dilemma!! dual el84 produces low vol. and distortion

D

drgroove

Member
My current project is a dual el84 pp with a standard AF amp/phase inverter set-up using the ubiquitous 12ax7's. Simple?
Rectifier is a 6ca4.

I am using a Hammond 6k56vg PT 540VCT 120ma and a weber OPT with an 8k primary and 8 ohm secondary.

The finished product produces low volume distorted output.

Questions:
1 is 540 VCT enough to power this? (The original schem called for 660VCT)
2. Wrong primary for the OPT? How do I determine primary res. for 2 EL84's?

All of the above is probably obvious to most of the experts here. For me it is perplexing.

Thanks for any help any of you can give me.
 
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Greetings from FixitLand!

Could we please see a schematic?

540 VAC CT (270-0-270) is not too low in my estimation, but it is lower than usual. That's not your problem.

8K (CT) OPT primary is fine for EL84 P-P. I used 10K CT for my rig. Anywhere from 8K to 12K is fine according to all the data sheets I read.

Have you a scope? Other test equipment?

Take care,
--
J. E. Knox "The Victor Freak"
 
I have no test equipment. Being a noob I wouldn't know what I was looking for.

Schematic attached
 

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Do you have a voltmeter at least? How do the voltages compare to whats on the schematic? They'll be lower of course since your B+ is lower, but it ought to be a certain percent lower across the board.

If you want to get the B+ closer to spec with that transformer, use silicon diodes instead of the tube rectifier. You won't have as much voltage drop.
 
Gadget73: I have a 660 vct transformer on order to use instead of the 540 vct. That will make it just like the schem. That will at least do away with the voltage question. I used the 540 vct because it was laying around my shop. Nature abhors a vacuum. :-)

Voltage!! I have no idea how to measure that. If I use a multi-meter where would I put the probes?

I thought of the Diode thing but not being as technically savvy as I need to be I should probably stay with the schem I think. If you can diagram out how to replace the tube rectifier using diodes and give me values for the diodes I would be very interested to see that.

One more question: Is the input on this schem ok to go with or would there be a better set-up for it. Is it ok to NOT include the "volume" pot and what is the other pot for? It is called "balance" and do I need it?

Thanks, Jon
 
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drgroove said:
Gadget73: I have a 660 vct transformer on order to use instead of the 540 vct. That will make it just like the schem. That will at least do away with the voltage question. I used the 540 vct because it was laying around my shop. Nature abhors a vacuum. :-)

Voltage!! I have no idea how to measure that. If I use a multi-meter where would I put the probes?

I thought of the Diode thing but not being as technically savvy as I need to be I should probably stay with the schem I think. If you can diagram out how to replace the tube rectifier using diodes and give me values for the diodes I would be very interested to see that.

One more question: Is the input on this schem ok to go with or would there be a better set-up for it. Is it ok to NOT include the "volume" pot and what is the other pot for? It is called "balance" and do I need it?

Thanks, Jon
Click to expand...

For diodes the 1n4007 will be fine. I just can't remember which pins on the rectifier sockets to use off the top of my head. Off to do a search but I'm sure someone with more knowledge will speak up first.
 
Should work fine with the lower voltage transformer. And your EL84s will last longer too - that circuit shows them cooking at 13.2 W apiece. Many modern EL84s won't be happy at that power level.

Get a $5 multi-meter from Harbor Freight. Measure voltages at each tube pin - black lead - meter(-) connected to chassis ground. Red lead in your hand, other one in your back pocket. Voltages are shown on schematic - all should be lower with your transformer.

Balance pot is unnecessary - you'd need a distortion analyzer to set it accurately - you can remove it and replace R7 with an 82K resistor to ground.
 
JAYMANNA: I have been using an I-pod (cranked) into a mic/pre-amp into the amp. My goal is to make this a stand alone to use with an outboard pre-amp as I am doing at present. Does this sound ok?

MR. BAVIS: How do I decrease the wattage to the EL84's. They do seem to be hotter than I would expect.

Voltages: (Thanks for teaching me how to do that.) All the voltages check out in a relative sense except pin 6 of v1 and pin2 of v2 which reads 130v instead of 75v. Pin 1 on v2 reads 270v vice 180v and pin 3 on v2 reads 210v vice 80v. Is this the problem?
 
drgroove said:
JAYMANNA: I have been using an I-pod (cranked) into a mic/pre-amp into the amp. My goal is to make this a stand alone to use with an outboard pre-amp as I am doing at present. Does this sound ok?

MR. BAVIS: How do I decrease the wattage to the EL84's. They do seem to be hotter than I would expect.

Voltages: (Thanks for teaching me how to do that.) All the voltages check out in a relative sense except pin 6 of v1 and pin2 of v2 which reads 130v instead of 75v. Pin 1 on v2 reads 270v vice 180v and pin 3 on v2 reads 210v vice 80v. Is this the problem?
Click to expand...

I'd remove the mic pre amp from that equation and just run the iPod direct in and use the volume control on the iPod.

Did you already replace the 33K resistor?

BTW, my wifes aunt and uncle live in Corvallis. What a beautiful part of the country. We hope to relocate to that area sooner rather than later ;)
 
REX E-THING: I did replace the 33k res. Minimal change. seems a bit less distorted.

I have tried both ways. Adding the mic/preamp maybe doubled the volume over just the i-pod alone but also increased distortion. Even with the pre-amp the volume is not near what it should be.

Thanks
 
EL84s are probably OK as-is, but may not be with higher voltage. Higher cathode resistor will reduce current, but as current goes down, voltage goes up too...

Voltages don't sound right. Check all the resistor values around the first stage. (you can measure them in the circuit if power has been off for a while, but they may read low, since there could be another circuit in parallel). If you're in doubt, unsolder one end.
 
REX: I will do that tomorrow.
Yeah it's pretty here but it rains incessantly. I might like to live where you do. Ha-ha... I grew up in so.cal. so the desert is my home.

Thanks for your help
 
drgroove said:
REX: I will do that tomorrow.
Yeah it's pretty here but it rains incessantly. I might like to live where you do. Ha-ha... I grew up in so.cal. so the desert is my home.

Thanks for your help
Click to expand...

Tom has good advice (as always) above.

If you like consistent 115+ degree days then this is the place for you ;) Fall has finally made it though so the gruesome a/c bill should end in about a month.....
 
Drgroove -- If I may, this is a Fisher circuit, and the value of the 33K resistor is correct for that unit as shown -- but correct for that unit also means when using the original Fisher output transformer as well. Since you are using a different output transformer, changing it to 2.2K to match the upper output tube is fine, but either way, it is ultimately not the source of your problem. Just realize however, that the Sams circuit is in fact correct for the value of this resistor.

Also realize that all of the voltages you gave relate to V1 and V1 only. You indicated that some of the pins measured related to V2, which is wrong. It is an easy mistake to make because V1 is in fact a dual triode tube shown as two separate tubes on the schematic, but in fact the tube designated as V2 on that schematic is actually the upper output tube, not the second half of V1 which is what you were really referring to. Both of the two most left hand audio tubes shown on the schematic are in fact all included in V1.

The other control you mention besides the level control is called an AC balance control. With the limited knowledge you have at this point, the best thing for you to do is to replace that control and the existing resistor associated with it with a single 82K resistor. This resistor would connect between pin #3 and ground, replacing the control and original resistor, with all other connections the same. While this change would eliminate the control, it will still leave the circuit balanced well enough without it.

As Tom said, the voltages you were indicating -- particularly on pins 1&3 are well off. I would check that the values of components installed on these pins are in fact what you think they are, and check your wiring there as well. If the voltages you indicated are in fact true, then this portion of the circuit is definitely a problem that must be addressed before going any further.

I hope this helps.

Good luck with your amp!

Dave
 
MR. GILLESPIE: Thank you much!!!!! One tube not two. Hmmmm.... No wonder it's a disaster. I have much study to do with schematics.
 
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