got buzzing and popping, volume drops, all of the above. So I tested the speaker out and I'm 110% sure it's blown since I had similar issues when I plugged another head into the cab.

Is it possible for a speaker to "stall" and produce no sound at all? As well, will a speaker on its way out exhibit thin and treble sounding tone more then warm thicker tone?


Thanks.

[Les Paul Lover]

This evening I replaced the power tubes with KT77's and biased to 70%. I still got buzzing and popping, volume drops, all of the above. So I tested the speaker out and I'm 110% sure it's blown since I had similar issues when I plugged another head into the cab.

Is it possible for a speaker to "stall" and produce no sound at all? As well, will a speaker on its way out exhibit thin and treble sounding tone more then warm thicker tone?


Thanks.

Man, that sucks. Yes, it's all possible.

I wouldn't use that cab until I was sure, you're not sure if that speaker exhibits the proper load for your amp anymore (you can test that with your multimeter though.

Crap, you've just dumped a small fortune on valves...... Hopefully that's all the issue is. Do you have accès to another cab? To make sure your head works just fine?

Do you have a muso friend who could let you use theirs to check it out?

What wattage was the speaker in your cab? Do you have any ideas how it might have got damaged ?

[Jbeaudin]

I think I know exactly what happened, but it took a ton of research and learning to figure it out.

Apparently, the largest cause of voice coil damage is caused by surges of current/electricity (I think current is the right one). Anyway, when I had issues of popping due to the power supply of my pedal board turning it off and on could have been all it took for those "surges" to do the speaker in. What I read, is that running your amp hard/loud typically will not damage the speaker, it has to do with surges...

The speaker is 40 years old, 1970's Fane Studio L 16 Ohm 200 Watts definitely more speaker then it needed (new ones in Canada cost $450... So this one is probably worth a small fortune since its vintage.) But, if I were to have the voice coil replaced, all vintage value would be lost since it is not original, and it's getting pretty hard to find a speaker repair guy these days...

I might try my hand at selling it on Reverb with a disclaimer to see what I can get for it.

You touched on one thing I need clarification on, with the speaker cabinet potentially putting out the wrong resistance, what could that possibly do to the amp? And would the symptoms be all or none? Such as will I lose tone due to a component wearing out, or will a component just fail entirely?

I'm going to check the resistance on the cab as soon as I press send because I am curious.

Thanks for all the help!

[Jbeaudin]

Ok, I just checked the speaker impedance and it is coming out at 10.6 Ohm's which tells me part of the voice coil is most likely shorting out making current easier to travel through thus reducing resistance. As well, I found that I have a weak solder joint which will be rectified when a new speaker goes in.

[jontheid]

A popular bias is 38 mA for a 50 watt or double that for a 100 watt. I forget what percentage this is somewhere between 60-70%.

This is not good advice at all.

You don't double the bias current for a 100 Watt amp.
The bias current is the current flowing from plate to cathode per tube in a fixed-bias amp.
If the tubes are well matched then the current will be the same in each tube.
Doubling the number of power tubes does not double the bias current.

Also saying 38mA equates to 60-70% bias is nonsense.
Bias current calculations depend upon plate voltage, which vary from design to design, and indeed from amp to amp of the same design. It also depends upon the maximum plate dissipation value of the type of power tube, which varies significantly.

If your plate voltage is 427V, then 38mA gives you 65% bias for an EL34. This is the same whether your amp uses 1, 2, 4, 8, 13, or 29 EL34s in the output stage.

Please don't give biasing advice when you obviously don't understand the principles involved.
Biasing a 100Watt EL34-based amp to 76mA will melt the output tubes.
Also biasing a 50 Watt 6V6 based amp (Rockerverb 50) to 38mA won't do any good either.

Thanks,
Jon

[Jbeaudin]

A popular bias is 38 mA for a 50 watt or double that for a 100 watt. I forget what percentage this is somewhere between 60-70%.

This is not good advice at all.

You don't double the bias current for a 100 Watt amp.
The bias current is the current flowing from plate to cathode per tube in a fixed-bias amp.
If the tubes are well matched then the current will be the same in each tube.
Doubling the number of power tubes does not double the bias current.

Also saying 38mA equates to 60-70% bias is nonsense.
Bias current calculations depend upon plate voltage, which vary from design to design, and indeed from amp to amp of the same design. It also depends upon the maximum plate dissipation value of the type of power tube, which varies significantly.

If your plate voltage is 427V, then 38mA gives you 65% bias for an EL34. This is the same whether your amp uses 1, 2, 4, 8, 13, or 29 EL34s in the output stage.

Please don't give biasing advice when you obviously don't understand the principles involved.
Biasing a 100Watt EL34-based amp to 76mA will melt the output tubes.
Also biasing a 50 Watt 6V6 based amp (Rockerverb 50) to 38mA won't do any good either.

Thanks,
Jon

Amen to that! I didn't listen to that advice. I did a ton of research before biasing the tubes. 440v plate 25 watt per tube - biased at 40mA.

Does anyone know if putting the multimeter to ohms and putting the leads on the speaker will result in the listed ohms on the speaker being the same as what the multimeter reads, or does there need to be a signal from an amp sent to get a proper reading?