Dual 50 Plexi/JCM 800

[purpletele]

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

[sluckey]

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.

[Ritchie200]

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!

[Ritchie200]

Nice work BV!  We need some more Marshalls around here in Fenderland....

Jim

[purpletele]

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

[sluckey]

Ooh, that's messed up bad!

[purpletele]

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.

[sluckey]

Which PT are you using?

[sluckey]

Here's a drawing I did for you. Not my neatest work but you should get the picture. 

[purpletele]

Here's a drawing I did for you. Not my neatest work but you should get the picture. 

Wow, thank you!

[purpletele]

Here's a drawing I did for you. Not my neatest work but you should get the picture. 

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

[sluckey]

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.

[purpletele]

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!

[MFowler]

I'm going to be building this dual 50 soon, I like the idea of switching between different era.

[purpletele]

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.

[Ed_Chambley]

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.

[purpletele]

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

[MFowler]

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

[purpletele]

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

[purpletele]

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

[sluckey]

Which PT are you using?

Worth repeating.

[purpletele]

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

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.

[sluckey]

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.

[DummyLoad]

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

[sluckey]

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.

[DummyLoad]

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

[purpletele]

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

[Big_Mike]

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.

[purpletele]

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

[sluckey]

I ordered a Classictone transformer that I used on the Fowler #39.

Which one?

I'm not yet convinced the transformer is an issue. 

[purpletele]

I ordered a Classictone transformer that I used on the Fowler #39.

Which one?

I'm not yet convinced the transformer is an issue. 

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.

[sluckey]

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.

[DummyLoad]

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

[purpletele]

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?

[sluckey]

Probably a 50W Marshall amp. 

If you're looking for something different from what you have, Matchless Chieftain and Clubman look interesting. Never heard either.

[purpletele]

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

[sluckey]

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.

[shooter]

EL34s?

could pin 6 be your pin3?

[purpletele]

EL34s?

could pin 6 be your pin3?

They are EL 34's

I will go look, that is possible.

Thanks

[purpletele]

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.

[shooter]

your voltage chart shows pin 6 >300vdc, my datasheet shows pin 6 N/C so jump it to pin3 

[sluckey]

your voltage chart shows pin 6 >300vdc, my datasheet shows pin 6 N/C so jump it to pin3 

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.

[shooter]

Yup, just thought of that, I'll go back2sleep

[sluckey]

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?

[purpletele]

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

[sluckey]

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.

[purpletele]

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

[sluckey]

OK, put the power tubes back in, remove the PI tube and recheck voltage at pin 3 of each power tube.

[purpletele]

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

[sluckey]

Put all tubes back in, disconnect the NFB wire, and recheck voltage on pin 3 of each power tube.