Fender Super Twin- New Project

[pbman1953]

HI All,


It's been a while so hope all are well and Happy New Year!


I was given some great advice on a search for a another tube amp for bass. I was highly recommended to look for a a mid 70's Fender Super twin. The amp is a combo amp with 185 watts at 4 ohm. The tubes are 6- 6l6's and a combo of 7025's, 12au7 & 12ax7 preamp tubes.


The goal would be to remove the chassis and buy a Mojo Tone cab to slip the chassis into. The cab are available. The combo cab has a pair of Fender 12's. I'll try to sell the cab locally probably.


In the shop I bought it from the amp had a typical hum so I knew it needed a cap job. The controls were all quiet. When I got home to try the amp I noticed the hum again and I use the tube match control to reduce the hum. The amp has never been worked on. I want to replace all the electrolytics first.  When I played the amp, with a bass, there was a buzz on the notes on the lower 2 strings. I expect the caps have a play in this. All of the paper Mallory caps are dis-colored. 




I sent pictures for inspection. I'll need a back up post to show most of the. I found  web site that stocks Fender cap replacement sets.




https://www.amprepairparts.com/fendercapkits.htm
 
Part #FCK-19




Questions
Besides the electrolytics, should I change the power resistors (39k x2) at the same time? If you have any other suggestions please expresses away! Thanks

[pbman1953]

More pics

[pbman1953]

even more

[sluckey]

If the 39Ks read the same resistance they will be fine. Doesn't matter if they read 50K as long as they both read the same.

[pbman1953]

42.7
43.7
Thanks!

[pbman1953]

I see a discrepancy with Fender between the picture inside the amp and the schematic. The picture shows from the the tube layout. For the pre-amp tubes right to left- 7025-7025- 12AU7. So you'd expect that V1 would be 7025, V2-7025 and 3- 12AU7. If you compare to the schematic, V1 is 7025, V2 is 12AU7 and V3 is 7025.


Did Fender print the wring tube layout? OR, did they label the schematic wrong?

[sluckey]

Did Fender print the wring tube layout? OR, did they label the schematic wrong?

I would say they are both correct. There are no tube designators on the layout so how can it be wrong? And there is no rule that says you must label the tubes in ascending order from right to left.

[pbman1953]

Will I  be ok doing this recap without a variac? I've never used one before I keep seeing videos that it's a good idea.

[sluckey]

yes

[pbman1953]

ok, thanks I found a new 10A for $80

[Willabe]

Will I  be ok doing this recap without a variac? I've never used one before I keep seeing videos that it's a good idea.

Use a light bulb limiter when you turn it on after installing the new caps. For a Twin, 100w bulb is fine. Don't go larger, the higher the bulbs wattage, the less protection to the amp.

[pbman1953]

How can I convert this Output matching control to a bias control?

[sluckey]

How can I convert this Output matching control to a bias control?

1. Post a link to the schematic so we don't have to track it down every time you ask another question.

2. Fix the amp according to the schematic and evaluate the bias circuit as Fender built it. Then...

3. Ask this question again if you think it's necessary or desirable.

[pbman1953]

Will do thanks-


https://el34world.com/charts/Schematics/files/Fender/Fender_super_twin_180w_schem.pdf




Will fix then ask

[pbman1953]

I haven't done the cap job yet, but my variac arrived. Can someone please give me the ABC's on how to charge the amp after the install


Thanks

[sluckey]

I haven't done the cap job yet, but my variac arrived. Can someone please give me the ABC's on how to charge the amp after the install

You don't need to charge anything up. New caps should be good to go right outta the box. Pull all power tubes and turn the amp on. If no smoke, check for B+ voltages at every filter cap. They should be high since the output tubes are out. If good so far, put the tubes back in.

Now check the bias balance...

I would put a one ohm resistor between the cathodes of V6, V7, V8 and ground. (Cathodes of V6, 7, V8 are tied together as a single tube). Now put another one ohm resistor between the cathodes of V9, V10, V11 and ground. (Cathodes of V9, V10, V11 are tied together as a single tube).

Now connect a DMM set to measure mV. Black probe to cathodes of V6, V7, V8. Red probe to cathodes of V9, V10, V11. Turn the amp on and let it warm up a bit. Finally, adjust the bias balance for zero volts (or as close as you can get) on the meter. If you can get zero, the two groups of tubes are balanced perfectly.

Be careful. This ain't no champ that will just jar your teeth if you make a slip. There's 500VDC in this amp and the PT can supply enough current to stop your heart!

[pbman1953]

"You don't need to charge anything up. New caps should be good to go right outta the box. Pull all power tubes and turn the amp on. If no smoke, check for B+ voltages at every filter cap. They should be high since the output tubes are out. If good so far, put the tubes back in."

I should rephrase about my charge statement. I saw a few videos that techs slowly bring up the varaic up to 25 volts , then let it sit for some minutes, then slowly increase the voltage in stages to finally arrive at 110. Should this be done?


"Now check the bias balance...

I would put a one ohm resistor between the cathodes of V6, V7, V8 and ground. (Cathodes of V6, 7, V8 are tied together as a single tube). Now put another one ohm resistor between the cathodes of V9, V10, V11 and ground. (Cathodes of V9, V10, V11 are tied together as a single tube).

Now connect a DMM set to measure mV. Black probe to cathodes of V6, V7, V8. Red probe to cathodes of V9, V10, V11. Turn the amp on and let it warm up a bit. Finally, adjust the bias balance for zero volts (or as close as you can get) on the meter. If you can get zero, the two groups of tubes are balanced perfectly.

Be careful. This ain't no champ that will just jar your teeth if you make a slip. There's 500VDC in this amp and the PT can supply enough current to stop your heart!"



I'm a bit confused since all #8 pins is grounded. Are you saying to lift them all off ground? Then connect all the 8's (per side) then connect a 1 ohm to ground per set? Then attach the probes to the pin side of the resistor?
I'll be using alligator clips for safety

On a side note after watching couple update videos on this amp. Some have cut out the 330pf treble cap at the input end. Plus some have changed the negative feedback from 820 ohm to 2.2k. Lastly, from a Talk bass Super twin Group, someone changed the 2.2k resistor, on the Distortion control, to a 1 meg ohm.  Many don't like the sound of the Distortion section and the 1M is supposed to soften it up. Do these changes only benefit guitar and not be relevant to bass?

[sluckey]

I should rephrase about my charge statement. I saw a few videos that techs slowly bring up the varaic up to 25 volts , then let it sit for some minutes, then slowly increase the voltage in stages to finally arrive at 110. Should this be done?

That's what I would do if I were attempting to reform the original filter caps. I would stretch this out over several hours. But since you will have all new caps I don't consider this necessary. However, you have a brand new Variac and I know you're itching to use it, so go ahead. Bring it up in stages and stop at 120VAC. Don't rely on a cheap built-in meter to set the final voltage to 120VAC. Measure it with your DMM. I like to use a variac to maintain the line voltage at 120V, especially when checking voltages and/or adjusting bias. Doing so gives me a good set of voltage readings that I can refer to at some later date.

I'm a bit confused since all #8 pins is grounded. Are you saying to lift them all off ground? Then connect all the 8's (per side) then connect a 1 ohm to ground per set? Then attach the probes to the pin side of the resistor?
I'll be using alligator clips for safety

Exactly. And by simultaneously connecting the probes to each set of cathodes you can easily set the balance to zero without having to measure one cathode then move to the other cathode and try to match, and repeating this over and over until they are reading the same voltage. After you have balanced to zero then you can measure/record the mV across each 1Ω for calculation purposes. Measuring/recording the voltage at pin 5 of each tube is also a good idea. (Don't ask about modyfing the bias circuit yet.)

On a side note after watching couple update videos on this amp. Some have cut out the 330pf treble cap at the input end. Plus some have changed the negative feedback from 820 ohm to 2.2k. Lastly, from a Talk bass Super twin Group, someone changed the 2.2k resistor, on the Distortion control, to a 1 meg ohm.  Many don't like the sound of the Distortion section and the 1M is supposed to soften it up. Do these changes only benefit guitar and not be relevant to bass?

That's all good stuff to play with ***AFTER*** the amp is up and running properly.

[pbman1953]

Thanks

[sluckey]

How can I convert this Output matching control to a bias control?

Since it's too cold to go outside I decided to play with your bias circuit. Here's a simple little mod to allow you to adjust the amount of bias in addition to the "matching" control. Win win!

NOTE!!! YOU MUST USE A 2 WATT POT. No cheap Alphas. I suggest PEC.

[shooter]

Since it's too cold to go outside

I just came in after 2 hours in 20 degree, with a wind-chill of 10
I like the circuit though

[pbman1953]

This may be a silly question, but you say turn on the amp, do I switch on standby as well. I get Power readings with only the power on.

[sluckey]

Power readings on what?

[pbman1953]

The 220 caps in series. I brought it up to 25 volts but then I had company over so I had to shut it down until tomorrow

[sluckey]

Look at the schematic to understand why.

[pbman1953]

I'm sorry,  I won't know why.

[sluckey]

Those two caps are not controlled by the STBY switch. They will be hot anytime the power switch is on.

Which variac did you get? Provide link.

[pbman1953]

You're right I see what you mean. Thanks


Here's the Variac-Beleeb 10 Amp Variable


https://www.amazon.com/dp/B07JZ2Z3PG?psc=1&ref=ppx_yo2_dt_b_product_details

[pbman1953]

Readings so far -Standby off

220uf caps
105v-  500v DC

120v-  564v DC
-------------------------------------------------------



1st 80uf from the hum balance pot


120V- -67




2nd- 80uf cap- before the output balance pot


93 v- -61


120v- -78.5



Standby ON
----------------------------------------------------


20uf- off the 500v line


105v-  400v DC


120v-   446v DC
------------------------------------------------------


20uf - after the 2.2k resistor


83v-  300vDC


120v- 430v DC

-------------------------------------------
20uf after the 10k resistor


106v- 300v DC


120v- 330v DC




 

[sluckey]

220v-  564v DC

Is it to leave it on at the 220 v point?

Please clarify. I don't know what that means. Your variac cannot put out 220v.

You may be interested in this thread...   https://el34world.com/Forum/index.php?topic=25028.0

[pbman1953]

Clerical mistake sorry-

120v




Please back to my modified results before your last post


Thanks

[pbman1953]

Balance test-


On the  2 volt DC scale setting I get .000

[sluckey]

I assume you have correctly installed two 1Ω resistors as planned? And all output tubes are plugged in? And the standby switch is in operate mode?

[pbman1953]

yes


Please notice the 3 pin8 wires coming to the 1 ohm, then the other side of the resistor is grounded. The clip is on the pin side not the ground side

[sluckey]

Where is the black probe connected?

Set your meter to read millivolts. You should be able to adjust the balance for a positive reading by turning the pot in one direction and a negative reading by turning the pot in the other direction. The final setting of the balance pot should be with a zero millivolt reading. Is this happening?

[pbman1953]

I sent you one half and the black is the same on the other set


My lowest DC setting is 200m. With that setting the lowest I get is 00.1 to 00.2, but there is a slight hum.


At 5.0 there's no hum

[sluckey]

OK. Set the balance to zero. Now connect the black lead to chassis ground and use the red lead to measure the millivolts on each 1Ω resistor. What are the two readings?

[pbman1953]

78.9


78.3

[sluckey]

That's very well matched. However, it's also very conservative/cold. That's only about 26.A per tube. Tubes should last a long time. I would expect closer to 40mA to 45mA per tube (120mA per side).

What is the voltage on pins 3, 4, and 5 of each power tube?

How does the amp sound?

[pbman1953]

left to right from the speaker jack-

Tube 1- hard to reach unless I'd have to remove the speaker jack

3-
4-
5-


Tube 2- 

3-541
4-539
5-64


Tube 3- 

3-536
4-536
5-64


Tube 4- 

3-530
4-521
5-58


Tube 5- 

3-514
4-515
5-58


Tube 6- 

3-513
4-514
5-57-58

This amp comes as I heard, powerful and has a scary attack. Great potential for the old school tube sound.

I'd like to changes 2 things so far sound wise-

Either I never use the distortion section or I'd like to calm it down. It sounds like a fuzz box if turned up too far. Maybe that's why someone suggested the 1M change from the 2.2k


I heard that this amp is mid heavy and it is. I know that there's a ton of EQ on this amp . The controls go from 1250hz to 435 and what's missing is 800-1k area.

One weird thing is if I hold a note to let it decay the decay gets distorted slightly but the main note doesn't

[sluckey]

It's biased very cold. Based on 530V plate voltage and 78mA per side of three tubes, each tube is idling at about 14 watts. If you like it as is, then you're done. If you want to run it hotter then consider the simple bias mod I suggested. For now, you could just tack a 3.3K 2watt resistor across the 2.7K that's on the balance pot. Then recheck plate and cathode voltages for all 6 tubes.

[pbman1953]

Interesting. When I brought the amp home and before the cap job I used the bias right just to see where the tubes were. After the cap job, not much of a change. Maybe a 1 watt increase.


I'll try the resistor for now and report back, probably tomorrow. Leaving for a gig


Can I reconnect the pin 8 grounds or waiting until I test with the 3.3k?


Thanks!

[sluckey]

Can I reconnect the pin 8 grounds or waiting until I test with the 3.3k?

Adding the 1Ω resistors is supposed to be a permanent mod. Clean it up and leave them in the amp. I suggest 1W/1% but 1/2W/1% is OK. Just connect the three cathodes together and install a resistor form one convenient pin 8 to a convenient ground lug. Putting a ground lug on one of the tube socket mounting bolts will work and look neat.

[pbman1953]

I have higher wattage resistors, luckily

[pbman1953]

Just got home and I see that the closest resistor I have is a 2.7k -2 w




I balanced again.




The new chassis measurement on each side is 152


473v DC seems to be the avg voltage at pins - 3&4

[sluckey]

That's about 24 watts per tube or 80% max. Too hot for my nerves. If you're gonna keep the amp you should give that bias mod some consideration.

Were any of the tubes red plating?

[pbman1953]

I didn't notice, but it was a quick test then I shut it down. I'll try some smaller values then report back tomorrow.

I think I want 18-21 watts


60-70% of dissipation


What is the math formula to get the watts?

[sluckey]

Don't go smaller. That will just run the tubes even hotter.

Just multiply the plate voltage times the cathode current. For your last numbers... that would be 473 x .152 = 71.9W for three tubes, or 23.9W per tube.

[pbman1953]

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


After a re- zero-


At the 1 ohm-


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?

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. It has a 3 prong if that matters in the issue.

Also, no red plating noticed

[Willabe]

I'd leave the hum pot.

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