Yes, that’s exactly what the last two stages are for.
Even with the choke fully connected, allowing the EF86 and 12AT7 to remain powered up, if I disconnect the individual wires to the screen grids of the power tubes, the trans goes quiet. I’m not sure what this means though and how to remedy this.
Agreed, very confusing. Let me know what you think.
Thank you!
Diyengineer,
You posted before I finished my edits to my post.
Please look at my latest post # 40.
Your symptom that when you disconnect the screens of the output tubes, the hum goes away might confirm my suspicions.
When there is no screen B+, the output tubes do not conduct current (or very little current).
If the plates of the KT66s (more accurately, the center tap of the output transformers) are connected to the Output of the choke, that would 'quiet down' the choke with the screens not powered.
If the KT66s xfmrs CTs are connected Before the chokes, then the no-current/very low current of non-powered screens would make a buzzing power transformer quiet down.
You ought to have this problem fixed very shortly. I can hardly wait to hear about a successfully fixed amplifier.
You posted before I finished my edits to my post.
Please look at my latest post # 40.
Your symptom that when you disconnect the screens of the output tubes, the hum goes away might confirm my suspicions.
When there is no screen B+, the output tubes do not conduct current (or very little current).
If the plates of the KT66s (more accurately, the center tap of the output transformers) are connected to the Output of the choke, that would 'quiet down' the choke with the screens not powered.
If the KT66s xfmrs CTs are connected Before the chokes, then the no-current/very low current of non-powered screens would make a buzzing power transformer quiet down.
You ought to have this problem fixed very shortly. I can hardly wait to hear about a successfully fixed amplifier.
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Thanks 6a3summer,
I wish I had wired this incorrectly, and the fix be that easy. But the output trans CT definitely is at the choke input.
I disconnected the choke and measured 585R. Then I measured a Vdrop of 5.8V across the choke. That’s 9mA.
Not sure what to try next.
Thanks for all the help!
I wish I had wired this incorrectly, and the fix be that easy. But the output trans CT definitely is at the choke input.
I disconnected the choke and measured 585R. Then I measured a Vdrop of 5.8V across the choke. That’s 9mA.
Not sure what to try next.
Thanks for all the help!
Diyengineer,
Unless the choke is poorly constructed, I doubt that 9mA makes it noisy.
Did you check if your stainless steel was non-magnetic, or if it is the magnetic kind?
Otherwise, that takes me back to a power transformer that either does not have the windings vacuum varnished to captivate them, or laminations and end bell that is noisy.
One more long shot: If your output tube currents are badly balanced, then the output transformers might hum from the unbalanced current, and that would also cause hum in the loudspeakers. That B+ ripple is 200mV or more, so if the two sides of the primary have unbalanced currents, that ripple will cause hum in the loudspeaker.
Unless the choke is poorly constructed, I doubt that 9mA makes it noisy.
Did you check if your stainless steel was non-magnetic, or if it is the magnetic kind?
Otherwise, that takes me back to a power transformer that either does not have the windings vacuum varnished to captivate them, or laminations and end bell that is noisy.
One more long shot: If your output tube currents are badly balanced, then the output transformers might hum from the unbalanced current, and that would also cause hum in the loudspeakers. That B+ ripple is 200mV or more, so if the two sides of the primary have unbalanced currents, that ripple will cause hum in the loudspeaker.
I really don’t think it’s coming from the choke, I had temporarily replaced it with the resistor of the same value, and the buzz was still there. It really sounds like it’s coming from the power transformer but goes away when the screens are disconnected.
I forget the alloy of stainless I bought. I’ll check if it’s magnetic or not when I get home.
The same exact noises heard from both mono blocks, so between the two transformers, maybe Hammond transformers are just not the greatest.
Thanks
I forget the alloy of stainless I bought. I’ll check if it’s magnetic or not when I get home.
The same exact noises heard from both mono blocks, so between the two transformers, maybe Hammond transformers are just not the greatest.
Thanks
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When the screens are disconnected, the power supply current is significantly reduced.
That means that the current to charge the input 220uF capacitor is also significantly reduced.
That would quiet down the power transformer.
Does your amp have plenty of power?
Would you be willing to use a Choke input power supply with an extra choke, (5H @ 200mA)? A choke input supply is has much lower transient currents, and would make the power transformer quieter.
To get back some of the lost power, you could also add a negative fixed bias supply, and 4 potentiometers for individual bias adjustments (and rock back and forth multiple times to get them balanced).
Probably you do not have room for fixed bias supply and potentiometers.
I am a fan of less power and more finesse sound and cooler amplifiers. I use choke input supplies, when I can.
But you may need the extra power.
That means that the current to charge the input 220uF capacitor is also significantly reduced.
That would quiet down the power transformer.
Does your amp have plenty of power?
Would you be willing to use a Choke input power supply with an extra choke, (5H @ 200mA)? A choke input supply is has much lower transient currents, and would make the power transformer quieter.
To get back some of the lost power, you could also add a negative fixed bias supply, and 4 potentiometers for individual bias adjustments (and rock back and forth multiple times to get them balanced).
Probably you do not have room for fixed bias supply and potentiometers.
I am a fan of less power and more finesse sound and cooler amplifiers. I use choke input supplies, when I can.
But you may need the extra power.
HI
this problem just from ac power trans it make noise .when turn on power switch...and output tubes receive to normal bias and warm up ...buzz starts from inside of trans !!!!
4 tubes in push pull needs good quality ac power trans .
can you change with trodial trans ?it is Silent no buzz ....
this problem just from ac power trans it make noise .when turn on power switch...and output tubes receive to normal bias and warm up ...buzz starts from inside of trans !!!!
4 tubes in push pull needs good quality ac power trans .
can you change with trodial trans ?it is Silent no buzz ....
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I don't think so...The transformer must be running hot. Something like 300mA secondary would be needed for this load.
Of course it did!
Sorry Diyengineer,
But I, for one, am guilty of not digesting completely what you have previously posted. You might need to re-cap...
I thought 6A3s had nailed it when he pointed out there is little current flow in the KT66s when the S' Grids are open. But you said ''Disconnected the output trans, buzz was still there'' so there is still a buzz with no anode current at all? Unless the screen grids were trying to pass 35mA each (Not recommended!)
Maybe a picture or two might help?
But I, for one, am guilty of not digesting completely what you have previously posted. You might need to re-cap...
I thought 6A3s had nailed it when he pointed out there is little current flow in the KT66s when the S' Grids are open. But you said ''Disconnected the output trans, buzz was still there'' so there is still a buzz with no anode current at all? Unless the screen grids were trying to pass 35mA each (Not recommended!)
Maybe a picture or two might help?
The amp has way more power than I actually need. Knowing what I know now, I wouldn't have chosen this amp design to build, and am starting two new monoblocks to replace these. But they do sound very good in spite of the small amount of buzz that you hear at low or no volume.
Nevertheless, I would like to try to fix the buzz, and have no problem trying a choke input supply. I don't see a need to gain back any power either, there is already more than enough. But if negative fixed bias would improve the sound quality, I could try it, I have plenty of room in my chassis for the pots. I would probably start by doing the choke input supply and if successful, add the negative fixed bias.
Being fairly new to this hobby, and while I do know what a choke input supply is, but have no idea how to design one for a particular circuit. If you could help me in selecting an appropriate choke and circuit design, I would be grateful.
I know some have mentioned changing the power trans but I'd like to try to stick to the one I have for now since I'm building two new amps anyways.
The power trans runs currently at 115-120F during extended listening(2-3 hours).
Thanks
Nevertheless, I would like to try to fix the buzz, and have no problem trying a choke input supply. I don't see a need to gain back any power either, there is already more than enough. But if negative fixed bias would improve the sound quality, I could try it, I have plenty of room in my chassis for the pots. I would probably start by doing the choke input supply and if successful, add the negative fixed bias.
Being fairly new to this hobby, and while I do know what a choke input supply is, but have no idea how to design one for a particular circuit. If you could help me in selecting an appropriate choke and circuit design, I would be grateful.
I know some have mentioned changing the power trans but I'd like to try to stick to the one I have for now since I'm building two new amps anyways.
The power trans runs currently at 115-120F during extended listening(2-3 hours).
Thanks
Diyengineer,
If you can get a 5H 200mA choke, then break the connection from the high voltage diodes output where they connect to the junction of the 200uF cap and the output transformer center tap.
Connect the diode outputs to one end of the 5H choke. Connect the other end of the 5H choke to the junction of the 200uF cap and the output transformer center tap.
You have just created a choke input power supply.
The B+ voltage will be lower now. You may need to adjust the output tubes cathode self bias resistors to be a little less resistance, in order to bring the output tubes cathode currents to be 30 mA to 35 mA each.
Be sure how you mount the 5H choke. It will have magnetic spray.
1. Do not mount the choke on magnetic steel, or use aluminum spacers of at least 1/4 inch between the chassis and the 5H choke. The Hammond 5H chokes I use have not had significant mechanical hum.
2. Be sure to keep the 5H choke coil oriented at right angles versus the output transformer coils.
Because of the lower B+, you may have to adjust the resistors of the input tube, and the driver tube. But even before you do that, you should be able to get some useful power to your loudspeakers.
If you can get a 5H 200mA choke, then break the connection from the high voltage diodes output where they connect to the junction of the 200uF cap and the output transformer center tap.
Connect the diode outputs to one end of the 5H choke. Connect the other end of the 5H choke to the junction of the 200uF cap and the output transformer center tap.
You have just created a choke input power supply.
The B+ voltage will be lower now. You may need to adjust the output tubes cathode self bias resistors to be a little less resistance, in order to bring the output tubes cathode currents to be 30 mA to 35 mA each.
Be sure how you mount the 5H choke. It will have magnetic spray.
1. Do not mount the choke on magnetic steel, or use aluminum spacers of at least 1/4 inch between the chassis and the 5H choke. The Hammond 5H chokes I use have not had significant mechanical hum.
2. Be sure to keep the 5H choke coil oriented at right angles versus the output transformer coils.
Because of the lower B+, you may have to adjust the resistors of the input tube, and the driver tube. But even before you do that, you should be able to get some useful power to your loudspeakers.
Alan4411,
Good point about the screens drawing lots of current when the output transformer center tap (and therefore the plates) were disconnected.
And that might be enough current to make the power transformer to still hum.
And it could quickly destroy the output tubes.
Diyengineer would need to disconnect voltage from Both . . . the output transformer center tap, And . . . from the screen grids (at the same time).
Good point about the screens drawing lots of current when the output transformer center tap (and therefore the plates) were disconnected.
And that might be enough current to make the power transformer to still hum.
And it could quickly destroy the output tubes.
Diyengineer would need to disconnect voltage from Both . . . the output transformer center tap, And . . . from the screen grids (at the same time).
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Earlier it was stated that the 200mA transformer was not capable of running this amp.
It certainly should be able to with a choke input power supply if the choke has critical inductance.
It is another thing for it to power the capacitor input filter, when the input cap is such a large capacitance. It probably will be quite warm because of the combination of cap input and the amp load currents.
It certainly should be able to with a choke input power supply if the choke has critical inductance.
It is another thing for it to power the capacitor input filter, when the input cap is such a large capacitance. It probably will be quite warm because of the combination of cap input and the amp load currents.
Thanks Summer,
If I’m understanding right, the new 5h choke will take the place of the first 220uf filter stage and the 20R series resistor. Will the existing 30h choke still be used to feed the later filter stages?
Also, I understand the 200mA rating but how did you arrive at 5H?
Are Hammond chokes considered good? Or is there other preferred brands?
The Hammond 193H should work I think.
193H - Hammond Mfg.
Thank you
If I’m understanding right, the new 5h choke will take the place of the first 220uf filter stage and the 20R series resistor. Will the existing 30h choke still be used to feed the later filter stages?
Also, I understand the 200mA rating but how did you arrive at 5H?
Are Hammond chokes considered good? Or is there other preferred brands?
The Hammond 193H should work I think.
193H - Hammond Mfg.
Thank you
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Thanks for the pictures Diyengineer.
That is a very tidy build, nicely laid out and a lot of planning I'm sure. You should be very pleased with it.
Only comments I would make is the heater wiring is a bit 'loose'. Lots more twists per inch and hard to the metal chassis next time
. Have a look at the Sticky Heater Wiring - the Good the Bad and the Ugly It does make a difference. And I guess the grounds all go back to the input socket and that the shielded cable from the pot is only grounded at one end?
Off the wall ideas, before you go down the extra choke route.
- Just try another 20 or 47 ohm from the end of the diode junction to the top of the first 220uF cap and replaces the wire there at the moment. It will take a bit of the 'pulse' out on each charging cycle. (That is where I believe 6A3s intends the choke to go too.) Another look at the circuit and now the picture shows two 220uF caps in series, so effectively only a 110uF - 20 ohm - 110uF filter.
- Then what about trying the KT66s in triode mode? You would need four 100 ohm 1 or 2 watt resistors. Take the screen grid feeds off pins 4 and insulate them. Now solder a 100 resistor from pin 4 back to pin 3, the anode/plate.
Triode mode! That takes the screen grid supply out of the equation completely, so if the transformer still buzzes you will be sure it is the transformer as others suggest.
In fact I would try triode mode 'test' first.
Alan
That is a very tidy build, nicely laid out and a lot of planning I'm sure. You should be very pleased with it.
Only comments I would make is the heater wiring is a bit 'loose'. Lots more twists per inch and hard to the metal chassis next time
. Have a look at the Sticky Heater Wiring - the Good the Bad and the Ugly It does make a difference. And I guess the grounds all go back to the input socket and that the shielded cable from the pot is only grounded at one end?Off the wall ideas, before you go down the extra choke route.
- Just try another 20 or 47 ohm from the end of the diode junction to the top of the first 220uF cap and replaces the wire there at the moment. It will take a bit of the 'pulse' out on each charging cycle. (That is where I believe 6A3s intends the choke to go too.) Another look at the circuit and now the picture shows two 220uF caps in series, so effectively only a 110uF - 20 ohm - 110uF filter.
- Then what about trying the KT66s in triode mode? You would need four 100 ohm 1 or 2 watt resistors. Take the screen grid feeds off pins 4 and insulate them. Now solder a 100 resistor from pin 4 back to pin 3, the anode/plate.
Triode mode! That takes the screen grid supply out of the equation completely, so if the transformer still buzzes you will be sure it is the transformer as others suggest.
In fact I would try triode mode 'test' first.
Alan
Thanks Alan,
I just tried triode mode with no luck, still the same buzz.
As for the additional cap, are you saying put it in series between the diode’s in the first filter stage? Or just an additional 20uf cap to ground?
From other research I’ve done, will the choke take the place of the 110uF- 20R- 110uF filter stage? Or go before it in series? I read through summer’s post several and just want to be sure, I believe Summer means the choke will replace the 110uF- 20R- 110uF stage and still feed the existing 30h choke and later stages.
Thanks for the advice on the heater wiring, and yes all grounds go to the insulated input jack, and the shielded wire is grounded at one end only at the input jack.
Thank you
I just tried triode mode with no luck, still the same buzz.
As for the additional cap, are you saying put it in series between the diode’s in the first filter stage? Or just an additional 20uf cap to ground?
From other research I’ve done, will the choke take the place of the 110uF- 20R- 110uF filter stage? Or go before it in series? I read through summer’s post several and just want to be sure, I believe Summer means the choke will replace the 110uF- 20R- 110uF stage and still feed the existing 30h choke and later stages.
Thanks for the advice on the heater wiring, and yes all grounds go to the insulated input jack, and the shielded wire is grounded at one end only at the input jack.
Thank you
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This I what I had in mind. An additional resistor (22 or so Ohms like the green one you have installed already) between the diodes' junction and the +ve of the first 220uF capacitor.
I think 6A3s suggests the choke goes in the same place, dotted on the picture, but please let him confirm his thoughts.
I think you have proved the transformer hums under load. I hold little hope either will make it silent...
Another point occurred to me drawing the circuit. It is important that the transformer centre tap goes directly to the -ve of the bottom 220uF capacitors then to ground buss. That way the capacitor charging 'pulses' do not get onto the ground points.
I think 6A3s suggests the choke goes in the same place, dotted on the picture, but please let him confirm his thoughts.
I think you have proved the transformer hums under load. I hold little hope either will make it silent...
Another point occurred to me drawing the circuit. It is important that the transformer centre tap goes directly to the -ve of the bottom 220uF capacitors then to ground buss. That way the capacitor charging 'pulses' do not get onto the ground points.
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