OK, we are stepping thro' this.
Anode = plate = pin 5
Screen = G2 = plan 6
Voltages should be measured when everything well warmed up.
But anyway...
323v on plate, say 15mA current (1/2 of that going thro' cathode resistor less a bit passing thro' the screen (yep, G2)
So Pd (plate dissipation) is about 4.8 watts in a 12 watt tube - nice and safe.
So questions to consider would be:
- if it sounds good, why change it?
- louder?
- cleaner?
- dirtier?
- just for fun?
- just to learn?
- are you happy to buy more tubes when these melt?
All good reasons to play with it... but I also need to know the screen voltage before I say any more.
Cheers
JimG
Anode = plate = pin 5
Screen = G2 = plan 6
Voltages should be measured when everything well warmed up.
But anyway...
323v on plate, say 15mA current (1/2 of that going thro' cathode resistor less a bit passing thro' the screen (yep, G2)
So Pd (plate dissipation) is about 4.8 watts in a 12 watt tube - nice and safe.
So questions to consider would be:
- if it sounds good, why change it?
- louder?
- cleaner?
- dirtier?
- just for fun?
- just to learn?
- are you happy to buy more tubes when these melt?
All good reasons to play with it... but I also need to know the screen voltage before I say any more.
Cheers
JimG
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Ummm.
350+ volts anode to ground....
What is the voltage rating of the 20u capacitors in the power supply?
They are marked as 300v on the schematic 😱
350+ volts anode to ground....
What is the voltage rating of the 20u capacitors in the power supply?
They are marked as 300v on the schematic 😱
Ok, pin 6 'screen' is 352v w amp cold. I really dont want to 'warm' it up too long with the 300v caps in there.
Why am I doing this?
1] its much to noisey with power supply noise.
2] I had a stock one about 10 years ago with a JBL D123 speaker and gigged with it for a few years.Loved that amp! So, I'd like to get this one as close to stock as possible with safety improvements like a fuse and quiet enough to record with.
3] I dont like that the 220 ohm resistor was replaced with 2 in series.What, he didn't have a 1k? Kind of 'hacky' looking to me.
And If I can use a stock 220/250 ohm I would like to.
The transformer,according to his repair invoice, was supposed to be wired 1 to 1 according to the original specifications, so why the need for a 1k bias resistor?
Q: if it sounds good, why change it? A: safety/noise
- louder? A: would be fine, I may put a more efficient speaker in there [looking for another 8ohm JBL D123]
- cleaner? A: As clean as can be please
- dirtier? A: No thanks, need a great clean platform for pedals
- just for fun? A: It is fun, but no
- just to learn? A:Learning is a plus, but no
Thanks for all your help thus far!
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Excellent.
220R is "stock" for the stock tubes. You aren't running stock tubes.
There is NO magic in the "stock" specifications of an old amp which was built as cheap as possible and is not a vintage collectors item. It IS a great platform to learn and have fun on, because it can be improved easily.
The cathode resistor has to suit
- the type of tube you have
- the brand of tube you have
- the voltages on P and G2 of the tube you have
There is no big deal about getting the right resistance or - more importantly - right wattage rating by using 2 resistors either in parallel or series.
There are more important things to address with this amp first.
I don't know how much over voltage 6AQ5 tubes can take (Tubelab probably does) but I would be wanting to get the screens down closer to 250v before I did anything else. Eg by replacing the 1k resistor (between) the OPT and the G2 supply with 10k and remeasuring. This will also smooth the PSU ripple to screens and preamp tube, possibly reducing some of the hum.
Hum? To start with
- Bigger smoothing caps than 20-22u. (There is no magic in the stock specs)
- A 100R resistor on each leg of the heater supply with the other end(s) attached either to ground or the output tube cathodes. (Reduces heater hum like magic 😀 )
Cheers,
(if you are not having fun I'm wasting my time, if this info is what you want I can keep going until you are really happy with the amp, and know why it's as good as it is) 🙂
JimG
220R is "stock" for the stock tubes. You aren't running stock tubes.
There is NO magic in the "stock" specifications of an old amp which was built as cheap as possible and is not a vintage collectors item. It IS a great platform to learn and have fun on, because it can be improved easily.
The cathode resistor has to suit
- the type of tube you have
- the brand of tube you have
- the voltages on P and G2 of the tube you have
There is no big deal about getting the right resistance or - more importantly - right wattage rating by using 2 resistors either in parallel or series.
There are more important things to address with this amp first.
I don't know how much over voltage 6AQ5 tubes can take (Tubelab probably does) but I would be wanting to get the screens down closer to 250v before I did anything else. Eg by replacing the 1k resistor (between) the OPT and the G2 supply with 10k and remeasuring. This will also smooth the PSU ripple to screens and preamp tube, possibly reducing some of the hum.
Hum? To start with
- Bigger smoothing caps than 20-22u. (There is no magic in the stock specs)
- A 100R resistor on each leg of the heater supply with the other end(s) attached either to ground or the output tube cathodes. (Reduces heater hum like magic 😀 )
Cheers,
(if you are not having fun I'm wasting my time, if this info is what you want I can keep going until you are really happy with the amp, and know why it's as good as it is) 🙂
JimG
Yep. R12 is the one I mean.
At the moment I am trying to protect your tubes.
If the screens melt you won't have any tone 😀
Because you are staying with the low value PSU caps An alternative strategy to changing R12 would be to add a resistor between the rectifier diodes and the 1st 22u cap (C7a on the schematic I am looking at). It should be in the 1k to 2k range and a 10 watter (2x 5 watt 2k to 4K in parallel an option). Adjust this until your screen voltage is close to the original (287v) spec.
Doing it this way should bring all your voltages closer to standard with 1 adjustment point, and your DC will have less ripple all the way thru the circuit.
I would look at adding the heater centre tap resistors at the 6.3v transformer leads. Metal film resistors of any wattage are good - no current flows thru these, they are voltage reference only.
At the moment I am trying to protect your tubes.
If the screens melt you won't have any tone 😀
Because you are staying with the low value PSU caps An alternative strategy to changing R12 would be to add a resistor between the rectifier diodes and the 1st 22u cap (C7a on the schematic I am looking at). It should be in the 1k to 2k range and a 10 watter (2x 5 watt 2k to 4K in parallel an option). Adjust this until your screen voltage is close to the original (287v) spec.
Doing it this way should bring all your voltages closer to standard with 1 adjustment point, and your DC will have less ripple all the way thru the circuit.
I would look at adding the heater centre tap resistors at the 6.3v transformer leads. Metal film resistors of any wattage are good - no current flows thru these, they are voltage reference only.
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I'll look into this first, as I dont have any 5 or 10 watt resistors.
Can I hook the two 100ohm heater wire resistors to the empty pin 7 lug of the 6AQ5's? Or should I solder them to the chassis?
Not really clear at what junction of the diodes and the 1st 20uf cap the 1k 10w resistor should go?
Pin 7 is not spare. You can see on the data sheet that both pin 1 & 7 are connected to the grid (G1)
See if you can connect the 100R resistors (value not critical) to the start of the yellow wires. Ground to somewhere handy in the circuit.
The circuit should attach to the chassis at one point only for least amount of hum (a whole new subject) 🙂
See if you can connect the 100R resistors (value not critical) to the start of the yellow wires. Ground to somewhere handy in the circuit.
The circuit should attach to the chassis at one point only for least amount of hum (a whole new subject) 🙂
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In your photo it looks like each diode has a one end each connected to a separate red/orange wire. The other ends of each diode are connected to each other and the 1st 20u cap.
My suggestion is to unsolder the cap ends of the diodes, and insert your voltage adjustment resistor(s) in the new gap between the diodes (still connected to each other) and the cap.
You may have to change this several times until you get the voltages you want. So don't cut any leads off short until you settle on final values.
My suggestion is to unsolder the cap ends of the diodes, and insert your voltage adjustment resistor(s) in the new gap between the diodes (still connected to each other) and the cap.
You may have to change this several times until you get the voltages you want. So don't cut any leads off short until you settle on final values.
They do have some current (6.3/200 = 31.5mA) but not too much power (0.1watts per resistor). I suppose 0.25watt resistors would be OK, but I would go for 0.5watt resistors.
Off to the local [one hour by bus + metro 🙁 ] electronics shop for parts.
Trying to figure out where best to ground those 100ohm resistors.Would a small wire 'O' terminal on each tube socket mounting screw be adequate?
Also, I will be mounting a speaker jack instead of attaching the OT speaker wires directly to the speaker. As for the location of the jack, does it matter on a noise/RF interference level where I put it [should it not be near anything] or can I simply find the most convenient location?
You should only provide one artificial centre tap (with the pair of 100 ohm resistors) per heater string. The two 100 ohms resistors are best placed near the PT. The connection to ground (from the junction of the two resistors) only carries negligible current and can be to your main signal ground. The speaker jack is usually placed close to the OT. Twist together the wires from the OT to the jack.
I wouldn't put the 'centre tap' resistors near a sensitive preamp tube, as they might induce hum.
On the 6AQ5 should be OK, but run a ground wire from there back to your preamp signal ground point.
On the 6AQ5 should be OK, but run a ground wire from there back to your preamp signal ground point.
There is negligible ground current from the artificial centre tap, so I expect it is OK to ground it any convenient point (as you have in the picture - if I'm interpreting it correctly).
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