Looking at a AB764 Bronco schematic, Fender placed 420vdc at the first power rail connection (to the OT). That's with a 5Y3GT, and only 117vac household voltage! Todays household volts can climb as high as 135vac....I guess they wanted to cook a 6V6GTA as hard as they could. They must have thought "Oh well, tubes are cheap. Go ahead and roast them for all they're worth". Good idea for you to roll it down a bit if vintage gear is to be saved.
Hi. If you are interested, I spent about two years intensely studying and working on three Champ amps: a 1973 Champ, a 1965 Vibrochamp, and a 1968 Bronco. The high plate voltage is typical, partly explained by the increase in A.C. line voltage over the years. There are two keys to this dilemma: The main cause seems to be a highly unregulated output from the power transformer. It seems to me Fender selected a power transformer that must have been selected for it's cost; so it is a "cheap" transformer for Fender to purchase. Secondly, this is also the power transformer for the Princeton and Princeton Reverb amplifiers. Those amps put a larger load on the power transformer, lowering plate voltages and the preamp voltages. I found two "fixes" for this. Interested??
Nice work Brad increasing the resistor. Were you tempted at all to increase the value of the rest of inline B+ resistors to compensate for the transformers unusually high output voltage? I enjoyed your spanish noodling too.
+Dan79istheman Yeah, a little tempted, but I let it cook like that for probably an hour and a half and everything seemed stable, so I let it go at that.
Those 70s Silverface Champ transformers are famous for putting out very high voltages. Leave as is. Just make sure you use a vintage NOS 6V6. I use a Sylvania 6V6 and it rocks. Modern JJs with the thicker glass that can handle 14 watts also sound good.
You should replace the first resistor on the power rail to 1K in order to bring the plate supply voltage down on that 6V6. You'd reduce the potential for overheating components along with making the amp sound much smoother. The older Champs all had between 350-360VDC on the 6V6.
+Garage Workshop Thanks for watching and taking the time to comment! I really appreciate it. I do a lot of videos, so please be sure to subscribe and stay in touch!
The Guitologist well i was wondering what kind of speaker considering ur playing style youd suggest i have the same one one in this vid is that an oford? mine seems to be in fairly original condition so in terms of value only is it wise to upgrade even lol i have a ton more but these are my main concerns on this hey btw thanks i know they r simple questions ...im not that knowledgible obviously but i am trying to learn so im sorry if they seem dumb thanks for taking time to reply ur work is impressive ....cheerz
My Champ had the same blown resistor. Replaced and it still sounds too bright. Anything you can think of to tame the treble? My other Champ sounds more mellower. Swapped tubes and speaker etc...still too bright. I replaced electrolytic caps too.
I've studied electronics, but how do you go on to learn what you do? I've got a faulty isp pro rack g that's cracking, you've inspired me to have a go at this, but as for valves how do you learn that stuff? thanks.
That death cap is on the fuse right? I think mine has been removed if so. Also what is the light blue cap that says, paxtron .047m 125 ac dvl? It is just to the left of the power transformer and is not present in the model you are looking at. How do I find out what year mine is? I do not see a date stamp anywhere.
I think the pilot light needs to be unsoldered and removed for that to come off. Maybe one of my viewers can answer that for sure. I do know you don't want to go tugging on it. They bend very easily. A rubber mallet can be used on a flat surface to straighten them back out though.
I currently have a '62 Gibson amp that is also having problems with excessively high voltage. I am getting ~400 DCV on the plates of the 6L6's when my variac is at 105VAC... At ~125VAC (wall volts here in Bloomington), the B+ is 480 DCV at the plates of the 6L6 (which imo is too hot, though OKAY), but the filter caps are only rated to 450V... Is there something I can do to fix this short of getting a new PT? Do you have any idea what's going on? I've been at this guy for a while and near pulling my hair out... Thanks for this video!
+Britt Rossman You can use a Zener diode on the power transformer secondary. There are tutorials online, although some people claim this approach compromises tone, I'm not sure either way. You could go with a different tube with higher max plate voltage and replace filter caps with 500V rated ones and raise the value of the resistors on the power rail like I did in this video to reduce the voltages downstream.
+The Guitologist The B+ taps directly from the Pi-filter to the CT of the OPT, with the next node being through an 0A2 regulator. No dropping resistors, just the choke...could it be bad? Everything beyond the regulator is just fine AFA voltage is concerned. According to the schematic, however, it is supposed to drop 75V, but I read that it drops almost 150V, which is some concern. But I guess I'd rather be reading 245V to the preamp tubes (before plate resistor) than over 300V...
+Britt Rossman Your choke is probably fine, and honestly, your voltages might be workable as long as your bias is correct. I would change at least the power cap on the first node to 500V power handling though and maybe go ahead and replace the cathode bias resistor (if there is one) to a hefty 5W to be on the safe side.
+Britt Rossman Also, make sure you're measuring your plate voltage from plate to cathode, not plate to ground when biasing your output tubes. I've made that mistake before and miscalculated.
+The Guitologist I think I'll start with the bias. I havent yet rebiased since dealing with bad resistors, coupling and filter caps... And the B+ CT was not grounded... no wonder there are so many issues here!
Could someone tell me what the values are on the blue caps? I can see mine are 600volt but the big blue cap all the way to the left has the value rubbed off.
Looking at a AB764 Bronco schematic, Fender placed 420vdc at the first power rail connection (to the OT). That's with a 5Y3GT, and only 117vac household voltage! Todays household volts can climb as high as 135vac....I guess they wanted to cook a 6V6GTA as hard as they could. They must have thought "Oh well, tubes are cheap. Go ahead and roast them for all they're worth". Good idea for you to roll it down a bit if vintage gear is to be saved.
Hi. If you are interested, I spent about two years intensely studying and working on three Champ amps: a 1973 Champ, a 1965 Vibrochamp, and a 1968 Bronco. The high plate voltage is typical, partly explained by the increase in A.C. line voltage over the years. There are two keys to this dilemma: The main cause seems to be a highly unregulated output from the power transformer. It seems to me Fender selected a power transformer that must have been selected for it's cost; so it is a "cheap" transformer for Fender to purchase. Secondly, this is also the power transformer for the Princeton and Princeton Reverb amplifiers. Those amps put a larger load on the power transformer, lowering plate voltages and the preamp voltages. I found two "fixes" for this. Interested??
Yes what's the answer?
absolutely interested!
With the over voltage it sounds like a few windings are shorted on the transformers Primary.
Have you ever heard back from this customer/individual on this Amp? I'm curious as all hell why the plate voltage was so high.
Nice work Brad increasing the resistor. Were you tempted at all to increase the value of the rest of inline B+ resistors to compensate for the transformers unusually high output voltage? I enjoyed your spanish noodling too.
+Dan79istheman Yeah, a little tempted, but I let it cook like that for probably an hour and a half and everything seemed stable, so I let it go at that.
Your variac is made for 110 coming out of the wall. Test the voltage in your home. And subtract the difference. Your variac is correct.
You would think he knew that
Those 70s Silverface Champ transformers are famous for putting out very high voltages. Leave as is. Just make sure you use a vintage NOS 6V6. I use a Sylvania 6V6 and it rocks. Modern JJs with the thicker glass that can handle 14 watts also sound good.
You should replace the first resistor on the power rail to 1K in order to bring the plate supply voltage down on that 6V6. You'd reduce the potential for overheating components along with making the amp sound much smoother. The older Champs all had between 350-360VDC on the 6V6.
Yeah, this one was a weird problem, and I honestly think in hindsight it was a mis-wound transformer from the factory causing the excess power.
Where specifically in the amp is the resistor you suggest changing to 1k
you should watch d lab for pointers
How bout a vid on tone stack LCR circuits? Different tube amp manufacturer circuits and component values?
Did you check the transformer wiring? Looks like several primary option
Nice video! Well done anp played :)
+Garage Workshop Thanks for watching and taking the time to comment! I really appreciate it. I do a lot of videos, so please be sure to subscribe and stay in touch!
i wish i could ask u a few questions about this particular amp ive heard u play and im sure id trust ur opinion so anyway cool vids and thx
Fire away.
The Guitologist well i was wondering what kind of speaker considering ur playing style youd suggest i have the same one one in this vid is that an oford? mine seems to be in fairly original condition so in terms of value only is it wise to upgrade even lol i have a ton more but these are my main concerns on this
hey btw thanks i know they r simple questions ...im not that knowledgible obviously but i am trying to learn so im sorry if they seem dumb thanks for taking time to reply ur work is impressive ....cheerz
why did you leave the power switch on neutral
couldn't you use zener diodes to drop the voltage a bit?
I have a question, why would you remove the dead cap? Thanks
@@GeeBee135 is it ok to just remove the cap on a three prong cord champ?
My Champ had the same blown resistor. Replaced and it still sounds too bright. Anything you can think of to tame the treble? My other Champ sounds more mellower. Swapped tubes and speaker etc...still too bright. I replaced electrolytic caps too.
I've studied electronics, but how do you go on to learn what you do? I've got a faulty isp pro rack g that's cracking, you've inspired me to have a go at this, but as for valves how do you learn that stuff? thanks.
Client changed the rectifier tube. Did you check to see he didn't put a 5AR4 where it should be a 5U4?
Rectifier tube on champ is 5y3GT
That death cap is on the fuse right? I think mine has been removed if so.
Also what is the light blue cap that says, paxtron .047m 125 ac dvl? It is just to the left of the power transformer and is not present in the model you are looking at. How do I find out what year mine is? I do not see a date stamp anywhere.
Sovtek 5Y3 rectifier by chance? Notorious for giving high voltages.
Not sure. Can't recall. Been awhile on this one. If it comes back in, I'll look for that. Thanks!
I'm trying to take off faceplate from my 1968 silver face twin having trouble around the pilot light area any tips?
I think the pilot light needs to be unsoldered and removed for that to come off. Maybe one of my viewers can answer that for sure. I do know you don't want to go tugging on it. They bend very easily. A rubber mallet can be used on a flat surface to straighten them back out though.
I currently have a '62 Gibson amp that is also having problems with excessively high voltage. I am getting ~400 DCV on the plates of the 6L6's when my variac is at 105VAC...
At ~125VAC (wall volts here in Bloomington), the B+ is 480 DCV at the plates of the 6L6 (which imo is too hot, though OKAY), but the filter caps are only rated to 450V... Is there something I can do to fix this short of getting a new PT? Do you have any idea what's going on? I've been at this guy for a while and near pulling my hair out...
Thanks for this video!
+Britt Rossman You can use a Zener diode on the power transformer secondary. There are tutorials online, although some people claim this approach compromises tone, I'm not sure either way. You could go with a different tube with higher max plate voltage and replace filter caps with 500V rated ones and raise the value of the resistors on the power rail like I did in this video to reduce the voltages downstream.
+The Guitologist The B+ taps directly from the Pi-filter to the CT of the OPT, with the next node being through an 0A2 regulator. No dropping resistors, just the choke...could it be bad?
Everything beyond the regulator is just fine AFA voltage is concerned. According to the schematic, however, it is supposed to drop 75V, but I read that it drops almost 150V, which is some concern. But I guess I'd rather be reading 245V to the preamp tubes (before plate resistor) than over 300V...
+Britt Rossman Your choke is probably fine, and honestly, your voltages might be workable as long as your bias is correct. I would change at least the power cap on the first node to 500V power handling though and maybe go ahead and replace the cathode bias resistor (if there is one) to a hefty 5W to be on the safe side.
+Britt Rossman Also, make sure you're measuring your plate voltage from plate to cathode, not plate to ground when biasing your output tubes. I've made that mistake before and miscalculated.
+The Guitologist I think I'll start with the bias. I havent yet rebiased since dealing with bad resistors, coupling and filter caps... And the B+ CT was not grounded... no wonder there are so many issues here!
Could someone tell me what the values are on the blue caps? I can see mine are 600volt but the big blue cap all the way to the left has the value rubbed off.
Hey, this is probably 7 months too late, but I just opened up my '74/'75 Champ and that cap says ".1 +/- 20% 600V ITW". I hope this helps!
Guitologist rules !
Why so many knobs? Ha!