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 Post subject: Re: 6V6 amp project- with a twist
PostPosted: Feb Mon 05, 2018 6:29 am 
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john8750 wrote:
Cant say I understand how it all works. If the cathode voltage was very low, don't that figure the plate current was low? Does the grid need to be lower than the cathode to conduct the tube into operation? This is a great learning experience for me.
All cathode and grid voltages are right in there, now.

First let's establish that when I say "grid volt" that's the voltage from grid to cathode. So if the cathode is +18 Volts and the grid is at 0 (ground) that's (0 - 18) -18 V "grid volt."

Now the idle current is a -18 grid volt. Any lower voltage (more negative) will be less current and any higher (less negative) will be more current. 'Max' current (under normal operation) will be when the grid volt is 0 (or near zero, but don't worry about that fine point). For the 6V6 with screen at 250 V (268 with the cathode at 18 V) that's 110 mA. They don't have 300 V (318 absolute, 300 V + 18 V cathode) graphed but it would be more, say on the order of 150 mA.

So if the grid volt were 0 (whatever the cathode and grid were) the tube should be conducting something on the order of 150 mA @ 300 V (assuming the power supply could handle it) for a power dissipation of 45 Watt, which is a teensy bit more (understatement) than 12 Watt max for the plate. That's red plate territory for sure.

But it gets worse. At a POSITIVE grid volt of 18 the tube conducts 50% more than at 0. It should have been dissipating in excess of 68 Watt!

There's still another twist. Tubes are generally operated with the grid negative because is it goes positive the grid becomes forward biased and it conducts, taking grid current. at 18 V positive it takes roughly 45 mA (which is why the normal tube preamp can't drive the grid positive) but there's a 1k resistor and the voltage drop would be 45 Volt, plus 18 volt, for 63 V and I don't see a rat's chance in hell that opamps on a 24 V supply could put out 63 V.

So, as I said, none of it makes any sense. The B+ power supply would be severely overloaded, the tube(s) should have been redplating, and the opamps couldn't drive the grid +18 anyway.


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 Post subject: Re: 6V6 amp project- with a twist
PostPosted: Feb Mon 05, 2018 6:33 am 
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john8750 wrote:
I could swap this transformer for the 12volt 1amp I have in there. It could power filaments with 450ma 24volts.
Will I have enough power left, 1.6amp output, for the IC supplies? Could use zeners for +- 20volt and +13volt supplies.
Don't need the 24volts.
https://www.ebay.com/itm/24-VOLT-HVAC-t ... SwmSdadQo9

Could use 78-79 IC regulators.
Why do you want to add/change another transformer in the first place? It's not going to make the solid state draw any less current, just at a lower voltage, but you have enough 'power' in the 120 V transformer as is. I don't see that you're 'solving' anything.


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 Post subject: Re: 6V6 amp project- with a twist
PostPosted: Feb Mon 05, 2018 7:15 am 
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john8750 wrote:
Pbpix wrote:
John:
What is the HV at now?
You said you were unable to get it over 300v I think?
Why is that I wonder?

Whatever B+ voltage you have .. put your scope on it (AC mode) and measure the peak-peak ripple voltage level.
I'm very curious.

As I have mine now I get less than 20mv pp ripple.

But I'm using larger filter caps.



Its right about 295-300 Peter. I need to consider where the rough sounding edges are coming from. Did you say that the opa604 had a harsh sound?

295-300v is the same for me but I'm using one less filter stage and a single 220 ohm resistor dropping 22 volts @100ma.
.... and I am getting only 20mv pp ripple.

what is your ripple voltage @ 150ma load ...when you put a scope probe on B+ (AC) range.

As for the opa604 being "harsh" .,..
No, I did not say that.
... but I did say that it seemed "tinny" .... and otherwise kind-of "flat".

Where, by comparison, I feel, the op275 has a much more round and robust sound coming through it ... and "full" rich tone quality about it.

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 Post subject: Re: 6V6 amp project- with a twist
PostPosted: Feb Mon 05, 2018 7:28 am 
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Flipperhome wrote:
john8750 wrote:
Cant say I understand how it all works. If the cathode voltage was very low, don't that figure the plate current was low? Does the grid need to be lower than the cathode to conduct the tube into operation? This is a great learning experience for me.
All cathode and grid voltages are right in there, now.

First let's establish that when I say "grid volt" that's the voltage from grid to cathode. So if the cathode is +18 Volts and the grid is at 0 (ground) that's (0 - 18) -18 V "grid volt."

Now the idle current is a -18 grid volt. Any lower voltage (more negative) will be less current and any higher (less negative) will be more current. 'Max' current (under normal operation) will be when the grid volt is 0 (or near zero, but don't worry about that fine point). For the 6V6 with screen at 250 V (268 with the cathode at 18 V) that's 110 mA. They don't have 300 V (318 absolute, 300 V + 18 V cathode) graphed but it would be more, say on the order of 150 mA.

So if the grid volt were 0 (whatever the cathode and grid were) the tube should be conducting something on the order of 150 mA @ 300 V (assuming the power supply could handle it) for a power dissipation of 45 Watt, which is a teensy bit more (understatement) than 12 Watt max for the plate. That's red plate territory for sure.

But it gets worse. At a POSITIVE grid volt of 18 the tube conducts 50% more than at 0. It should have been dissipating in excess of 68 Watt!

There's still another twist. Tubes are generally operated with the grid negative because is it goes positive the grid becomes forward biased and it conducts, taking grid current. at 18 V positive it takes roughly 45 mA (which is why the normal tube preamp can't drive the grid positive) but there's a 1k resistor and the voltage drop would be 45 Volt, plus 18 volt, for 63 V and I don't see a rat's chance in hell that opamps on a 24 V supply could put out 63 V.

So, as I said, none of it makes any sense. The B+ power supply would be severely overloaded, the tube(s) should have been redplating, and the opamps couldn't drive the grid +18 anyway.



24volts are a possibility, 24volt positive supply rail, with a malfunctioning opamp because of a wiring problem. I did not notice anything else wrong. The voltages were actual as I measured them. Now, it seems to be ok. Might be good thing the opamp could not drive the tube. I have no idea. Wish I could fill in the blanks.

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 Post subject: Re: 6V6 amp project- with a twist
PostPosted: Feb Mon 05, 2018 7:32 am 
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Pbpix wrote:
john8750 wrote:
Pbpix wrote:
John:
What is the HV at now?
You said you were unable to get it over 300v I think?
Why is that I wonder?

Whatever B+ voltage you have .. put your scope on it (AC mode) and measure the peak-peak ripple voltage level.
I'm very curious.

As I have mine now I get less than 20mv pp ripple.

But I'm using larger filter caps.



Its right about 295-300 Peter. I need to consider where the rough sounding edges are coming from. Did you say that the opa604 had a harsh sound?

295-300v is the same for me but I'm using one less filter stage and a single 220 ohm resistor dropping 22 volts @100ma.
.... and I am getting only 20mv pp ripple.

what is your ripple voltage @ 150ma load ...when you put a scope probe on B+ (AC) range.

As for the opa604 being "harsh" .,..
No, I did not say that.
... but I did say that it seemed "tinny" .... and otherwise kind-of "flat".

Where, by comparison, I feel, the op275 has a much more round and robust sound coming through it ... and "full" rich tone quality about it.



I will measure that for you later Peter. Right now just getting it working is a big job.
Tinny and flat sounds pretty harsh to me. :lol:

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 Post subject: Re: 6V6 amp project- with a twist
PostPosted: Feb Mon 05, 2018 7:39 am 
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Flipperhome wrote:
john8750 wrote:
I could swap this transformer for the 12volt 1amp I have in there. It could power filaments with 450ma 24volts.
Will I have enough power left, 1.6amp output, for the IC supplies? Could use zeners for +- 20volt and +13volt supplies.
Don't need the 24volts.
https://www.ebay.com/itm/24-VOLT-HVAC-t ... SwmSdadQo9

Could use 78-79 IC regulators.
Why do you want to add/change another transformer in the first place? It's not going to make the solid state draw any less current, just at a lower voltage, but you have enough 'power' in the 120 V transformer as is. I don't see that you're 'solving' anything.



Do you mean it will still consume 313ma to power about 90ma, without the dropper cap circuits?
I could sure do without all the heat producing ballast resistors used with those dropper cap circuits.
And if I go back to 100 ma for the tubes, 25ma CCS, I could use a smaller power transformer, say 50watt.

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 Post subject: Re: 6V6 amp project- with a twist
PostPosted: Feb Mon 05, 2018 7:40 am 
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john8750 wrote:
I will measure that for you later Peter. Right now just getting it working is a big job.
Tinny and flat sounds pretty harsh to me. :lol:

Also measure the voltage drop across each 1ohm cathode resistor ( to get idle current) should be 37ma.
Then as you confirm that each one is at 37ma please also record the dc volts on the cathode ie 18v

Do that for each tube A, B , C, D
and please show the current for each and the cathode voltage for each

So like this:
A= 37ma , 17.5v
B= 37ma, 18.2v
C= 36.7ma, 19.2v
D= 37.2ma, 20.1v

etc.

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 Post subject: Re: 6V6 amp project- with a twist
PostPosted: Feb Mon 05, 2018 7:46 am 
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john8750 wrote:

I could sure do without all the heat producing ballast resistors used with those dropper cap circuits.

I don't understand why the resistors in the IC supplies are so hot.

please measure the actual voltage dropped across those "hot resistors" indicating which ones are which.

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 Post subject: Re: 6V6 amp project- with a twist
PostPosted: Feb Mon 05, 2018 8:03 am 
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John:

Please mark the voltages you have at all the RED Vs on the schematic .... ok?

Attachment:
6v6 twist red v.png
6v6 twist red v.png [ 121.42 KiB | Viewed 748 times ]

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Last edited by Pbpix on Feb Mon 05, 2018 8:05 am, edited 1 time in total.

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 Post subject: Re: 6V6 amp project- with a twist
PostPosted: Feb Mon 05, 2018 8:04 am 
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I will get the measurements tomorrow. The shop is closed for the night. All those power supply resistors are getting hot. The 470 in the 13volt supply is the hottest. That resistor is dropping 30volts. Supply load is 39ma. What about the zener, must be seeing a good load.

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 Post subject: Re: 6V6 amp project- with a twist
PostPosted: Feb Mon 05, 2018 8:16 am 
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john8750 wrote:
I will get the measurements tomorrow. The shop is closed for the night. All those power supply resistors are getting hot. The 470 in the 13volt supply is the hottest. That resistor is dropping 30volts. Supply load is 39ma. What about the zener, must be seeing a good load.


That's a lot of wasted current going up in heat.
Esp If the load only draws 39ma.

If you are actually dropping 30v across 470 ohms ... that means 64ma

So 64ma - 39ma = 25ma extra left over just wasted going through that Zener.... It's just being wasted there until or just in case your load should ever require more.

25ma of EXTRA current seems like a waste. That represents an extra 65% of the live load as spare current. All going into that poor 13v Zener. You can easily keep the Zener alive with only 2ma or 5 or 10ma. And if you know your load never is actually varying beyond 39ma... what a waste.


Maybe I could see 5ma, or10ma or even maybe 15ma ..... but 25 is a lot.

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 Post subject: Re: 6V6 amp project- with a twist
PostPosted: Feb Mon 05, 2018 6:34 pm 
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Got all the voltages this morning. Amp is working very well, but not using tone control yet. So far, I had a bad pot, open solder joint at the tone amp IC, bad solder joint at one of the preamp IC's.

MP3 connected and amp working:

LV1-----38ma-----18.51V-----Grid .o44V
LV2-----39ma-----17.62V-----Grid -.033V
RV1-----36ma-----17.16V-----Grid .057V
RV2-----38ma-----17.66V-----Grid -.051V

Attachment:
PS voltages.png
PS voltages.png [ 224.48 KiB | Viewed 725 times ]

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 Post subject: Re: 6V6 amp project- with a twist
PostPosted: Feb Mon 05, 2018 9:15 pm 
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I put the tone amp back in the system. Only one channel works, balance pot not working. Another bad pot? So I did a check on all 4 pots. Pulled the chip and found 3 pots not working. Followed the circuits back to the pot, looks like bad pots. No voltage on the middle wiper on all three pots. Now the volume pot, I just replaced was ok.
I looked closely and saw a small rivet that connects the wiper plate to the solder lug. Sure enough, just touching that rivet made contact. So, a shot of contact cleaner got all the pots working again.
These 50K pots were in the surplus supply where I bought them. 50 cents each. I guess still not a good deal. I will order new pots.
Attachment:
pott.jpg
pott.jpg [ 127.82 KiB | Viewed 724 times ]

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 Post subject: Re: 6V6 amp project- with a twist
PostPosted: Feb Mon 05, 2018 10:57 pm 
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john8750 wrote:
I put the tone amp back in the system. Only one channel works, balance pot not working. Another bad pot? So I did a check on all 4 pots. Pulled the chip and found 3 pots not working. Followed the circuits back to the pot, looks like bad pots. No voltage on the middle wiper on all three pots. Now the volume pot, I just replaced was ok.
I looked closely and saw a small rivet that connects the wiper plate to the solder lug. Sure enough, just touching that rivet made contact. So, a shot of contact cleaner got all the pots working again.
These 50K pots were in the surplus supply where I bought them. 50 cents each. I guess still not a good deal. I will order new pots.
Attachment:
pott.jpg
Man, you've had a bad time with pots!


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 Post subject: Re: 6V6 amp project- with a twist
PostPosted: Feb Mon 05, 2018 11:10 pm 
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john8750 wrote:
Got all the voltages this morning. Amp is working very well, but not using tone control yet. So far, I had a bad pot, open solder joint at the tone amp IC, bad solder joint at one of the preamp IC's.

MP3 connected and amp working:

LV1-----38ma-----18.51V-----Grid .o44V
LV2-----39ma-----17.62V-----Grid -.033V
RV1-----36ma-----17.16V-----Grid .057V
RV2-----38ma-----17.66V-----Grid -.051V

Attachment:
PS voltages.png
All the cap dropper supplies are right spot on with my simulations, so that's good. B+ is a tad low but that's probably due to being a different transformer than I simulated, plus my simulation of transformers not being precise, and transformers having tolerances as well.

The CCS current seems to have a rather wide variance from a high of 39 mA to a low of 36 mA. Is that with 10% resistors or are they all 1%?


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 Post subject: Re: 6V6 amp project- with a twist
PostPosted: Feb Mon 05, 2018 11:18 pm 
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Flipperhome wrote:
john8750 wrote:
I put the tone amp back in the system. Only one channel works, balance pot not working. Another bad pot? So I did a check on all 4 pots. Pulled the chip and found 3 pots not working. Followed the circuits back to the pot, looks like bad pots. No voltage on the middle wiper on all three pots. Now the volume pot, I just replaced was ok.
I looked closely and saw a small rivet that connects the wiper plate to the solder lug. Sure enough, just touching that rivet made contact. So, a shot of contact cleaner got all the pots working again.
These 50K pots were in the surplus supply where I bought them. 50 cents each. I guess still not a good deal. I will order new pots.
Attachment:
pott.jpg
Man, you've had a bad time with pots!




I hear ya Flip. I have some new ones commin. That's an easy fix.
Do you think I damaged my tubes with the grid-cathode problem? I fixed it, sounds ok.
Could be, being the small wooden chassis, is building up the heat underneath. Maybe a larger metal chassis would be better.
Or, just for a power amp. Build a separate preamp box. I mean, the amp circuit is so simple by itself.
I know what you are saying right now, ' here we go again, every one of John's projects drags us to hell and back '
Sorry about it. I hope you guys have fun helping me. I sure do enjoy it.

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 Post subject: Re: 6V6 amp project- with a twist
PostPosted: Feb Mon 05, 2018 11:21 pm 
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Flipperhome wrote:
john8750 wrote:
Got all the voltages this morning. Amp is working very well, but not using tone control yet. So far, I had a bad pot, open solder joint at the tone amp IC, bad solder joint at one of the preamp IC's.

MP3 connected and amp working:

LV1-----38ma-----18.51V-----Grid .o44V
LV2-----39ma-----17.62V-----Grid -.033V
RV1-----36ma-----17.16V-----Grid .057V
RV2-----38ma-----17.66V-----Grid -.051V

Attachment:
PS voltages.png
All the cap dropper supplies are right spot on with my simulations, so that's good. B+ is a tad low but that's probably due to being a different transformer than I simulated, plus my simulation of transformers not being precise, and transformers having tolerances as well.

The CCS current seems to have a rather wide variance from a high of 39 mA to a low of 36 mA. Is that with 10% resistors or are they all 1%?



Sorry Flip, Carbon 1ohmers. tolerance, 50%. I will isolate them and get an exact measurement.

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 Post subject: Re: 6V6 amp project- with a twist
PostPosted: Feb Mon 05, 2018 11:48 pm 
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OMG- You all will not believe this. I never changed the power transformer. Just using the old one I had there. Don't know the rating of it. But not 100va, might be half that. I was just playing the amp, went to feel the power tranny for heat, about a thousand degrees, very hot. So, let me mount the correct transformer. Could be why your figuring was off, Flip.
I think I will also remodel the amp, power amp only.

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 Post subject: Re: 6V6 amp project- with a twist
PostPosted: Feb Mon 05, 2018 11:49 pm 
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Well until you get precision 1 ohm resistors you could actually open the cathode ckt and place an amp meter in series for the 4 test readings.

BTW... did you measure the ripple voltage on the B+?

Also... I think you can cut down the heat on that 470 ohm resistor in the 13v supply if you replace that 3.3uf dropper cap with something smaller like 2uf or 2.5uf.

You need to drop that 40v down to about 20v .... then it will be better.

If you can get it down to 20v .... and your Zener is 13v, then you need to only drop 7volts.

So if your load normally draws 39ma and a max of possibly 48ma, then you can provide for a max of 50ma.

To drop 7 volts @ 50ma = 140 ohms

7v x 50ma = .35 watts ( triple = 1 watt)

-----------------------------

If you can't get the voltage down to 20v.... using a 2uf dropper cap, maybe you can get down to 30v
30v -13v = 17v you have to drop at 50ma
then use 340 or 330 ohms

17v / 330 ohms = 51ma
In that case you'd need to drop 17v at 50ma = .88 watts ( triple 2.6watts)

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 Post subject: Re: 6V6 amp project- with a twist
PostPosted: Feb Mon 05, 2018 11:54 pm 
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john8750 wrote:
I hear ya Flip. I have some new ones commin. That's an easy fix.
Do you think I damaged my tubes with the grid-cathode problem? I fixed it, sounds ok.
Could be, being the small wooden chassis, is building up the heat underneath. Maybe a larger metal chassis would be better.
Or, just for a power amp. Build a separate preamp box. I mean, the amp circuit is so simple by itself.
I know what you are saying right now, ' here we go again, every one of John's projects drags us to hell and back '
Sorry about it. I hope you guys have fun helping me. I sure do enjoy it.
I doubt you damaged the tubes. For one, I don't believe those numbers from when you had positive volt on the grid anyway. Don't misunderstand, I believe that's what you THINK they were but something must have been amiss with the measurement or else, as I said, you'd have seen a rash of other, rather spectacular, problems real quick.

I've been meaning to suggest that your 'heat' problem is likely due to the case and confining 'hot' components in a small area so they mutually warm each other by radiation. "Free air" dissipation depends on spacing and, well, FREE air circulation. Without it they will heat the air, it 'stays' there, and everything (in the area) gets hotter than if it were allowed to circulate. It doesn't seem like much but you'd be surprised how fast it mounts up. I ran into that with my first amp, also in a wooden chassis, and before I vented it I was seriously cooking the insides.


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