Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: purpletele on February 28, 2019, 08:29:47 pm
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I started the Sluckey Dual 50. I have made good progress while I have been in building mode.
Question for Steve: The schematic has 1 Meg Log Pots for the Master, Bass and three volumes, the Visio drawing indicates 1 Meg Audio.
Do you remember the logic of your pot selection and more specifically, what should I use to make this amp work as designed?
Thanks
BV
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Log taper and audio taper are the same. I used the pot tapers that were shown on the 2204 schematic but the 1987 did not show the tapers. No worry though. Volume pots are almost always log/audio taper because your ears are log tapered.
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I can tell you my ears are just like how I got them. I'm not sure what you young bucks do with your piercing and plugging and now tapering... Kids...
Jim
Get off my lawn!
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Nice work BV! We need some more Marshalls around here in Fenderland....
Jim
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Advice Needed
I am working on the Dual 50 and I would respectfully ask to have someone take a look at the proposed HT wiring for errors.
Mucho Gracias
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Ooh, that's messed up bad!
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Ooh, that's messed up bad!
I didn't feel comfortable with it. It's completely different than the way Fowlers is wired.
I'll take another attempt at it.
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Which PT are you using?
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Here's a drawing I did for you. Not my neatest work but you should get the picture. :icon_biggrin:
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Here's a drawing I did for you. Not my neatest work but you should get the picture. :icon_biggrin:
Wow, thank you!
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Here's a drawing I did for you. Not my neatest work but you should get the picture. :icon_biggrin:
Thanks Steve,
The drawing that you have shows a 15K resistor jumping across the Bias diade to the bias cap. My drawing doesn't show that, what purpose would that resistor have?
BV
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Obviously I grabbed an older version than the one you have. If you look closely you will find that same 15K on your board too. I'm hoping you will concentrate on the stuff I added to the left of the board.
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Obviously I grabbed an older version than the one you have. If you look closely you will find that same 15K on your board too. I'm hoping you will concentrate on the stuff I added to the left of the board.
Thanks, I'll focus on wiring correctly!
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I'm going to be building this dual 50 soon, I like the idea of switching between different era.
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I'm going to be building this dual 50 soon, I like the idea of switching between different era.
I am using my standard chassis which is 17.5 x 7.5. It is so much nicer to work on than the JTM 45 that I used for #39.
The photo below is recent, Sluckey saved my bacon again, I was disoriented with his HT Power config.
I am getting ready to start building some buildings, so I am trying to get my amps in a semi completed stage, where it is just tweaking that needs to occur.
I become obsessed and connot focus on anything else, so I needed to get that out of my system.
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Looking at your ground buss in the preamp, you only have 1 chassis connection, correct?
I wish I had a photo, but I think this explanation will suffice. When I build a Marshall Style, I will bend the solder lugs on the pots which ground up and back and twist them 90 degrees. This needs to be done before any heat is pt on the lug or it may become brittle and snap when you bend it. Then I can just run the ground buss through them and attach it to the input ground. It is then out of the way and makes my builds look cleaner.
Have not built anything on a board in a few years since I mainly build stupidly simple amps. I have built too many amps now 3 times and have to sell them off. Now I build mainly preamps since I have 4 tube power amps. You mentioned chassis space on one of you posts. Radial caps with vertical board, or bend the leads on the Axial. Also, the tone stack parts do not need a board. Haven't met one yet I cannot build on the pots. Here is a link to a few.
http://monster.partyhat.co/article/amplifier-tone-stacks/
Check put the Moonlight stack for something nice and simple for building little grinders.
I used to use Copper taken from Romex, but for years I have been using Electric Fence Wire and prefer it. It is stiff and stays where I put it.
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Looking at your ground buss in the preamp, you only have 1 chassis connection, correct?
I wish I had a photo, but I think this explanation will suffice. When I build a Marshall Style, I will bend the solder lugs on the pots which ground up and back and twist them 90 degrees. This needs to be done before any heat is pt on the lug or it may become brittle and snap when you bend it. Then I can just run the ground buss through them and attach it to the input ground. It is then out of the way and makes my builds look cleaner.
Have not built anything on a board in a few years since I mainly build stupidly simple amps. I have built too many amps now 3 times and have to sell them off. Now I build mainly preamps since I have 4 tube power amps. You mentioned chassis space on one of you posts. Radial caps with vertical board, or bend the leads on the Axial. Also, the tone stack parts do not need a board. Haven't met one yet I cannot build on the pots. Here is a link to a few.
http://monster.partyhat.co/article/amplifier-tone-stacks/
Check put the Moonlight stack for something nice and simple for building little grinders.
I used to use Copper taken from Romex, but for years I have been using Electric Fence Wire and prefer it. It is stiff and stays where I put it.
I only have one chassis connection for the Pre-Amp.
Good info thanks Ed
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Also, to get the buss wire nice and straight to put through those pot lugs:
You cut a length of buss wire chuck one end into a drill and hold the other end of the buss wire with a pliers and let the drill twist it straight.
Mark
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Also, to get the buss wire nice and straight to put through those pot lugs:
You cut a length of buss wire chuck one end into a drill and hold the other end of the buss wire with a pliers and let the drill twist it straight.
Really? I'll try that thanks. That bugs me to have a waving buss
Mark
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I lost a bet with myself. I thought I could get this amp built and turn it on without issue.
The amp went together very well once the HT wiring Life Preserver was published.
The amp is up and running and there is sound getting through, I have an issue with voltage at V5 & V6, I am only getting 357 V and I am expecting at least 430V.
The bias is really high and I can only go so far in either direction before the values reverse and start climbing.
The heater voltages are good, the pre-amp voltages are a little wonky.
I installed the mixing resistor for Note 5 Alternate Switching as I was loading the board. I am not using it but I don't think that unconnected resistor in that parallel circuit will cause issue. Please verify that for me.
I had an issue with the board and my stand up 25 K bias trim pot, so I added some turrets to make everything fit nicely. I thought I hooked it up correctly but I have to consider otherwise.
Please let me know if you spot something obvious or have ideas for testing
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Which PT are you using?
Worth repeating.
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Which PT are you using?
Worth repeating.
I have a PT from Doug's Shop, the Heyboer 50.
https://www.mojotone.com/amp-parts/Power-Transformers/British-Style-Plexi-800-50-Watt-Power-Transformer (https://www.mojotone.com/amp-parts/Power-Transformers/British-Style-Plexi-800-50-Watt-Power-Transformer)
I found that I had the bias trim pot set incorrectly, so now I am running at 385V and the bias is steady.
I feel like it is still missing some power and I have a little in and out of power and signal that I cannot locate with chop sticks yet. It occurred to me that it might be a bad ground connection that I am looking for.
The amp sounds and feels a little tinny, but it is getting close.
The channel switching is working well.
Not sure what to do next to trouble shoot other than to keep looking for a bad connection with chop sticks.
I check voltages in the morning.
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Hmmm, 360-0-360 @75mA... I know it's supposed to be the correct PT but it really seems light weight for a 50 watt amp. Voltage is gonna sag considerably. Voltage will even be low at idle.
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Hmmm, 360-0-360 @75mA... I know it's supposed to be the correct PT but it really seems light weight for a 50 watt amp. Voltage is gonna sag considerably. Voltage will even be low at idle.
maybe they mean 150mA? - which would be more realistic for 2 x EL34 PT. Mojo states it's a 5+ lb. part - that hardly seems like a 75mA transformer...
--pete
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Here's Mark Huss's 2204 schematic with voltages...
https://mhuss.com/MyJCM/JCM800_2204.pdf
And tube voltage chart...
https://mhuss.com/MyJCM/vchart.html
Keep in mind his amp uses 6550s rather than EL34s so his voltages will be a lot higher than yours.
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M. Huss states that the PT is a drake 1202-324 in that schematic linked to. so, either heyboer is building a weak part or Mojo is confused about the ratings, or there's an issue with the brian's build?
the classictone replacement (40-18023) for the drake 1202-324 is stated as a 690VCT @ 150mA secondary.
--pete
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M. Huss states that the PT is a drake 1202-324 in that schematic linked to. so, either heyboer is building a weak part or Mojo is confused about the ratings, or there's an issue with the brian's build?
the classictone replacement (40-18023) for the drake 1202-324 is stated as a 690VCT @ 150mA secondary.
--pete
Steve/Pete,
Thanks for the info. I had used a Classictone for Fowlers #39. Doug had the Heyboer on sale so I bought two without comparing the spec's, so shame on me.
I am going to go through and test the voltages and play it some more, but after I set my bias trim pot correctly the amp settled down and seems to be operating correctly but it feels like it is starving for voltage.
Considering that I come across or create issues with my level of experience it is hard to say with 100% confidence that I still have a wiring issue.
Based upon the enlightenment on the transformer specs, it makes sense that the amp is underperforming.
If my voltages look steady throughout then I'll probably order another transformer and pawn the two that I have.
Thanks for the advice, I really appreciate it.
Brian V
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Really interested in how this project turns out... I love my Plexi 6v6, but someday I would like to make a full-on 50 watter. The ability to switch between Plexi and 2204 mode in one amp is really a killer idea.
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Really interested in how this project turns out... I love my Plexi 6v6, but someday I would like to make a full-on 50 watter. The ability to switch between Plexi and 2204 mode in one amp is really a killer idea.
Note to self, check the transformer specs before you buy two.
I ordered a Classictone transformer that I used on the Fowler #39. I think the amp is going to come out really nice.
Just a little stumbling block for me. I'll have this amp running by Tuesday
BV
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I ordered a Classictone transformer that I used on the Fowler #39.
Which one?
I'm not yet convinced the transformer is an issue. :dontknow:
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I ordered a Classictone transformer that I used on the Fowler #39.
Which one?
I'm not yet convinced the transformer is an issue. :dontknow:
I ordered one just to be prepared, I ordered the Classictone 40-18023. I am sure I can use it on another project if I don't need it here.
I still need to go through and retest all of the voltages and play it some more and then chop stick it. Work got in the way today, so I'll play with it later tonight.
I missed the chance to crank it up and see how that felt, so tomorrow morning I'll open it up a bit.
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I'm beginning to think this is more about how a manufacturer chose to describe the product rather than the actual specs of the product.
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I'm beginning to think this is more about how a manufacturer chose to describe the product rather than the actual specs of the product.
i concur. i don't think anyone makes a 75mA part with that much material with that voltage rating. it would be interesting to know what the DC resistance of the windings are on the classistone and the heyboer parts.
--pete
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I'm beginning to think this is more about how a manufacturer chose to describe the product rather than the actual specs of the product.
i concur. i don't think anyone makes a 75mA part with that much material with that voltage rating. it would be interesting to know what the DC resistance of the windings are on the classistone and the heyboer parts.
--pete
What would be the ideal amp for my stock of Heyboer 50 Watt 360 75mA transformers?
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Probably a 50W Marshall amp. :wink:
If you're looking for something different from what you have, Matchless Chieftain and Clubman look interesting. Never heard either.
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I have testing and playing the Dual 50.
I definitely have a Plate voltage issue, so I am focusing there.
Node Voltages
A I have that sealed with shrink wrap, the plate voltage is 3 Volts or less.
B 360 V
C 241 V
D 241 V
E 241 V
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A I have that sealed with shrink wrap, the plate voltage is 3 Volts or less.
You can test node A at the junction of the diode cathodes on the lower left corner of the board, or you can test it on the Node A cap can (lower left corner of chassis). The only thing between the plates and node A is the OT. Either a bad connection or a bad OT.
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EL34s?
could pin 6 be your pin3?
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EL34s?
could pin 6 be your pin3?
They are EL 34's
I will go look, that is possible.
Thanks
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EL34s?
could pin 6 be your pin3?
I just checked the orientation of the socket and that is correct.
I also just measured 470 V at the OT connection at the HT fuse.
I am wondering if the OT is not grounded properly, I have the ground wire going to a speaker jack, and I don't have that grounded further. I'll will continue that ground to my main CT ground just to try it.
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your voltage chart shows pin 6 >300vdc, my datasheet shows pin 6 N/C so jump it to pin3 :icon_biggrin:
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your voltage chart shows pin 6 >300vdc, my datasheet shows pin 6 N/C so jump it to pin3 :icon_biggrin:
Slow down. Look at the layout and the pic of his amp. Pin 6 is not used by the tube but the 1K/3W screen resistor is conveniently mounted on the socket, between pin 4 and 6.
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Yup, just thought of that, I'll go back2sleep
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I also just measured 470 V at the OT connection at the HT fuse.
Turn the amp off. Measure resistance from the OT center tap at the HT fuse and pin 3 of each EL34. Should be some low resistance, one hundred, two hundred, etc. What have you?
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I also just measured 470 V at the OT connection at the HT fuse.
Turn the amp off. Measure resistance from the OT center tap at the HT fuse and pin 3 of each EL34. Should be some low resistance, one hundred, two hundred, etc. What have you?
40 ohms and 38 ohms respectively
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Hmm, the plot thickens. You have 470V at the OT center tap. And the windings are only 40Ω. But you only have 3V at the plate. Very interesting! Pull the output tubes and recheck the voltage at pin 3 of each tube.
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Hmm, the plot thickens. You have 470V at the OT center tap. And the windings are only 40Ω. But you only have 3V at the plate. Very interesting! Pull the output tubes and recheck the voltage at pin 3 of each tube.
485 V
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OK, put the power tubes back in, remove the PI tube and recheck voltage at pin 3 of each power tube.
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OK, put the power tubes back in, remove the PI tube and recheck voltage at pin 3 of each power tube.
415 V and 412 V
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Put all tubes back in, disconnect the NFB wire, and recheck voltage on pin 3 of each power tube.
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Put all tubes back in, disconnect the NFB wire, and recheck voltage on pin 3 of each power tube.
With NFB disconnected I get 409 V and 407 V
I put it on standby to calculate the bias, set up the meter and now I get an indication of a short from the light limiter.
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How does the amp sound with NFB disconnected?
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How does the amp sound with NFB disconnected?
I didn't get there, I wanted to set the bias and then play it. I switched to Standby and clipped my lead onto my 1 ohm resistor and the light limiter started glowing a lot brighter than it had been.
I switched everything off to contemplate
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Which OT do you have? Be specific. Post a link.
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Which OT do you have? Be specific. Post a link.
I flipped the switch and let the light limiter absorb the rush, it settled down to a normal low glow but it startled me, Shocked would be the wrong word.
As the amp was warming up it jumped in and out of a solid signal and settled to a strong quiet idle.
The amp sounds and plays like it is within the right range of power and touch.
The JCM side sounds real Ice Pickey, the JMP side sounds good, I will try with a single coil and I can tell if the tone is right or thin
I posted the Classictone 40-18025 below. I don't think i'm shorting out, but I spooked for a second.
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You must have the black wire on the secondary connected to chassis ground. I suggest using the power ground point. Reconnect the NFB wire. If the amp screams then reverse the OT red and blue plate leads.
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You must have the black wire on the secondary connected to chassis ground. I suggest using the power ground point. Reconnect the NFB wire. If the amp screams then reverse the OT red and blue plate leads.
The black wire is connected to a speaker ground only, I will extend the ground from the jack to the power ground.
I dropped some voltage where everything is at 395 V now. I described the sequence in the note above.
I'll reconnect the NFB and see what happens.
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I described the sequence in the note above.
Huh? What? Where?
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You must have the black wire on the secondary connected to chassis ground. I suggest using the power ground point. Reconnect the NFB wire. If the amp screams then reverse the OT red and blue plate leads.
The grounding on the power ground helped with the surge.
The amp is back at 1 volt or less at the plates
**Correction, plate voltage is jumping in and out.
397 V at the fuse OT connection 397 V at the plates.
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Plug the amp straight into the wall. You don't need the lamp limiter now. Did you swap the OT plate leads? And?
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Plug the amp straight into the wall. You don't need the lamp limiter now. Did you swap the OT plate leads? And?
Oh man did it yell at me!!
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Swap the plate leads again.
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Swap the plate leads again.
I must have a bad connection that I cannot find.
After I reset the Plate Leads I was getting 427 V. I played it for a while and then it dropped out while I was on the JCM side. It went to 390 V. The JMP side stayed strong and deep but the JCM side was weak and thin.
After all of that I was checking voltages and my Klein meter kept jumping to AC after I would get a low reading on the Plate.
Funny stuff going on. I get 1.5 volts on V6 and it goes right to AC mode when I hit pin 3 V4
This amp is winning tonight! I am beginning to think the OT is failing. I didn't have any event that would make me think I fried it, but it sure seems suspicious.
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Klein meter kept jumping to AC
I'm kinda beginning to suspect that meter. I've never had a meter to jump from dc mode to ac mode. every meter I've ever used required you to turn a knob to go from ac to dc. Maybe your Klein wants a fresh battery.
Tell me more about this Klein. Exact model number so I can look up the specs and manual.
EDIT... Never mind. I think your meter has a SEL button to toggle between AC and DC mode. This just sets some logic state in the meter brain. This logic state is being upset when you try to measure the plate voltage on this amp for some reason. If fresh batteries don't fix it, there is likely nothing you can do about that. Try another meter if you have one.
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yep never had a meter go to ac from dc either.do you have a spare or borrow one from someone for a few quick tests?
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Klein meter kept jumping to AC
I'm kinda beginning to suspect that meter. I've never had a meter to jump from dc mode to ac mode. every meter I've ever used required you to turn a knob to go from ac to dc. Maybe your Klein wants a fresh battery.
Tell me more about this Klein. Exact model number so I can look up the specs and manual.
EDIT... Never mind. I think your meter has a SEL button to toggle between AC and DC mode. This just sets some logic state in the meter brain. This logic state is being upset when you try to measure the plate voltage on this amp for some reason. If fresh batteries don't fix it, there is likely nothing you can do about that. Try another meter if you have one.
I changed the batteries in my Klein meter.
Fired the amp up, here is the sequence:
1. Once it is warmed up I get 465 v on the plate.
2. Pull off and measure V5 Plate and the voltage is on a sequence voltage drop, but it goes from 465 to 1.25 Volts and then drops at a decimal at a time until it is less than a volt.
Then I get disgusted and turn it off.
I think I fried the transformer with the bias trim pot being backward.
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You gotta ask why that Klein only misbehaves when you have the NFB wire connected? Seriously, try another meter before you order an OT.
If I had that amp and also had a super duper 50W Plexi (probably with the same OT), I'd temporarily swap OTs.
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You gotta ask why that Klein only misbehaves when you have the NFB wire connected? Seriously, try another meter before you order an OT.
If I had that amp and also had a super duper 50W Plexi (probably with the same OT), I'd temporarily swap OTs.
I'll try another meter tomorrow.
Said What?? My super duper Plexi is working great, don't want to disturb the mojo.
Thanks for the help
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Back In The Game
I had tested the plate voltage with another meter, but it was a cheapy. It had just kind of redlined and didn't know how to react.
I had to let it go for a few days, but I did order a spare.
I performed a test bench retrofit of a new OT, (I didn't mount it). The amp warmed up and was in quiet idle at 400 V on V6 and 315 on V5. That didn't look promising.
I disconnected the NFB and I have a steady 410 V at each plate and the bias is steady at 44 mV.
Summary: I am not sure that the OT is fried, but the new one that is hooked up temporarily is at least holding a voltage while the NFB is connected.
NFB: My first thought is do I have the bias system hooked up correctly.
Bias: At 410 V I set the bias at 43 mV but I am bottoming out on the low end adjustability on the bias system.
Tone: The amp plays well on both channels. The tone for each channel is reflective of its design. I didn't have a chance to turn up the MV, but at lower levels the amp sounded distinctly like a JCM 800 and a 1987x and it sounded nice.
The amp feels like it is operating at 92%. The NFB is obviously an issue. I have posted a new photobelow.
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short? see area circled in red in attachment.
--pete
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I don't like the looks of many of the solder joints on the turrets looks bare of solder to me.
Mark
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Your 1Ω bias resistors mounted on the sockets don't appear wired correctly. The correct way to wire them is to jumper pins 1 and 8 together. Then connect the 1Ω resistor between pin 8 and chassis ground (preferably use the power ground point). Do this for each tube socket.
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short? see area circled in red in attachment.
--pete
Thanks Pete,
Those turrets are close, but I can slide a straight edge between the gap, so nothing is touching.
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Your 1Ω bias resistors mounted on the sockets don't appear wired correctly. The correct way to wire them is to jumper pins 1 and 8 together. Then connect the 1Ω resistor between pin 8 and chassis ground (preferably use the power ground point). Do this for each tube socket.
Thanks, I'll try that this afternoon.
The amp is sounding nice, it's almost there.
BV
BV
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Your 1Ω bias resistors mounted on the sockets don't appear wired correctly. The correct way to wire them is to jumper pins 1 and 8 together. Then connect the 1Ω resistor between pin 8 and chassis ground (preferably use the power ground point). Do this for each tube socket.
Thanks, I'll make that change now.
The amp is sounding nice, it's almost there.
BV
BV
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Afternoon Tests
I changed the 1 Ohm resistor for V5 and V6, I used a solder post to tie the ground and the 1 ohm resistors together.
Things are still wonky:
1. With the NFB disconnected the voltage starts at high 300's and bleeds voltage like a metronome.
2. If you play it when it first heats up is sounds good, after a few minutes it is real tinny and you can tell the voltage has dropped.
3. When I measure the voltage after playing it is really low and continues to drop decimals of voltage while powered up.
4. If I start up with the NFB connected, then the voltage is low to start, mid 300's and it drips away like a faucet.
Photos of the grounding on the power tubes.
Thanks for any ideas. It took me a while to dream up this issue. :happy2:
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with the NFB
the dripping, and metronome are with your ears, or meter?
without any tubes in, monitoring your "last" tap VDC, what happens in 15min With NFB, and 15min Without NFB
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with the NFB
the dripping, and metronome are with your ears, or meter?
I have a meter on it and visually watching the voltage drop.
without any tubes in, monitoring your "last" tap VDC, what happens in 15min With NFB, and 15min Without NFB
I'll test it.
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with the NFB
the dripping, and metronome are with your ears, or meter?
without any tubes in, monitoring your "last" tap VDC, what happens in 15min With NFB, and 15min Without NFB
Shooter I get the following:
514 V on the plates with the NFB connected
517 V on the plates with the NFB dis-connected
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so put all the pre tubes in and repeat
if it's steady, add the PA tubes, repeat, somebody, or some thing is sucking current and it should be getting mighty warm. did you fix/replace your sketchy meter?
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so put all the pre tubes in and repeat
if it's steady, add the PA tubes, repeat, somebody, or some thing is sucking current and it should be getting mighty warm. did you fix/replace your sketchy meter?
Busy day at the Jungle today.
I'll do the steps as suggested. I changed the batteries in the Klein.
I was looking at the following as a second meter:
https://www.amazon.com/SSEYL-FLUKE-Digital-Multimeter-F17B/dp/B00SF3SQ50/ref=sr_1_fkmrnull_1?crid=3LFK3V3IKEV6E&keywords=fluke+17b%2B&qid=1552692307&s=gateway&sprefix=fluke+17b%2Caps%2C205&sr=8-1-fkmrnull (https://www.amazon.com/SSEYL-FLUKE-Digital-Multimeter-F17B/dp/B00SF3SQ50/ref=sr_1_fkmrnull_1?crid=3LFK3V3IKEV6E&keywords=fluke+17b%2B&qid=1552692307&s=gateway&sprefix=fluke+17b%2Caps%2C205&sr=8-1-fkmrnull)
These meters look like they are built in Asia somewhere and built for the Amazon market. Any other recommendations?
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so put all the pre tubes in and repeat
if it's steady, add the PA tubes, repeat, somebody, or some thing is sucking current and it should be getting mighty warm. did you fix/replace your sketchy meter?
493 V NFB Connected Pre-Amp Tubes installed Voltage is steady and not dropping
495 V NFB Dis-Connected Pre-Amp Tubes Installed. Voltage is steady and not dropping.
475 V NFB Connected Power Tubes installed. Voltage drops like a Zeppelin
475 V NFB Connected Power Tubes Installed. Voltage drops like a Zeppelin.
BV
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It's time to beg, borrow or buy another meter to be sure, same check.
If it still drops without NFB, I'd be inclined to swap out the tubes, then the filter cap feeding the PA tap.
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I'm suspecting some HF oscillation when NFB is connected is confusing his meter. There's just no way for voltage to really drop that much without seeing some smoke somewhere. Probably need a scope to prove/disprove my suspicion.
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without seeing some smoke somewhere
that's why I put in the 15min disclaimer :icon_biggrin:
I suspect you're correct since he didn't mention any smoke
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without seeing some smoke somewhere
that's why I put in the 15min disclaimer :icon_biggrin:
I suspect you're correct since he didn't mention any smoke
No smoke, and I let it run for at least 15 minutes.
How did I get a case of High Frequency Oscillation and what should I do to correct that?
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High Frequency Oscillation
you know the gremlin with the spiked hair smoking a cig, that's the guy you're after :icon_biggrin:
typical, positive feedback, induced signals, bad DC filtering, harmonic tubes. I've chased him a few times, he's a B :cussing: ch to kill :icon_biggrin:
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without seeing some smoke somewhere
that's why I put in the 15min disclaimer :icon_biggrin:
I suspect you're correct since he didn't mention any smoke
No smoke, and I let it run for at least 15 minutes.
How did I get a case of High Frequency Oscillation and what should I do to correct that?
a shot in the dark: try removing the plate to plate (47pF SM) cap from the LTPI - it's the cap connected from pin 1 to pin 6 on the LTPI tube.
you may have a high(er) level preamp signal in proximity to a lower level preamp signal and they are relatively in phase.
an oscilloscope really helps to troubleshoot these kinds of issues.
pull the the LTPI tube and power up - is B+ stable? if yes, then LTPI or something before the LTPI is the issue. if B+ is not stable, then the issue is with the output stage.
--pete
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a shot in the dark: try removing the plate to plate (47pF SM) cap from the LTPI - it's the cap connected from pin 1 to pin 6 on the LTPI tube.
you may have a high(er) level preamp signal in proximity to a lower level preamp signal and they are relatively in phase.
an oscilloscope really helps to troubleshoot these kinds of issues.
pull the the LTPI tube and power up - is B+ stable? if yes, then LTPI or something before the LTPI is the issue. if B+ is not stable, then the issue is with the output stage.
--pete
Pete,
I get the 475 V and then it drops immediately as before.
Thanks
BV
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what does it drop to - e.g. bottoms out at?
--pete
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what does it drop to - e.g. bottoms out at?
--pete
The voltage dropped quickly to 335 V.
It has stayed at 335 V for about 4-5 balls in the water at 17. (15 -20 minutes)
BV
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I'm suspecting some HF oscillation when NFB is connected is confusing his meter. There's just no way for voltage to really drop that much without seeing some smoke somewhere. Probably need a scope to prove/disprove my suspicion.
How do you resolve such a matter and what would create a HFO?
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I don't know that it's even an issue. It may only occur when you put your meter probe on the plate socket pin. I've had amps that would emit a screeching/arcing sound when my meter was connected to the plate of one tube but would behave quite well when connecting to the other tube plate. Never resolved that. Just said to self, don't do that.
So, other than this plate voltage issue, how does the amp behave otherwise? Sound good, bad, or ugly? Any red plates?
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Just said to self, don't do that.
yup, same! My 100X probe ALWAYED messed up plate readings. I just made my I readings via Rk and accepted the math, and audio measurements at speaker.
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I don't know that it's even an issue. It may only occur when you put your meter probe on the plate socket pin. I've had amps that would emit a screeching/arcing sound when my meter was connected to the plate of one tube but would behave quite well when connecting to the other tube plate. Never resolved that. Just said to self, don't do that.
So, other than this plate voltage issue, how does the amp behave otherwise? Sound good, bad, or ugly? Any red plates?
The amp sounds decent for maybe a minute, then it drops voltage and gets real thin and tinny.
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then it drops voltage
are you wired like the layout on page 1, reply 8? specifically the "A" cap?
If so can you clip your meter + on cap, - on chassis (Not a stud, or ground point, just gator-clipped to chassis metal), set for VAC and watch it from power on, for ~~~~5min or so
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The amp sounds decent for maybe a minute, then it drops voltage and gets real thin and tinny.
Is this happening with your meter hooked up?
Or is it getting thin sounding and your thinking it's because the voltage has dropped?
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The amp sounds decent for maybe a minute, then it drops voltage and gets real thin and tinny.
Is this happening with your meter hooked up? Or is it getting thin sounding and your thinking it's because the voltage has dropped?
It does happen when the meter is hooked up.
I can also not be on the meter and just play it after it warms up, it sounds full then it shifts gears and feels real thin and tinny, that timing is consistent with the meter readings of the voltage.
Thanks for thinking about it.
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then it drops voltage
are you wired like the layout on page 1, reply 8? specifically the "A" cap?
If so can you clip your meter + on cap, - on chassis (Not a stud, or ground point, just gator-clipped to chassis metal), set for VAC and watch it from power on, for ~~~~5min or so
I thought that would be the next suspect. I'll run the test and get back.
Thanks for the input.
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It does happen when the meter is hooked up.
IS this what doesn't happen;
feels real thin and tinny
in other words; it works fine without meter?????
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It does happen when the meter is hooked up.
IS this what doesn't happen;
feels real thin and tinny
in other words; it works fine without meter?????
No, it performs the same with or without the meter hooked up. It warms up fine, plays fine for a minute and then gets weak.
BV
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ok, I'm back one the same page, nut I still got your meter in the 70% chance bad bucket
what kinda AC on the cap?
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ok, I'm back one the same page, nut I still got your meter in the 70% chance bad bucket
what kinda AC on the cap?
When I am hooked up as described my Alternating Current is going from 210 V to 77 V then to 28 V.
When I switch over to DC it starts at 497 V and bottomed out at 341 V
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I think that connecting a meter to the power tube plate is a bad idea, as it effectively puts a massive transmission antennae there, invoking feedback loops to earlier stages.
It's no surprise then that the amp oscillates, resulting in crazy readings (and potentially damaging levels of Vac on to the meter).
Just measure the HT at its feed to the OT, the Vdc at the plate/s is hardly going to be that much lower.
I'll hazard a guess that the crazy readings stop if the same test is repeated with LTP tube removed (thereby breaking the most problematic loops)?
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I think that connecting a meter to the power tube plate is a bad idea, as it effectively puts a massive transmission antennae there, invoking feedback loops to earlier stages.
It's no surprise then that the amp oscillates, resulting in crazy readings (and potentially damaging levels of Vac on to the meter).
Just measure the HT at its feed to the OT, the Vdc at the plate/s is hardly going to be that much lower.
I'll hazard a guess that the crazy readings stop if the same test is repeated with LTP tube removed (thereby breaking the most problematic loops)?
Thanks PDF,
I had performed the test as you suggested previously, but I did it again to see what occurs.
I am getting the exact same result when I read the voltage from the OT feed at the fuse.
The voltage starts in the high 470's and then just about 45 seconds it starts ticking away volts. Then it will drop quickly to 345 V and hold.
When I play the amp I can hear and feel the amp lose power and volume which is consistent with the visual on the meter.
Thanks for the input
BV
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Is the filter cap charging up at turn on and then discharging somehow?
You get the same decreased voltage reading at the screens?
Mark
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Is the filter cap charging up at turn on and then discharging somehow?
MFowler might be on to something?
I think at this point, I'd change all the B+ caps.
You could try disconnecting the B+ line after the screen grid B+ filter node, and pull all the tubes except the power tubes. That would eliminate everything down stream from there.
Maybe it's a new/bad cap that's leaking?
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If it's one of the filter caps, there's two ways to know. 1. High voltage capacitor tester, which most people don't have. 2. Take caps out of circuit, tack in new ones (different values are okay for now, as long as they can stand the voltage).
Note, however, that it has been observed that the problem only happens with the power tubes installed. This might indicate the following: 1. It has to do with the power tubes themselves but if I remember correctly different pairs have been tried. 2. It has to do with circuitry associated with the power tubes. A. This could something like oscillation happening in the power amp, but i guess removing preamp tubes or disconnecting nfb should affect this, which apparently it doesn't. Can you conform this? B. It could also be a physical problem. Look carefully at the rear of the sockets when you insert and remove the tubes (Amp off). Anything touching anything? getting really close? Any dirt/gunk/deposit on the sockets? burn marks? Take closup pics. C. It could also have to do with the power tubes' current draw and a possibly associated thermal issue (although you say there are no signs of heat). When it happens, do all voltages go down, e.g. preamp? And have you by any chance measured the ac at the pt secondary (at input of recitifier)? Does it also go low?
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Is the filter cap charging up at turn on and then discharging somehow?
You get the same decreased voltage reading at the screens?
Mark
That seems logical, I had ordered a new 50/50 Cap to test, I'll grab some of the on-board caps as well.
BV
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Is the filter cap charging up at turn on and then discharging somehow?
MFowler might be on to something?
I think at this point, I'd change all the B+ caps.
You could try disconnecting the B+ line after the screen grid B+ filter node, and pull all the tubes except the power tubes. That would eliminate everything down stream from there.
Maybe it's a new/bad cap that's leaking?
It is worth investigating. That is where I will be focusing on the next few days.
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If it's one of the filter caps, there's two ways to know. 1. High voltage capacitor tester, which most people don't have. 2. Take caps out of circuit, tack in new ones (different values are okay for now, as long as they can stand the voltage).
Note, however, that it has been observed that the problem only happens with the power tubes installed. This might indicate the following: 1. It has to do with the power tubes themselves but if I remember correctly different pairs have been tried. 2. It has to do with circuitry associated with the power tubes. A. This could something like oscillation happening in the power amp, but i guess removing preamp tubes or disconnecting nfb should affect this, which apparently it doesn't. Can you conform this? B. It could also be a physical problem. Look carefully at the rear of the sockets when you insert and remove the tubes (Amp off). Anything touching anything? getting really close? Any dirt/gunk/deposit on the sockets? burn marks? Take closup pics. C. It could also have to do with the power tubes' current draw and a possibly associated thermal issue (although you say there are no signs of heat). When it happens, do all voltages go down, e.g. preamp? And have you by any chance measured the ac at the pt secondary (at input of recitifier)? Does it also go low?
ST,
All good points and test options, thank you. I'll respond after I work through some of the suggestions.
BTW, I did not try another set of tubes, but I will today.
BV
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BTW, I did not try another set of tubes, but I will today.
Try swapping the power tubes 1st.
You have EL34's in there? They have 3 grids, that when they heat up can move and that could cause them to short.
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BTW, I did not try another set of tubes, but I will today.
Try swapping the power tubes 1st.
You have EL34's in there? They have 3 grids, that when they heat up can move and that could cause them to short.
Willabe,
I just put in new tubes and it performed just as the others. The voltage built up to the high 400's and then started dropping.
BV
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this will be a ugly hail-Mary suggestion
pull all the pre an PI tubes
If you don't have a spare 32uf+32uf PS cap, use any collection of caps that will get you to >500vdc, >20uF. make that your "new" PA tube PS tap. (It don't have to be pretty). BUT you NEED to be safe!
disconnect NFB and tape off.
disconnect the secondary wires on the OT. Use gator clips to connect them directly to the speaker, which is isolated from amp, chassis, etc.
ONLY measure at your new "tap", not plates.
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Don't forget... Once he disconnects the NFB wire, everything is fine.
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ya, my thought, get the cap outta the way cuz it ain't a prob, but it's proven now.
then add the NFB back with gator clip once the speaker jacks & wiring are gone from the equation.
then it's wonky meter time :icon_biggrin:
can the coupler caps from PI be safely grounded on the left side? my "uneasiness" there is DC creep that could effect bias :dontknow:
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...when I read the voltage from the OT feed at the fuse.
The voltage starts in the high 470's and then just about 45 seconds it starts ticking away volts. Then it will drop quickly to 345 V and hold...
So the above happens only if the NFB loop is connected?
Is that true even with no tube in the LTP phase splitter?
While it is happening, what is the mV across the 1ohm power tube cathode resistors?
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...when I read the voltage from the OT feed at the fuse.
The voltage starts in the high 470's and then just about 45 seconds it starts ticking away volts. Then it will drop quickly to 345 V and hold...
So the above happens only if the NFB loop is connected? No, it happens either connected or not connected
Is that true even with no tube in the LTP phase splitter? It is true
While it is happening, what is the mV across the 1ohm power tube cathode resistors?
The bias setting is stable at 52 mV, however as the amp was in the peak voltage stage (Assuming the same pattern) the bias was around 75mV. I heard a mild pop through the speakers and the bias voltage dropped and stabilized at 52 mV.
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Don't forget... Once he disconnects the NFB wire, everything is fine.
I somehow missed this...
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Don't forget... Once he disconnects the NFB wire, everything is fine.
I somehow missed this...
That is not true anymore. Originally there was a big difference with the NFB connected vs. not connected. I am not sure what happened but the NFB connected or not no longer has a significant difference.
I have a new cap can coming tomorrow and some more 47 UF 450V caps by Saturday, I think that will be the next test.
Thanks for checking it out.
BV
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I think that will be the next test.
while you're waiting you can still eliminate the secondary of the OT. gator clip the OT wires directly to a "free-standing" speaker, NO nfb.
IF PA tap is stable, gator clip the spkr - (neg) to your cap neg, recheck.
IF it's still stable, add the PI tube, recheck.
IF it's still stable, use a 2nd gator clip from speaker + (pos) to the NFB point, recheck.
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...The bias setting is stable at 52 mV, however as the amp was in the peak voltage stage (Assuming the same pattern) the bias was around 75mV. I heard a mild pop through the speakers and the bias voltage dropped and stabilized at 52 mV.
It would be good to see what is happening to the bias supply output voltage over the course of the above pattern, ie the feed to the 220k pair of power tube grid leak resistors.
If that is stable, what is happening to the bias voltage at the power tube control grids, socket terminals #5?
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I think that will be the next test.
while you're waiting you can still eliminate the secondary of the OT. gator clip the OT wires directly to a "free-standing" speaker, NO nfb.
IF PA tap is stable, gator clip the spkr - (neg) to your cap neg, recheck.
IF it's still stable, add the PI tube, recheck.
IF it's still stable, use a 2nd gator clip from speaker + (pos) to the NFB point, recheck.
I had installed a new fresh OT last week and that obviously didn't change things, but it eliminated the possibility that I damaged it early on.
I'll still follow the instructions and run the test, I want to see the reactions.
Thanks,
BV
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this will be a ugly hail-Mary suggestion
pull all the pre an PI tubes
If you don't have a spare 32uf+32uf PS cap, use any collection of caps that will get you to >500vdc, >20uF. make that your "new" PA tube PS tap. (It don't have to be pretty). BUT you NEED to be safe!
disconnect NFB and tape off.
disconnect the secondary wires on the OT. Use gator clips to connect them directly to the speaker, which is isolated from amp, chassis, etc.
ONLY measure at your new "tap", not plates.
I missed this note.
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Ya, got that you swapped OT, I'm still in the HF osc camp or meter gone wild, since the only thing left in the PA is cap and secondary wiring/jacks......
I do have a nagging little helper whispering something about the bias circuit, or DC creep, but I'm ignoring him for now :icon_biggrin:
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Ya, got that you swapped OT, I'm still in the HF osc camp or meter gone wild, since the only thing left in the PA is cap and secondary wiring/jacks......
I do have a nagging little helper whispering something about the bias circuit, or DC creep, but I'm ignoring him for now :icon_biggrin:
The meter gone wild seems to be a potential culprit, but the amp acts the same whether I measure the voltage or plug in and play the amp.
I am looking at putting $300 into a nice meter if anyone has suggestions I would certainly appreciate it.
I placed a close up of the bias circuit and trim pot. I had the trim pot set up incorrectly originally, then I added turrets to set it the way it is now, which I think is correct, but maybe not.
I found a 32 /32 500V cap that I can run tests with, so I'll start with that.
Thanks for the help
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does the bias supply drift with the B+? e.g. does bias voltage at pin 5 fall? you may simply have an issue with the bias supply. check the pot for drift or temporarily tack in a fixed resistor to validate.
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does the bias supply drift with the B+? e.g. does bias voltage at pin 5 fall? you may simply have an issue with the bias supply. check the pot for drift or temporarily tack in a fixed resistor to validate.
I had a small bias pot, like you have, in my BF Princeton reverb that went bad. I would turn on the amp and after a 30 seconds or so, Iwould hear a small pop and the bias would shift.
You said you hear a small pop.
Tack in a R in place of the -bias pot like DL wrote.
-Bias cap could be bad too.
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Wow, this is getting interesting.
1. I swapped the 'A' Cap with a 32/32 500V Cap without change. I changed it to the original and tested with same results.
2. I swapped the 'B' Cap with the 32/32 500 V Cap without change.
3. I changed out the 25 K bias pot with a new one and the same result.
4. I have a speaker here that I am going to run the isolation test that Shooter recommended.
5. I hooked up the secondary to the speaker +. I clipped the ground to the speaker ground.
6. I pulled the pre amp and PI tubes.
7. Flipped the switch and the circuit loaded to 490 V and started dropping quickly.
8. I shut it down and poured a glass of wine. Time to play a working amp.
Thanks for the help
BV
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Has this amp been plugged into that lamp limiter this whole time?
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Has this amp been plugged into that lamp limiter this whole time?
No, not the whole time. I plugged it in for the speaker isolation test.
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1. I swapped the 'A' Cap with a 32/32 500V Cap without change. I changed it to the original and tested with same results.
2. I swapped the 'B' Cap with the 32/32 500 V Cap without change.
When you did this, 1 & 2, did you have the rest of the down stream B+ disconnected and the PI and preamp tubes out?
If you didn't disconnect the down stream B+ filter caps, then you don't know if it's 1 of them that's bad and bleeding the B+ off. Trying to isolate, divide/conquer, just the power tubes/PT B+ OT CT node and screen node/OT.
If that stuff is OK, then go through the -bias circuit test's/voltage readings the guys suggested.
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1. I swapped the 'A' Cap with a 32/32 500V Cap without change. I changed it to the original and tested with same results.
2. I swapped the 'B' Cap with the 32/32 500 V Cap without change.
When you did this, 1 & 2, did you have the rest of the down stream B+ disconnected and the PI and preamp tubes out?
If you didn't disconnect the down stream B+ filter caps, then you don't know if it's 1 of them that's bad and bleeding the B+ off. Trying to isolate, divide/conquer, just the power tubes/PT B+ OT CT node and screen node/OT.
If that stuff is OK, then go through the -bias circuit test's/voltage readings the guys suggested.
Thank you I'll be back on it later today.
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I am studying the Hoffman Bias section and I think I have this particular Piher 25KL Trim Pot installed incorrectly.
1. I have the middle leg of the trim pot tied one of the Pair of legs, and that connects to the 47K resistor.
2. I haven't been able to determine the designation of the legs on this vertical trim pot, but I am thinking the middle leg is really the conductor that should be connected to the two 220K resistors.
I attached a photo and a cut sheet. Please let me know if I botched the trim pot installation.
Thanks BV
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You have a choice. Do you connect the center leg to this outer leg or the other outer leg? Doesn't matter.
Then you have another choice. Do you connect the outer leg and center leg (that are connected together) to the 47K or to the 220K grid resistors? Again, it doesn't matter.
The bottom line... you want the negative voltage on pin 5 of the output tubes to change when you turn the pot. If it does change then don't worry. Be happy.
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You have a choice. Do you connect the center leg to this outer leg or the other outer leg? Doesn't matter.
Then you have another choice. Do you connect the outer leg and center leg (that are connected together) to the 47K or to the 220K grid resistors? Again, it doesn't matter.
The bottom line... you want the negative voltage on pin 5 of the output tubes to change when you turn the pot. If it does change then don't worry. Be happy.
Thanks, digging deep looking for clues.
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I know. And I feel for you. I keep thinking we are overlooking something simple. It seems to be a power amp problem. I would pull all the preamp tubes, leaving only the PI and PA tubes in and troubleshoot that. Maybe open up that other 50W amp you just built. Put them side by side and make some voltage comparisons. I suspect the power amps in both have nearly identical circuits.
I want to see some voltage readings for ***EVERY*** pin on those EL34s while the amp is misbehaving.
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I know. And I feel for you. I keep thinking we are overlooking something simple. It seems to be a power amp problem. I would pull all the preamp tubes, leaving only the PI and PA tubes in and troubleshoot that. Maybe open up that other 50W amp you just built. Put them side by side and make some voltage comparisons. I suspect the power amps in both have nearly identical circuits.
I want to see some voltage readings for ***EVERY*** pin on those EL34s while the amp is misbehaving.
I'll run some more tests today,
Thanks, it is all in good fun.
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it is all in good fun.
:thumbsup:
that goes miles in tough troubleshooting!
along with your favorite "adult distraction" and a good amp :icon_biggrin:
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The first test I ran today was the stability of the filament voltage.
Check this video out as I start up the amp cold. half way through the video you hear a series of knocks, that is the amp making the noise as it is dropping voltage.
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Positive Update
1. I installed the Classictone 40-18023 PT after I ran the Filament test today.
2. The amp is now running stable but it is running hot.
3. The plate voltage is 478 and I went to set the bias to 34 and I could only go down to 66 mV.
4. I replaced the 220K V Bias resistor with a 100K Bias resistor and couldn't get a reading. I need a 180K Resistor.
The amp plays really stiff but the voltage is stable.
5. How can I reduce the bias voltage to balance the circuit?
6. Getting closer, long day of quiet amp work, with multiple bursts of loud guitar throughout the day.
7. My pre-amp voltages are a little hot as well.
8. Current voltage chart is attached for review
How is that for perseverance :violent1:
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>I replaced the 220K V Bias resistor with a 100K Bias resistor and couldn't get a reading. I need a 180K Resistor.
use 2 x 330K 1W in parallel. should be close enough to trim.
--pete
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>I replaced the 220K V Bias resistor with a 100K Bias resistor and couldn't get a reading. I need a 180K Resistor.
use 2 x 330K 1W in parallel. should be close enough to trim.
--pete
Thanks Pete!
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This is sounding encouraging. :thumbsup:
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This is sounding encouraging. :thumbsup:
:BangHead: Still losing juice. The dual 330K resistors worked great to get me in range for the bias. It was looking very good for a few fleeting moments, then I started leaking again..
I'll try rolling through the 47/450 caps. I have a 47 uF/500 V on hand to use.
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On the video as the power tubes warmed up it popped and reduced the voltage, has to be bad power tube or tubes.
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On the video as the power tubes warmed up it popped and reduced the voltage, has to be bad power tube or tubes.
Mark,
I just found it, or at least I think I have. The 47 uF/450 V cap in the Plexi Pre-Amp section must have been leaking. The replacement held the voltage as I rolled the Cap through the 3 on-board caps.
The voltages all look much more in line , plate Voltage of 478, etc.
I also found I have a bad connection somewhere that is affected when I push down on the board. So I'll be looking for that, but I am encouraged about the steady voltage!
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Success
I got it running smooth and stable. I have had it on and off for quite a few hours today.
I have to assume that the 47 uF 450V cap was definitely one of the culprits.
I like the Plexi side best.
I remember why I didn't have a JCM 800, too harsh!! I had used 500 pF silver micas on the JCM and plexi circuit instead of 470 pF as designed.
Any suggestions to take the iciness away from the JCM, it's a bit sterile and will make your eyeballs bleed. I want to soften it and smooth it out if possible.
Thanks, that was quite a ride.
I have a couple of head shells that I can use but I thought Red looked cool.
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Glad you hung in there! You can probably cut some of the harsh ice pick by removing those two 470pF caps on the voltage dividers in the V2 circuit. Maybe remove the .001 from the volume pot also. The red head looks bold and daring! :headbang:
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what He said;
if you're feeling froggy, un-do the select switch and replace with a blend pot for testing, it might be worth it
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Probably not worth it since the two channels would be out of phase.
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:think1:
it's still early :BangHead:
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what He said;
if you're feeling froggy, un-do the select switch and replace with a blend pot for testing, it might be worth it
that won't work well - signals would be out of phase and cancel.
--pete
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Glad you hung in there! You can probably cut some of the harsh ice pick by removing those two 470pF caps on the voltage dividers in the V2 circuit. Maybe remove the .001 from the volume pot also. The red head looks bold and daring! :headbang:
Thank you Steve.
In classic Marshall form, the amp sounds really good when it is flush with volume.
Thanks for the voicing advice, I'll work on that tonight.
BV
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since I failed the Captain Obvious test :icon_biggrin:
a simple quickie, put a 12AU7A in V2's spot.
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since I failed the Captain Obvious test :icon_biggrin:
a simple quickie, put a 12AU7A in V2's spot.
I'll try that, thanks for the tip
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Here is the first video that I could make of this really cool amp. My friends latched onto it, which is great, in fact it's used on a new recorded EP.
I had Ben Fargen install a MV for me and it came out really nice.
Emmit the Gargoyle made it into another video.
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HI Brian! Long time, was wondering how you were doing?
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HI Brian! Long time, was wondering how you were doing?
Hi Joe, doing well. I'll send you a note later this evening
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It's really good to see everyone putting time and effort into helping our fellow builder out in a nice civil manner. This, in spite of the obvious frustration bubbling just under the headlines. Good, informative stuff. Group hug! Ha.