Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: pbman1953 on January 15, 2022, 08:55:00 am
-
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 (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
-
More pics
-
even more
-
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.
-
42.7
43.7
Thanks!
-
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?
-
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.
-
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.
-
yes
-
ok, thanks I found a new 10A for $80
-
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.
-
How can I convert this Output matching control to a bias control?
-
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.
-
Will do thanks-
https://el34world.com/charts/Schematics/files/Fender/Fender_super_twin_180w_schem.pdf
Will fix then ask
-
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
-
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!
-
"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?
-
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.
-
Thanks
-
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.
-
Since it's too cold to go outside
:laugh:
I just came in after 2 hours in 20 degree, with a wind-chill of 10
I like the circuit though
-
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.
-
Power readings on what?
-
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
-
Look at the schematic to understand why.
-
I'm sorry, I won't know why.
-
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.
-
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
-
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
-
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
-
Clerical mistake sorry-
120v
Please back to my modified results before your last post
Thanks
-
Balance test-
On the 2 volt DC scale setting I get .000
-
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?
-
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
-
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?
-
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
-
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?
-
78.9
78.3
-
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?
-
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
-
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.
-
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!
-
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.
-
I have higher wattage resistors, luckily
-
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
-
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?
-
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?
-
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.
-
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
-
I'd leave the hum pot.
Some preamp tube are noisy, heater to K. That hum will come in handy someday.
-
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!
-
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
:thumbsup:
-
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
-
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
-
You need a 3.3K to 5.6K resistor to tack across (connect parallel) the 2.7K. NOT A 1K OR 1.5K!!!
-
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
-
My replies can only be as accurate as the info you provide. :icon_biggrin:
-
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
-
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.
-
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?
-
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.
-
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!
-
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.
-
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!
-
Input jack question-
On input 2 there's a 1m resistor connected. Is that for higher output guitars , like an active?
-
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.
-
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?
-
With the 5.6 and 2.7 parallel, what is the outcome value?
1822Ω
-
Ah, so with a range of 0-4k (2.5 + 1.5), I should be in range for a better outcome
-
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.
-
:thumbsup:
-
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 (https://www.talkbass.com/wiki/fender-studio-bass-club-wiki-page/#wikiPage))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 (https://music-electronics-forum.com/showthread.php?t=15622)]
-
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.
-
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
-
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.
-
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/
-
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
-
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. :grin:
-
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.
-
The closest I could find to 180k was 182K
-
What's the percentage rule when replacing resistors? Do the grid resistors have to be disconnected to test?
-
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?
-
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
-
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.
-
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
-
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.
-
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?
-
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?
-
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
-
Your playing bass through the amp, I wouldn't go hotter for bass.
-
I took out the 1.5 k, to get the pot closer to the center.
I have it now at 19.5
-
I took out the 1.5 k, to get the pot closer to the center.
What did you put in it's place?
-
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
-
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.
-
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?
-
...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
-
I'm cool leaving them, just ordered the 3 watters
Thanks!
-
After the screen resistor change, do I need to re-bias?
-
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
-
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.
-
Gotcha!
Re-bias?
-
You've probably read this before...
If it squeals or howls you'll likely need to swap OT primary plate leads.
-
After the install there wasn't any howl and it's quite, but there's a little hum.
I sent a chart to show the voltages and they are very close.
The odd finding is that cathode is off with the first and last tube and that's why I'm writing. I'd suspect the hum would disappear if tubes 1 & 6 had a cathode reading like the rest.
Tubes 1-3 is one side and 4-6 is the other side.
I'm curious what you all think. Unless the too hotter tubes are on the way out. Before the OT update they were all very close.
-
try swapping tubes 3 and 5? that might balance out the sides
-
try swapping tubes ... balance out the sides
^That's^ how I bias up an Ampeg SVT-CL
-
I'm back with a question-
This amp has been great but I wonder if I softened the attack. Before most of the work the amp had a scary attack to it. I performed a cap job throughout with a kit designed for this amp. The kit was part for part at the most of the same values. If they were not , they were slightly higher for the filter caps.
In the front end for V1A pin8, the 750 cap I subbed with a 10 uf. No other stock values were changed besides the resistor off pin1 of V4B- from 220k to 180k
Thanks
-
I'm back with a question-
Well, what's the question?
-
Did any of these changes effect the attack sensitivity?
-
The 1st stage cathode bypass cap, to my ears, is a big tone shaping component.
If 750 means 0.75uF changed to a 10uF, that'd be a big difference.
-
750 means 750µF. Using such a huge size on the input triode is supposed to reduce or eliminate heater induced hum. The Fender 5F6A used a 250µF. All the old Sunn amps used 250µF as well.
-
Using the 10uf was suggested from an amp tech to calm down the gain. I could install the 250 and try a 12at7 to see if that controls the gain more.
Another thing I noticed between the Fender, Sunn and Traynor YBA series:
The resistor for the v1 pin 8-
Fender & Sunn use a 1.5 k
Traynor uses a 820 ohm
What are the differences?
For V4B, should I stay with the 180k on pin 1? If I remember ,that's a Fender update.
Thanks
-
Any suggestions on my last post?
-
> Any suggestions on my last post?
Two resistors and a cap? Hot-up the iron and try it.
-
It's true I could experiment, but I'm simply asking for an answer to my question. I know it's a cop out but I can live with that
Thanks
-
It's true I could experiment, but I'm simply asking for an answer to my question. I know it's a cop out but I can live with that
Sometimes the answer is "just because" or "just the way it is".
-
:laugh:
those were my canned answers to "why did it break?"
"Boss, you pay me to keep things runnin, to fix broke things, I want $1,000/hr to figure out "why" it broke :icon_biggrin:
-
I've just acquired one of these Super Twins (just chassis and iron) and plan on rebuilding it to spec, or possibly with a standard Twin output section. Either way, I'd like to use the multi-band EQ and have a silly question; The pots in the EQ section show a fourth wire going to ground (in addition to the usual upper, lower and wiper connections). I've not seen this configuration before. Is this just a wire soldered to the chassis of the pot, or some other type of pot? I've studied all of the photos but can't see the details on the pot wiring and there doesn't seem to be a layout diagram available anywhere.
Any help would be appreciated, thanks
-
The fourth wire is a fixed tap that's connected to the resistive element in the pot. It's not connected to the metal housing of the pot. Treat with care. May be impossible to find a replacement.
-
that's unfortunate, because I didn't get any of the pots with the chassis. Oh well, I guess I'll have to go in another direction. I'm open to suggestions. Thanks for the info and the quick reply.
-
Fender must have bought boat-loads of those tapped pots during that time.
I just removed 2 out of a 79 Pro Reverb I am rebuilding; one for the MV & one in a power tube balance circuit. Different values than you would need anyway, but looks like they used them all over the place.