Fender Super Twin- New Project

[pbman1953]

I'd leave the hum pot.

Some preamp tube are noisy, heater to K. That hum pot could/will come in handy someday.

ok, thanks!

[sluckey]

I installed the 1 ohm resistors permanently with a 1k @ 2watt and 16 gauge wire. Not sure why my results are different today.

Just to be sure, that 1K resistor is tacked across the 2.7K, right??? You don't need any 16 AWG wire. 20AWG is fine.

119 & 117 @ 501 volts = 19.9 & 19.5 watts
After this, there still is a hum. I can balance that out with the pot but my figures will be off. Will moving tubes around help?

Numbers look good. If you adjust the balance pot for no hum, what are the numbers? Moving tubes around may help but I'm a little skeptical. BTW, there's a more accurate way to adjust for no hum. Rather than using your ears, connect your meter across the speaker jack and adjust the balance for minimum AC VOLTS. This is also an accurate way to adjust the HUM pot.

I need to buy parts for your bias adjustment. Can I remove the "hum" pot and place the your bias pot there? The hum pot seems to do nothing.

Leave the hum pot. Here's the pot you need for the bias mod. You will not find it any cheaper.

     https://www.ebay.com/itm/164384667101?hash=item264616f5dd:g:NmEAAOSwLOlfYNRn

Also, no red plating noticed

[pbman1953]

Just to be sure, that 1K resistor is tacked across the 2.7K, right??? You don't need any 16 AWG wire. 20AWG is fine.-yes, you're right about the wire but I had it



If you adjust the balance pot for no hum, what are the numbers?-   158 & 137-   496v




connect your meter across the speaker jack and adjust the balance for minimum AC VOLTS-  1mv AC


Hum control- on the 200m scale-  1.8

[pbman1953]

This thing is still running too hot-




After another -re-zero-


497V
151 mv
153mv


that's 25 watts, too hot


The only difference was the higher gauge grounding wire and a higher strength 1 ohm. Looks I need a 1k or a 1.5 play pay with. This I can get locally until I get the new pot

[sluckey]

You need a 3.3K to 5.6K resistor to tack across (connect parallel) the 2.7K. NOT A 1K OR 1.5K!!!

[pbman1953]

Sorry, my mistake- not the 1 or 1.5 but I did have a 2w- 5.6


After the re-zero-


106 & 106 @ 510v-  18W

[sluckey]

My replies can only be as accurate as the info you provide.   

[pbman1953]

Can't thank you enough!




After the re-zero there is a little hum. To eliminate the hum ,it's a slight turn and here's the off balance-


500v


100 & 106- on the cooler end

[sluckey]

That's only about 16.7 watts per tube. When you get that pot from eBay and do the mod, you'll be able to dial in the amount of bias you want. I would not put the bias pot on the back even though it has a locking nut. I'd put it on the top side of the chassis, somewhere near the balance pot if possible. Makes it less tempting to tweak the pot.

Post a hi-rez pic of the inside of the chassis and another pic of the outside of the chassis that shows where to balance/matching pot is located.

[pbman1953]

I'd guess it's ok to use for now right? The bias pot will be the fine tuner. I'll order that today. Thanks for the link!


Can we talk sound tweaking yet?

[sluckey]

I'm not good with sound tweaking for other people. Plenty of opinions/suggestions from others.

You have a big selection of tone knobs. I suggest you play with the knobs for about a month before changing anything. I'd expect this amp to have a painfully clean sound meant for lots of big speakers. A 2x15 cab might be the cat's meow. No clue about the distortion circuit.

Will you be playing bass with this amp? If so, you probably don't need to change anything.

[pbman1953]

That's ok as far as sound , you've done plenty, Thanks! The pot is already been set up to ship, so I'll go get 1.5k resistor.


Yes, this is purely for bass.


I'll be back to chime in for the bias pot  install. As far as surface space, there's an open spot behind a power tube and the board. This pot looks like a smaller diameter so it should be fine.


Thanks Again!

[sluckey]

I'll go get 1.5k resistor

Pick up a 1.2K/2W and a 1K/2W just in case the 1.5K/2W isn't quite right.

[pbman1953]

I'll go get 1.5k resistor

Pick up a 1.2K/2W and a 1K/2W just in case the 1.5K/2W isn't quite right.

On the list!

[pbman1953]

Input jack question-


On input 2 there's a 1m resistor connected. Is that for higher output guitars , like an active?

[sluckey]

On input 2 there's a 1m resistor connected. Is that for higher output guitars , like an active?

No, both inputs act the same. These input jacks do not operate like HI/LO jacks found on the typical Fender inputs. The 1M is simply the grid return resistor for V1A.

[pbman1953]

On input 2 there's a 1m resistor connected. Is that for higher output guitars , like an active?

No, both inputs act the same. These input jacks do not operate like HI/LO jacks found on the typical Fender inputs. The 1M is simply the grid return resistor for V1A.

Understood


Another question about the 2.7 resistor as it is now. You had me attach the 5.6k across the 2.7 which helped raise the temp , as to say for the bias. But, your diagram has the of the pot and 1.5k in series to add to 4k adjustable. Then by placing the resistor across cuts the value output. With the 5.6 and 2.7 parallel, what is the outcome value? Is it close to the 4k, where you want me?

[sluckey]

With the 5.6 and 2.7 parallel, what is the outcome value?

1822Ω

[pbman1953]

Ah, so with a range of 0-4k  (2.5 + 1.5), I should be in range for a better outcome

[sluckey]

Ah, so with a range of 0-4k  (2.5 + 1.5), I should be in range for a better outcome

The range would be from 1.5K to 4K. Going down to zero would be dangerous. Be prepared to use the 1K or 1.2K if needed to get your happy bias in the center of pot rotation. I suggest setting the pot to center then connect the 1.5K first and check bias. If you have to turn the pot too far from center to get your desired bias point, replace the 1.5K with the 1.2K. Repeat with the 1K if necessary. The idea is to get your desired bias point to occur with the pot in the center of rotation. Then you can easily adjust hotter or cooled if you want.

[pbman1953]

 

[pbman1953]

There's a Talkbass page dedicated to this amp . I noticed a couple tweaks some suggest. One I did one I didn't yet


1- Waiting for your opinion on this
changing the plate load resistor on the inverting part of the phase inverter (V4B) to 180k should balance the feed to the output tubes better, a la the "classic" AA864 & AA165 Bassman and AB763 ***Reverb amps.

2-Since it was easy, I did this one and it was like he said but very soft. actually made the sound fuller.
Distortion Channel(top)Apparently, Fender tried to fix the original crappy channel with another crappy one.On the cathode of V3, after the 4.7K cathode resistor, they changed the circuit from just a 3.3K resistor to a 2.2K R paralleled by a diode into a coil (marked 01194 4 on the scheme). I changed the 2.2K resistor to a 1 meg ohm. It changed the whole channel for the better. It is not a heavy metal distortion, kind of a soft distortion. [Source]

[sluckey]

1- Waiting for your opinion on this
changing the plate load resistor on the inverting part of the phase inverter (V4B) to 180k should balance the feed to the output tubes better, a la the "classic" AA864 & AA165 Bassman and AB763 ***Reverb amps.

I agree. And just to be sure everyone is on the same page, they're talking about changing the 180K that's connected to V4 pin 1.

[pbman1953]

Thanks, that's what I needed the pin number, BUT. I looked here in the schematic page for the AA864 and AA165. Both Pin 1's from the 12at7 doesn't have a 108k resistor.


Please correct me if I'm reading it wrong

[sluckey]

Most of the Fender LTP PI circuits have an 82K on pin 1 and a 100K on pin 6. You won't find a 220K on any of them except that super twin. The important thing is to set the RATIO of those plate resistors. Fender found that a ratio of 82K/100K (.82) give fairly equal signals at the plates.

Changing the 220K on pin 1 of your amp to 180K gives a RATIO of 180K/220K which is .82 and that gives equal signal at the plates, just like the AB763.

[Willabe]

Both Pin 1's from the 12at7 doesn't have a 108k resistor.

There's only 1 pin 1.

https://www.effectrode.com/knowledge-base/the-12at7-tube/

[pbman1953]

Both Pin 1's from the 12at7 doesn't have a 108k resistor.

There's only 1 pin 1.

You're right, I meant both schematics from the 165 & 864

[pbman1953]

Most of the Fender LTP PI circuits have an 82K on pin 1 and a 100K on pin 6. You won't find a 220K on any of them except that super twin. The important thing is to set the RATIO of those plate resistors. Fender found that a ratio of 82K/100K (.82) give fairly equal signals at the plates.

Changing the 220K on pin 1 of your amp to 180K gives a RATIO of 180K/220K which is .82 and that gives equal signal at the plates, just like the AB763.

I'm fine with that.  I didn't want to make a move without permission and clarification. 

[Willabe]

You're right, I meant both schematics from the 165 & 864

Most of the Fender LTP PI circuits have an 82K on pin 1 and a 100K on pin 6. You won't find a 220K on any of them except that super twin.

[pbman1953]

The closest I could find to 180k was 182K

[pbman1953]

What's the percentage rule when replacing resistors?  Do the grid resistors have to be disconnected to test?

[sluckey]

What's the percentage rule when replacing resistors?  Do the grid resistors have to be disconnected to test?

Look at the resistor. The fourth band indicates tolerance. Gold=5%, silver=10%, and no fourth band means 20%. Many times you will have to disconnect one lead to get an accurate measurement. And other times you don't have to disconnect. It all depends on the circuit. Which grid resistors are you concerned about?

[pbman1953]

Since I had it back in front of me, since the bias pot arrives today, I tested the 6- 470 ohm 2 w resistors. They are all silver banded.


Here they are-


492
489
491
501
680-535- this one was weird. At first it was 680 then I decided to remove the tube so it would be dead on one side. Then it reduced to 640 but leaving the leads on it went to 535. Going back to it after a couple minutes it's 542
491

[sluckey]

I would replace the weird one for sure. This is a good opportunity to replace all six with some 470Ω 3 watt metal oxide resistors. Hoffman sells them.

[pbman1953]

Great idea. I won't have a cab to install to for weeks, so this is a good time to get the bugs out. Are there any others that should be tested?


Thanks

[sluckey]

I would need to have my hands on the amp to make that call. I usually don't change components without a good reason. I suggested upgrading the screen resistors mainly because you will likely be running the amp hotter now. The resistors I recommended are flame proof.

[pbman1953]

I'm testing the new bias pot/1.5 k resistor 




470/121-  18.9w


470/116-   18.1w


This is my first experience 6L6, should I hit for 19-20 or leave it?

[sluckey]

If you like how it sounds then let it be. 21 watts would be 70%. Are you able to adjust hotter and cooler? And balance?

[pbman1953]

I still did your procedure.


Balance
Individual measurement per side (mv) the Wattage per side


I had the pot started half way, like you said with the 1.5k. That first measurement was cold- 11 watts. Then after 3 increased increments, I am where I am now. There is some more pot travel to go, but not much.


This locking pot is cool though.


On the resistors, Since I have a nice new set of filter caps, shouldn't I give them a nice new set of resistors too? Those 39k's

[Willabe]

Your playing bass through the amp, I wouldn't go hotter for bass.

[pbman1953]

I took out the 1.5 k, to get the pot closer to the center.


I have it now at 19.5

[sluckey]

I took out the 1.5 k, to get the pot closer to the center.

What did you put in it's place?

[pbman1953]

I took out the 1.5 k, to get the pot closer to the center.

What did you put in it's place?

1k, they didn't have a 1.2 in stock

[jmccanna]

Hello,

Long time since I have been in the forum. I picked up a Super Twin today.  There was no doghouse over the power caps. Previous owner reports it starts up fine in standby but as soon as he puts it to full power it blows a fuse.  A few of power caps look newer but not necessarily new.  The rectifier is a set of four really small diodes that appear to be within what was the doghouse enclosure.  I have yet to take out the chassis out to examine it closely.

I had all the tubes tested and they are good.  So glad I found this discussion.

[pbman1953]

I want to update a few parts. Here at Hoffman they have the 3 watt 470 ohms but not in 39K. Can I use the 47k? Or use the 27k?

[sluckey]

...not in 39K. Can I use the 47k? Or use the 27k?

Why? You got something against carbon comp? They are only being used as bleeders across the series connected reservoir caps. They still look brand new.

If you must, then use the 47K or go to AES and buy some 39K 2W metal oxide resistors.

     https://www.tubesandmore.com/products/resistors-2-watt-metal-oxide-power

[pbman1953]

I'm cool leaving them, just ordered the 3 watters


Thanks!

[pbman1953]

After the screen resistor change, do I need to re-bias?

[pbman1953]

The last piece of the project is new OT with 4 & 8 ohm outputs. What is the procedure ? Should I bring it up with a Variac slowly? Re-bias?


What's customary?


Thanks

[tubeswell]

What's customary?

Holding hands and singing kum-ba-yah


 If its a new OT, just wire it in and switch it on. If it blows up, send it back and get a replacement. If you're afraid of making a mistake, triple-check your wiring first.