Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: kwm488 on May 09, 2011, 12:18:07 pm
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hi
my old 1976 fender silverface twin reveb. here have 2 80uf 75v cap here, but i jsut can buy 100uf 100v cap only. can i use it to replace ? is this a bias cap? what different if i replace it from 80uf to 100uf? the 80uf cap is the white color in the photo, thank
http://i23.photobucket.com/albums/b383/kwm488/21c60cd1.jpg
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Yes. You're within about 20% for the uF rating, and the higher voltage is a plus. Alternatively you could put 2X 47uf 100 volt caps in parallel, for 94uF @ 100V. Doug has them for only 75¢ ea.
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hi
thank you for your reply. but i don't want to parallel two cap, because i don't like the look. sorry about this.
BTW, do you mind to let me know is it a bias cap?
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Yes that is the bias cap.
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thank so much. I am sorry, do you mind to let me know what result if I replace the bias cap to higher or lower valve ?
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Smaller uF cap value = less ripple filtering, and vice-versa. If you stay reasonably close in value, I doubt you'll hear a difference (the bias voltage does end up on the tube's grid); and operation of the circuit will be fine. When retrofitting a new cap to an old hard to find uF value, the general rule of thumb is to use a higher, rather than lower, uF value. A higher voltage rating will give the cap more stamina; but don't use a smaller voltage rating.
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hi
i replace the bias cap today. and i test these cap, it is 1 of result 98uf , another one is 128uf. i think another cap is dead, because the value is hugh different with 80uf. am i correct? because my amp volume sometime very weak.
sorry for another question.my amp also have some 25uf/25v cap in the preamp current. should i need to change it? because i just always see the people change the fender cap board cap and bias cap only.
thank
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I would go with the 98uF bias cap. Not much different from 80uF.
Low vol could be a number of issues, including a bad tube. As long as the chassis is already open, probably the best thing is (with tubes in) to measure & write down voltage readings at all filter caps, and all tube pins. Post your readings.
If the amp is due for a cap job, you might as well do it. Some guys replace the preamp tube bypass caps, some don't. When doing a cap job I usually also replace all the resistors in the B+ supply including plate resistors.
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hi
my amp also have volume problem (sometime volume weak) after i replace power tube and Electrolytic cap. can you give me some idea to solve this problem.
also, twin reverb have 2 channel. but i never use channel 1. can i don't install the tube for channel 1?
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You need to post your voltages.
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hi
i will check and post the voltage tomorrow.
but can i don't install preamp tube for channel 1? because i never use channel 1.
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but can i don't install preamp tube for channel 1? because i never use channel 1.
Yes. Gain will increase slightly on channel 2.
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hi
i think i fix the volume problem. today, i try to replace channel 2 all preamp tube. the volume problem is disappear. why i don't try to replace preamp tube before? because i remember i change the tube to RCA already before, but i am wrong. it is original 70s fender tube, and the tube word is red color, so i misunderstand it is RCA tube already.
and the old fender tube. it is good. i think it is RCA or GE tube, but mark fender logo.
in fact, will preamp cause the volume sometime weak problem?
i try to don't install preamp tube to channel 1, no sound totally. so i exchange the channel 1 and 2 preamp tube.
sorry for once more question. i already order sozo new fender blue molded cap. have anyone try it? should i replace all the cap to sozo? is my amp blue color cap good or bad? because my amp is 35 year old, so i buy the cap in case something.
(http://i23.photobucket.com/albums/b383/kwm488/IMG_3581.jpg)
(http://i23.photobucket.com/albums/b383/kwm488/IMG_3565.jpg)
(http://i23.photobucket.com/albums/b383/kwm488/IMG_3566.jpg)
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> will preamp cause the volume sometime weak problem?
Guitar pickup gives a weak signal. The job of the preamp is to bring the weak signal up to a medium signal. (The power-amp, 6L6 and tube before it, brings medium signal up to BIG signal.)
So, yes. If the preamp is not doing its job, the weak signal is not boosted as much as we need, and the sound will be weak.
Preamp problems can be tubes, resistors, capacitors, or connectors. We clean connectors (input jack) and change tubes first, because it is easy. Preamp tubes "can" run 50+ years, or fail the first day.... if another tube does not fix the problem, put the old tube back because it is probably OK.
Next: Change old cathode caps, because they go bad, do not cost much, and on a Fender they are easy to change. Then use volt-meter to check for voltages which are very-wrong (100V where should be 190V, 0V where we expect 1.9V, etc.... 170V for 190V is probably not a problem).
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Hi
Thank you for your very detail answer. As the cap, should I change it to sozo?
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Thank you for your very detail answer. As the cap, should I change it to sozo?
Absolutely not! Doug Hoffman sells the only caps that will work in an old Fender amp. :wink:
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HI
NOW, i find out the volume problem is come out from the preamp tube. the problem is solved. sorry to ask one stupid question, should i need to replace the old cap to the amp? because the old cap is not dead yet. i think it is wasted, also, i think old cap sound better than new cap. sorry for this question, but i really hope to know
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Which cap? Do you mean the cathode bypass cap to that preamp tube. If the cap is still functional, then this is a "buckdancer's choice". I.e., for a dollar, you get to dance with the girl of your choice, if you get my drift.
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hi
sorry , i have written clearly. i mean the electrolytics cap. my photo show that the old electrolytics cap changed to new sprague atom cap. but i know the volume problem come out from the preamp tube now. so can i put the old electrolytics cap to replace the new sprague atom electrolytics cap. i think old electrolytics cap may sound better. i know this may a stupid question.
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Further clarity: the cathode bypass cap is an electrolytic cap also. But now I know you mean the B+ filter caps. Do not re-install the old B+ filter caps. They look old and are past their useful lives. They are now prone to sudden failure. This can harm tubes and trannies. You may wish to put the original caps aside and save them to help preserve the vintage value of the amp.
Old caps (and resistors) drift in value over time, maybe substantially. Restoring an old amp with parts of stock value may alter the tone of the old amp. You can measure the actual value of the old components, and replace them with new parts of those values, instead of stock values.
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thank of your information . as the resistor , i also want to change it for in case. but which parts of resistor i need to change. i will change the 470ohm which mount in the power tube socket. and should i change all resistor in B+ filter cap? how about bias cap resistor?
thank
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There are different practices. My personal recommendation is in Reply#7 above: "When doing a cap job I usually also replace all the resistors in the B+ supply including plate resistors."
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hi
i will change the resistor too.
but should i change the film cap in my amp?
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Usually the film caps are not changed unless faulty.
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hi
sorry to ask more. what problem i will get if the film cap be faulty?
and i bought sozo fender cap already. i consider i should change it now or change it after the cap be faulty. so i hope to know what happen if the cap is faulty
thank
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If a film cap is faulty it will leak DC instead of blocking it. This will make pots scratchy; or throw off the bias on the next tube stage.
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hi
i have question. should i mod this amp to blackface twin reverb? this is silverface with master volume. because i search user review, silverface with master volume is not good if compare no master volume silverface and blackface.
because i need to buy extra cap and resistor, i hope to know more suggestion.
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This is a very subjective question.
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hi
i think i will mod my silverface amp to blackface. i see the layout and this mod web.
http://www.webphix.com/schematic%20heaven/www.schematicheaven.com/mods/silver2blackmod.html (http://www.webphix.com/schematic%20heaven/www.schematicheaven.com/mods/silver2blackmod.html)
but something they haven't metioned, my silverface is 2 of 330uf and 3 of 20uf Electrolytic capacitor. but blackface is 2 of 70uf and 3 of 20uf cap. so should i replace it from 330uf to 70uf? what different here?
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I think I would do all the Blackface mods except the filter caps first. Then maybe A/B the different filter cap values.
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hi
i remove the master volume finally. and i mod this amp from silverface to blackface 80%. i think sound better much too much.
next step, i will change the silverface bias system to blackface. but i have question. i really don't understand why blackface use 50uf/50v for bias cap , but silverface use 100uf /100v for bias cap. what different?
and i plan to add presence control pot to this amp, because it can install in master volume hole. do anyone can teach me how to do this?
thank
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Bias caps of 50uF to 100uF are OK; either value may be used with good results. 100V is better than 50V; a 100V cap will feel less stress from the bias voltage.
For presence control, just follow the schematic. You may have to alter the configuration of the PI circuitry and NFB loop. Layout is another issue. You may need to find room for components. Don't blindly assume that any empty eyelets are really free for use. There may be hidden wire connections under the board. Refer to the layout diagram for that model of amp (but don't assume it to be 100% accurate!). If the amp squeals when you're done, then the OT connections to the power tubes.
To aid in building the circuitry correctly, place a copy of the schematic by your workplace (I put it overhead), and trace over each leg of the circuit as you complete it with a highlighting magic marker.
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Hi
I am sorry. I don't know much how to add presence control in twin reverb. Also, which amp model schematic I can follow? Do you mind to give me more detail. I really need your help, please
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See: Bassman 5f6A schematic for a presence control. Also search this Forum & the Schematics section.