Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: pbman1953 on July 05, 2015, 08:28:58 am
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Hi
Has anyone converted a YBA-3 from 6ca7 to 6550?
Here's the process-
Disconnected the bias voltage from pin 1 of the output tube sockets, shorted pin 1 to pin 8 (ground)
- Removed the two 47Ω screen resistors, installed four 1kΩ 5W screen resistors
My questions-
Is this mod worth it? With this mod will the sound be closer to a Sunn type sound. Meaning smoother with less grunge. Before I do anything I will check the bias to be sure it's not running (hot) on the higher of the scale for the stock tube.
I'm not a tech but have worked on many amps including cap upgrades. I am very careful. All I need is some direction and I know I can perform the mod.
Step one says to connect pin 1 to pin 8
Above step two is to remove the 2- 47 ohm resistors. The install 4- 1k 5W , but does't explain what connection type.
Thanks for any help
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I don't think this mod will make the amp sound any closer to a Sunn amp than it does with the 6CA7s. Bass may be a bit stronger. Sunn used a completely different preamp and power amp circuit. BTW, a lot of Sunn amps used 6CA7s, but the circuit was still the same as the 6550/KT88 amps.
Disconnected the bias voltage from pin 1 of the output tube sockets, shorted pin 1 to pin 8 (ground)
This step will result in running the suppressor grid of a 6CA7 at zero volts rather than -50V. I don't think you'll be able to hear a difference. But... The 6550 has no suppressor grid and pin 1 'may' be connected to the tube base if the base is metal. Connecting pin 1 to ground makes sense. Doing so will allow you to use either tube type.
Removed the two 47Ω screen resistors, installed four 1kΩ 5W screen resistors...
...Above step two is to remove the 2- 47 ohm resistors. The install 4- 1k 5W , but does't explain what connection type.
This is probably a good idea. If pin 6 is not used this is how I would do it...
First remove all wires and or resistors from pin 4 of all four tubes. Now connect all pin 6s together and also connect a wire from one pin 6 to R30 (If R30 had been connected directly to pin 4 of a socket just reconnect it to pin 6 of that same socket). Finally, mount a 1K/5W resistor between pin 4 and 6 of each power tube socket. (Leave the leads a little long so the resistor will sit high above the socket.)
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My replies-
Disconnected the bias voltage from pin 1 of the output tube sockets, shorted pin 1 to pin 8 (ground)
So are you saying, don't bother with this step?
This is probably a good idea. If pin 6 is not used this is how I would do it...
All pin 6's are being used and are already tied together. On the left side the first pin 6 is connected to V3 preamp tube. On the right and last in the row is connected to the Standby switch and light
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So are you saying, don't bother with this step?
No. What I said was... "Doing so will allow you to use either tube type."
All pin 6's are being used and are already tied together. On the left side the first pin 6 is connected to V3 preamp tube. On the right and last in the row is connected to the Standby switch and light
WHAT? That's not what I see in the pics at all. There is nothing connected to pin 6 on the left or right socket. The two sockets in the middle have a 1500Ω connected between pin 6 and pin 5.
But, since two of the sockets are using pin 6 you'll need to just use a terminal strip or standoff turrets to mount the 1K/5W. Or you could still do it as I said above but you'll need to 'fly' the 1500Ω resistors to pin 5, thus freeing up pin 6 for the two middle sockets.
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In your second pix above...the way you have the lead from your 5W screen resistor (connected to pin 4) crossing over tube socket pin 5.....
makes me rather nervous! I realize that often, posted pix are very lacking in their portrayal of depth. But, if that lead touches your pin 5 that output tube is probably destroyed in seconds.
If you threaded that lead through pin 6 and ran on to pin 4, then seriously bent pin 5 down towards the chassis...OK.
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I looked closer at the sockets-
Starting from the left, socket closest to V3 preamp tube
1- pin 6 is un-used
2 & 3 (middle sockets)- you are correct -pin 6 & 5 are connected with a 1.5k resistor on each socket
4- pin 6 - un-used
While working the mod I will make sure nothing is close to pin 5 and any un-used pin 5's will be pushed away from any exposed wire leads- Thanks for noticing that
Ok, let's re-cap the steps-
1) On all tubes jump pins 1 & 8
2) Remove both 5 watt resistors
3) Remove remaining wires from the other pin 4's, which is the 1k/20 watt feed
4) tie all pin 6's
5) Take one wire from the 1k / 20 watt to only one pin 6
6) Install a 1k/5w between each pin 4 & 6
I have height in the chassis to be able to " fly" each resistor
One question - does the resistor (1.5k) connection between pins 5 & 6 , on the middle sockets, change? Meaning, when I tie all pin 6's together that connection still stays?
Thanks
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...any un-used pin 5's will be pushed away (There won't be any unused pin 5.)
1) On all tubes jump pins 1 & 8 (and be sure pin 8 is connected to ground)
One question - does the resistor (1.5k) connection between pins 5 & 6 , on the middle sockets, change?
Yes. Unsolder the wire and the end of the resistor connected to pin 6 to free it up. Leave the resistor connected to pin 5 and swing the resistor body around so it is pointing away from the socket. Now solder the dangling wire to the dangling end of the resistor. I would use heatshrink tubing to insulate the solder joint and the resistor body. Do this for both middle sockets.
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Great, thanks for the help!
I'm not sure if this amp can handle them but I was wondering if I could use kt 88 in this amp. I already a Sunn 2000s with 6550 tubes.
Thanks
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KT88 and 6550 is pretty much the same thing.
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"1) On all tubes jump pins 1 & 8 (and be sure pin 8 is connected to ground)"
All pin 8's are tied together and with checking with my meter, pin 8 does ground out to chassis.
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I just bought the 1k's and will probably do the work later. I do have a mached pair of 6550's and need another pair. While looking can I test the mod with 2- 6550 and 2 el34's?
I won't stress it .. Also, I'm going to remove the spring clips, because they are too small, and install spring tube retainers
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I just noticed that all the 8's are jumped to each other and the 1's are jumped to each other.
I suppose all I have to do is jump one set and all the 1's & 8's will be joined to ground?
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While looking can I test the mod with 2- 6550 and 2 el34's?
I would not mix them. Test it with 4 6CA7s or just two 6550s. You'll probably have to tweak the bias for each type tube.
I suppose all I have to do is jump one set and all the 1's & 8's will be joined to ground?
That's electrically correct. It would certainly look nicer to just jumper pin 1 to pin 8 on each socket and do away with half that duplicate wiring.
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Hmmmm, I get a hum and the 6550's get real red
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Shut it down, their red-plating, it will kill them running them that hot! :w2:
You have to adjust the -bias, increase it to cool them off.
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with the bias control maxed it still hums and tube are red, but as soon as I see red I shut down
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I just checked with my Bias Rite,
Voltage is just over 500v
Idle current is off the chart
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Leave the power tubes out until you are sure you have -dcv on all the power tube grid pins and have enough -dcv.
Take the power tubes out and measure the -dcv at each grid on the power tube sockets with the -bias set full up.
Post what you get.
If you have -dcv then;
Here's a hi-lighted drawing of you amps -bias. Blue circle shows 10K adjust pot with a 47K R below it. (You might have to make that R lager, maybe 68K or a 100K.)
The red circle shows a series R of 15K, you might need to make that 10K or a little less.
You might need to play with both a little.
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I just checked with my Bias Rite, Voltage is just over 500v. Idle current is off the chart.
Leave the power tubes out until you are sure you have -dcv on all the power tube grid pins and have enough -dcv.
You already should know that the idle current is 'off the chart' because the plates are red hot. No need to test them for current and cook them more. :w2:
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Big typo, should be -DCV not -acv from the bias supply. Very sorry! :BangHead:
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Not to sound ignorant but want to be safe because I can do this but I'm not a full fledged tech.
Tubes are out I have a meter . Placing the DC on 1000?
You want me to test pins 4 &5 ?
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To be safe use your clip on gator meter probes. With amp off clip on pin 5 which is the power tubes control grid and the other clip to ground. (Pin 4 is the screen grid.)
Keep your hands out/off of the chassis, turn on the amp, take reading, write it down, turn off amp, wait a few seconds for B+dcv to drain off, like 30 or so, then repeat.
Yes DCV on meter and it should be around 50 to 100-dcv.So you don't need to use the 1000 v setting.
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Ok, meter is at 1000 volts DC setting
Black to ground
pin 4- 567 volts
pin 5 - 0 volts
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Ok, forget about pin 4, that's the screen grid and has nothing to do with the (negative) -dcv on pins 5.
So you have no -dcv on pins 5?
Was the amp working before with no red plating?
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Yes, I did exactly the steps that I wrote above
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Possibly jumping pins 1 & 8 changed things?
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This may be it- I didn't remove the bias wire from pin 1
QuoteDisconnected the bias voltage from pin 1 of the output tube sockets, shorted pin 1 to pin 8 (ground)
This step will result in running the suppressor grid of a 6CA7 at zero volts rather than -50V. I don't think you'll be able to hear a difference. But... The 6550 has no suppressor grid and pin 1 'may' be connected to the tube base if the base is metal. Connecting pin 1 to ground makes sense. Doing so will allow you to use either tube type.
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Possibly jumping pins 1 & 8 changed things?
It should not have and you have the tubes out so pins 5 should be isolated.
On the other hand if the amp was working before you changed some things then maybe you miswired something of broke off a wire?
Or your not hooking up your meter correctly?
Look at this drawing, measure for -dcv in the yellow and green circles and post -dcv's for them.
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This may be it- I didn't remove the bias wire from pin 1
QuoteDisconnected the bias voltage from pin 1 of the output tube sockets, shorted pin 1 to pin 8 (ground)
That should be it alright. The -bias is being shorted to ground.
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So the wire is purely removed and shrink , unused?
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I don't know how it's actually wired in the amp but looking at the schemo, you leave the -bias wire to feed the ends of the 220K grid leak R's where they come together.
Any other wire that goes from that junction over to pin 1 take it out or if you might want to go back to stock, then yes just tape/shrink wrap 1 end.
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Now pin 5 has - 50 volts DC
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Actually -97.2
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Actually -97.2
OK, that's good. You need to check all 4 power tube sockets pin 4. Doh!!!!! PIN 5
Here's another drawing but I think you have it fixed, get rid of red wire.
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It's quiet now and I shrink off that wire
The only left over is that the bias is all the way down but the idle current high. In the past I've always got 30-40 miliamps, it's showing in the mid 70's
Pin 4's
1- 518
2- 526
3- 524
4- 516
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Once your sure you have -bias on all power tubes pin 5, then leave the -bias full up, -97.2-dcv so the tubes run the coolest and then use your bias right to set the bias for idol current.
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pin 5
-106
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You have to measure EACH pin 5 on all 4 power tube sockets for -dcv.
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Pin 4's.....
My mistake again. Very sorry. Pin 5. :BangHead:
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how do i get the idle current down?
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The only left over is that the bias is all the way down but the idle current high. In the past I've always got 30-40 miliamps, it's showing in the mid 70's
More negative dcv will idol the tubes cooler, ie, -60dcv is more than -40dcv.
Are you sure you have -dcv on all 4 pin 5's?
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yes, but niw has gone down to -55
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The -bias supply should be stable and should not be going up and down.
Power tubes draw no current from the -bias so they should be stable.
Did you move/bump the -bias adjust pot?
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No, the pot is solid, still -55, when it's all the way down. If turned up it goes lower but not much, only to 53 or 52
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OK, somethings wrong, you had as much as -106dcv.
You might have a partial short, like a single strand of thin wire or a little blob of solder shorting somewhere.
Look at my reply #27 and with the power tubes out take -dcv readings for the yellow and green circles and post them.
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Will post in the AM. I'm done for today.
Thanks!!!
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Here's what measured
On off
Yel Circle -59 -108.9
Green Cir. -74 -135.8
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What do you mean by "ON" and "OFF"? Is that the standby switch? Power switch? Tubes in versus tubes out?
Those negative voltages should not change. They should be pretty much rock solid.
BTW, did this amp work properly before you started this mod?
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on = power is on
off = amp in standby mode
I think I have a bad socket .
This amp did work before he mod , but possibly problems emerging
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Measure for ACV in the red and blue circle in stand by mode and post. They should be 0 acv's.
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If I'm right it's the 2 brown wires feeding the diodes
If yes-
On the left one 425
On the right one- 462
In standby on AC volts setting
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Yes the PT B+ secondary wires going to the B+ diodes.
In stand by they both should be 0 acv's.
The stand by switch in that amp, looking at the schemo, is wired up to the B+ center tap (CT). It disconnects the CT from ground so there should be no acv voltage when the CT is disconnected from ground.
(Unless for some reason lifting the CT allows the B+ to float up where it want's but will allow no current?)
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Measure for DCV's in the red circle (after the B+ diodes, junction of B+ diodes and the 1st B+ filter cap) in stand by mode. It should be 0 dcv's.
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Let me know if this is the spot. If so 71 volts DC scale
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Yes I think that's it.
See if you can find C26. That cap's positive end goes to where the B+ diodes come together.
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You enlarged the pic in reply 53 too much.
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OK, I had overlooked the fact that the STBY switch breaks the CT of the HT winding. That explains the high negative bias voltage when in standby mode. The voltages you have are normal.
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OK, I had overlooked the fact that the STBY switch breaks the CT of the HT winding. That explains the high negative bias voltage when in standby mode. The voltages you have are normal.
Ok,
Then if you still can't get the current draw down in the power tubes you need to change a resistor or 2 as explained below;
Leave the power tubes out until you are sure you have -dcv on all the power tube grid pins and have enough -dcv.
Take the power tubes out and measure the -dcv at each grid on the power tube sockets with the -bias set full up.
Post what you get.
If you have -dcv then;
Here's a hi-lighted drawing of you amps -bias. Blue circle shows 10K adjust pot with a 47K R below it. (You might have to make that R lager, maybe 68K or a 100K.)
The red circle shows a series R of 15K, you might need to make that 10K or a little less.
You might need to play with both a little.
R37 and R36 are the resistors to change.
I'd also change the 2 bias filter caps if there original. In fact I'd change ALL the electrolytic caps if there original.
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Here's 2 links to our host Doug's library of tube amp information on biasing an amp;
http://el34world.com/charts/fenderservice5.htm (http://el34world.com/charts/fenderservice5.htm)
http://el34world.com/charts/Biascircuits.htm (http://el34world.com/charts/Biascircuits.htm)
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I don't think there is anything wrong with the bias supply. There is -74v at the output of the diode. That is applied to a voltage divider (15K, 0-10K pot, and 47K). The pot will swing the bias voltage from about -56v to -58.5v. Not much swing but it should make the 6CA7s run cool.
I suspect the tubes that were in the amp while the bias voltage was shorted to ground are probably now damaged.
You say your idle current is about 70mA. Is that for one tube? Two tubes? All four tubes? What is the procedure you used to measure idle current?
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You say your idle current is about 70mA. Is that for one tube? Two tubes? All four tubes? What is the procedure you used to measure idle current?
Actual is about 68ma,
2 tubes, I'm in the hunt for a pair unless both of these are damaged
I measured using my Bias Rite
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Put your EL34s back in it. Do they work properly?
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seem to
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I decided to bias the El's and in order to get close to match i need the bias control is swung the complete opposite direction as the 6550's. full turn clockwise
But all tube fall in a good range between 60%- 70%
Does that mean anything? Because of the full turn
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Does that mean anything? Because of the full turn
It means, like Sluckey said, there's not much 'swing', ie, change of adjustment with the bias pot, you only have about 2.5dcv's to play with. And that to get the EL34's and 6550's to bias up at about 60% to 70% of max dissipation you have to turn the bias full up or full down.
And that's just with those 2 pairs of tubes. Different EL34's and 6550's might draw more current or they might draw less current even if their the same brand as you have now.
IMO, it would be best to get some more swing in the bias pot. It would be nice to get at least 20dcv to 25dcv more swing centered around what you have now so your covered with different tubes in the future.
R37 and/or R36 (see reply #57) need to be changed to a different value to do this.
Sluckey (and others who can do the voltage divider math) will know what value(s) to use.
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I think you have to lower the value of R37, maybe by ~half to get more swing, BUT that will bleed some voltage off to ground so then to make up for that you have to lower the value of R36 to, maybe 10K?
Wait for someone else to reply to be sure.
:think1:
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Looks like I need more on the upper to use the small draw tubes and less at the other way for high draw tubes
Thanks for all the help!!!
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Looks like I need more on the upper to use the small draw tubes and less at the other way for high draw tubes
Yes, I think you got it now. That's the 'swing'. :icon_biggrin: (Tubes that draw LESS current need less -bias dcv's to tame/cool them off, tubes that draw MORE need more -dcv to cool them down.)
(After you get the -bias sorted out, I'd still change ALL those e-caps if their original.)
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Replace the 10K pot with a 50K pot.
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Replace the 10K pot with a 50K pot.
That's cheatin', that's the easy way. :laugh:
Can't you also do it by changing R36/R37?
(Fender BF AB763 amp's used a 10K bias pot and they had enough swing.)
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Can't you also do it by changing R36/R37?
(Fender BF AB763 amp's used a 10K bias pot and they had enough swing.)
The value of the pot needs to be equal to or greater than that 47K resistor in order to get a wide swing. You could try making the other resistors 10K and see what swing that gets you. My preference would be a 50K pot.
Fender didn't need a huge swing because he never intended to use different tubes that require such a different bias voltage.
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The value of the pot needs to be equal to or greater than that 47K resistor in order to get a wide swing.
OK, thanks. :icon_biggrin:
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I don't think there is anything wrong with the bias supply. There is -74v at the output of the diode.
May be I missed it, where did the -74V figure came from? The schematic above says -66V. Assuming a plate voltage of 540V and a screen voltage of 515V, then the bias needs to be about -66V for 70% dissipation, so I hope the -74V figure is correct. Also, were the above screen voltages measured with the 1k/5W resistors already installed?
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Look at reply #46. :icon_biggrin:
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Look at reply #46. :icon_biggrin:
Got it, so it's good and bad - if the bias voltage got higher, it would also mean that rest of the voltage did too, so what is the plate voltage now, and again, were the screen voltages measured with the larger screen resistors installed and hopefully close to the proper bias applied?
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all measurements were done after the resistor install
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I decided to take my tubes out of my Sunn 2000. They seem to be fine. I should of mentioned before that I have a heat beat type buzz that I had with both sets. It goes up and down with volume.
Picture the amp, service side up on my desk. If I come close to the amp , like inches away the noise quiet down some. If I waver my hand mainly around the V3 preamp tube it all but goes away. I have a florescent above me and I turned it off but had no effect. Bias has no effect. If I remove V3 it goes away. I changed out V3 and there was no change
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Heart beat not Heat beat :think1:
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I'm here at the parts store all they have is a 20k or 100k.
Can I get by with a 20k?
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Ended up using the 100k, I know there's a lot of turns but much better adjustments and dialed right in
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I have done this mod to my personal yba-3 custom back in 2012. It has been rock solid since. It can be done.
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Ended up using the 100k, I know there's a lot of turns but much better adjustments and dialed right in
What range of voltage do you have at each end of the pot? 100K may be a little touchier to adjust than a 50K pot but otherwise should be OK.
What do you mean by "I know there's a lot of turns"? Is this a multi-turn pot?
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Not sure about the range but I could really dial both tubes
Turn wise it wasn't so bad, it's listed as a 28 turn pot
Any ideas on that crazy heart beat buzz I wrote about?
Plus my amp socket was fine. The amp didn't like one of the Bias Rite Meter sockets. I have to get with Weber anyway due to that crazy 1000 volt reading. The Bias Rite has been a great performer, over all. If I use it once a year that's a lot. I like it because it's safe
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Not sure about the range but I could really dial both tubes
That's what's ultimately important. It would be nice to have a note on the schematic that states what the range of negative voltage is with the pot set to zero ohms and 100KΩ. Might come in handy in the future during times of mild stress.
Turn wise it wasn't so bad, it's listed as a 28 turn pot
Well that should allow you easy and precise control of the bias.
Any ideas on that crazy heart beat buzz I wrote about?
None. Was it there before the mod? Does it do it with either tube types?
Has this mod been worth it?
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None. Was it there before the mod? Does it do it with either tube types?Has this mod been worth it?
No, the buzz is a new effect but if I hover my hand mainly over the v3 preamp tube it subsides.
Worth it ? Yes, 6559's are my kind of sound. The El's and even the 6KG6 , from my YBA-3A super are grungy. I need the smoother larger sound of the 6550. The amp is scary sensitive in terms of dynamic range
Figure it this way and to be honest I'm a home hobbyist that has always loved electronics. I never was schooled in it maybe I should of followed that into a career. I was in high end car audio for many years and changed out all kinds of parts. Improved EQ and CD player with better IC's alla Analog Devices.Made hundreds of custom length rca's. Helped Jim Fosgate with his Dolby Prologic 2 for the car. Had some fun and with great sound systems.
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Any ideas on that crazy heart beat buzz I wrote about?
:dontknow:
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Forget about the buzz, I moved the amp away from my desk and it's super quiet
Can't thank you guys enough!!
Case closed, for now :laugh:
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HI Guys
This is probably the best upgrade ,besides new caps, I've ever done. Best of both world installing the 6550's. On one hand I get the fullnest I wanted in a Sunn type sound. Then I get the pre-amp driven sound of a Traynor, focused and strong.
For other YBA owners with a Master and Volume it's real simple- Actaully many of you may know this but for those who want to experiment-
Sunn- Volume control on 3-4, then use the master as the main volume control
Pre-amp driven- Volume control on 8-9, then use master volume as main volume
Real big- Master on 10, then use volume control as main
Over driven- volume on 10, then master as main
No worries, Traynor/ Yorkville documented this
Thanks to all you guys!!!