It is currently Mon Mar 16, 2020 3:35 pm

All times are UTC - 7 hours



Post new topic Reply to topic  [ 33 posts ]  Go to page 1, 2, 3  Next
Go to page 1, 2, 3  Next
Author Message
Post subject: opinions choosing 69 Twin replacement bias cap
Posted: Sat Apr 19, 2014 8:56 am
Offline
Amateur
Amateur

Joined: Thu Apr 11, 2013 2:55 pm
Posts: 183
So I'm recapping a 69 Twin Reverb and reflecting upon my choice of a generic 47mfd-160vdc@ 85°C (from antique electronics) that I replaced for the original paper 50mfd-70vdc on the bias supply board.

Unfortunately, one of the three JJ 20mfd-500vdc capacitors already installed as exact value replacements under the dogpan broke at it's wire entrance into the cap (even though carefully handled), and I have a replacement on order.............
And will not be ready to test the amp until then. Not impressed with JJ caps so far.

Anyway, my goal is to stay as close to original values, which leads to my question.
The closest available value......and generally recommended bias supply cap is the 100mfd-100vdc type........but I chose the generic 47mf-160v to stay as close to my stock 50mf-70v cap..

But will this 160v rating have any undesirable effects on the bias control circuit or capacitor (higher esr or heat)?
Or will my tubes be safer with the 100mf-100v caps?
Is there enough of a difference to even worry? (keeping in mind my bias is not adjustable).

Thanks


Top
Profile
Fender Play Winter Sale 2020
Post subject: Re: opinions choosing 69 Twin replacement bias cap
Posted: Sat Apr 19, 2014 9:25 am
Offline
Rock Icon
Rock Icon
User avatar

Joined: Fri Feb 05, 2010 4:31 am
Posts: 14049
Location: Province de Québec, Canada
100 MFD or close is always my choice for a quiet amp , not less than 75 volts . But if your amp is quiet you can keep the original value if you want .

More voltage is only more expensive cap.

Original 1969 Twin reverb have a balance pot that you can modified easely on adjustable bias pot .
Or if you want to keep the balance pot , bias is adjustable with the 15K resistor to ground .
Higher value= more grid negative bias = cold tubes bias


Top
Profile
Post subject: Re: opinions choosing 69 Twin replacement bias cap
Posted: Sat Apr 19, 2014 10:18 am
Offline
Amateur
Amateur

Joined: Thu Apr 11, 2013 2:55 pm
Posts: 183
Yes,, the amp was quiet with the original 50mfd-70v cap., and i' m trying to not alter the circuit any further than by replacing the cap..

Guess from what I've read generally, a higher capacitance results in, amongst other things, a higher circuit voltage (too cold a bias).............while a higher voltage rating results in more esr (or lower current in the bias-hotter bias).

Yes, more filtering equates to quiter..........but in a bias circuit I presumed it could also lead to a colder running bias.
I'm reasoning that sticking with the stock capacitance would not run my tubes too cold so as to have to mod the bias ckt..
Further reasoning that the higher esr in over spec. 160v rated capacitor, would actually run the tubes a little hotter than Fender's likely colder setting.


Top
Profile
Post subject: Re: opinions choosing 69 Twin replacement bias cap
Posted: Sat Apr 19, 2014 11:24 am
Offline
Rock Icon
Rock Icon
User avatar

Joined: Fri Feb 05, 2010 4:31 am
Posts: 14049
Location: Province de Québec, Canada
yomammaaho wrote:
Yes,, the amp was quiet with the original 50mfd-70v cap., and i' m trying to not alter the circuit any further than by replacing the cap..

Guess from what I've read generally, a higher capacitance results in, amongst other things, a higher circuit voltage (too cold a bias).
Not here , a better filtering = less AC ripple and more DC but it is millivolts here we talk about
............while a higher voltage rating results in more esr (or lower current in the bias-hotter bias).

Yes, more filtering equates to quiter..........but in a bias circuit I presumed it could also lead to a colder running bias.
I'm reasoning that sticking with the stock capacitance would not run my tubes too cold so as to have to mod the bias ckt..wrong

Further reasoning that the higher esr in over spec. 160v rated capacitor, would actually run the tubes a little hotter than Fender's likely colder setting. Wrong .

First you must know what is your bias now. What tubes ? Plate voltages ? Cathode current ( bias ) ?


Top
Profile
Post subject: Re: opinions choosing 69 Twin replacement bias cap
Posted: Sat Apr 19, 2014 11:51 am
Offline
Amateur
Amateur

Joined: Thu Apr 11, 2013 2:55 pm
Posts: 183
Will post those when replacement dog pan cap received.


Top
Profile
Post subject: Re: opinions choosing 69 Twin replacement bias cap
Posted: Sat Apr 19, 2014 1:37 pm
Offline
Rock Icon
Rock Icon
User avatar

Joined: Tue Jun 29, 2010 1:20 pm
Posts: 9640
Location: Indiana
+1 to everything said by stratele52

The values (Farads and voltage) were more limited back when these amps were built, so they used what was available. Now that we have more choices, it is generally better to up the Farad value of capacitor in the power supply and bias supply, for more filtering and less noise. For the power supply (under the dog house), if you increase the Farad value too much, it can change the character of the amp, but slightly higher values can be beneficial. For the bias supply, if you increase the Farad value of the cap, there will be no negative change in the tonal character, and very little to no change in the bias. You should never use lower Farad value caps for replacements. When you have the choice, always increase the farad value rather than decrease, even by a little.

It is always a good idea to use higher voltage value caps, sturdier, longer lasting, and the higher voltage value has zero affect on tone or bias level, but can be a great benefit for reliability.

Unless you are using exact replacements, or repros, the components aren't going to be original anyway, so using higher values as described here won't hurt a thing.

BTW, F&T and Sprague Atoms are very good capacitors, and the preferred choice of many members here. Both available from AES.

_________________
---> "The amp should be SWITCHED OFF AND UNPLUGGED before you do this!" <---

Por favor, disculpe mi español, no se llega a la práctica con mucha frecuencia.


Top
Profile
Post subject: Re: opinions choosing 69 Twin replacement bias cap
Posted: Sat Apr 19, 2014 5:36 pm
Offline
Amateur
Amateur

Joined: Thu Apr 11, 2013 2:55 pm
Posts: 183
Thank you.
My primary concern here is replacing the 50mfd-70v on the bias supply with the closest available equivelant value so that I can avoid possibly having to alter the fixed bias on this 69 drip edge.

That's why I picked the 47 mf-160v cap, being that the 47 is very clse to the 50mf original value.......while a 100 mf (might) slightly increase the bias voltage, causing me to ( have to) alter the bias resistance.

Maybe the few additional milliamp difference in the bias ckt. caused by using a 100 mf cap (could) be the difference between running the tubes too cold, necessitating having to alter the ckt. resistance.

Until my replacement dog pan cap comes in so that I can turn it on and crunch the numbers, maybe the question would be better stated ( given the suggested 100 mf-100v cap)
Does using the 100mf-100v bias cap on a fixed bias Silverface generally make necessary having to alter the bias circuit according to those who have used it?


Top
Profile
Post subject: Re: opinions choosing 69 Twin replacement bias cap
Posted: Sat Apr 19, 2014 6:50 pm
Offline
Rock Star
Rock Star
User avatar

Joined: Sat Jul 03, 2010 2:18 pm
Posts: 6544
Really, it's not that critical to be exact with the filtering cap in the bias supply. Most electrolytic caps run in a range of -10% to +20% tolerance. Esp generic variety. In the TR, the main rectifiers and bias supply rectifier are solid-state. You do not need to be spot on, wrt individual & overall capacitance, as you do with tube rectification.

Size is a consideration. And original OEM caps of the BF and SF era were big, for their capacitance. Modern technology has allowed for tighter wrapping of the internal guts. So, the same capacitance is in a smaller package. 100mfd/100VDC is a good choice for the bias supply.

Higher voltage rating usually means the cap can withstand more heat, for longer period of time, and still be within nominal values. So, to go the highest VDC rated cap in the mfd range will yield better filtering over the long haul.

And 22mfd/500VDC for OEM 20mfd/500VDC caps is fine, too.

" But will this 160v rating have any undesirable effects on the bias control circuit or capacitor (higher esr or heat)? Or will my tubes be safer with the 100mf-100v caps? Is there enough of a difference to even worry? (keeping in mind my bias is not adjustable)."

No undesirable effects with either a 100VDC or 160VDC rated cap. Higher mfd & VDC ratings, up to a point, will last longer in the bias supply. Don't go crazy with the mfd value, such that the bias supply lags behind the main PSU, during fire-up.


Top
Profile
Post subject: Re: opinions choosing 69 Twin replacement bias cap
Posted: Sun Apr 20, 2014 4:38 am
Offline
Amateur
Amateur

Joined: Thu Apr 11, 2013 2:55 pm
Posts: 183
These are some good answers here. Helps me narrow my choices.
Here is an answer from an ebay seller in regards to which bias supply cap to use to replace my stock 50mf-70v cap. The choice was between a 50mf-50v.......or a 100mf-100v:

MOST silverface amps have a double bias cap of a higher value - it sounds like you have one of the transitional models. There are countless variations of the SF coircuit - many undocumented even by Fender! The standard blackface recap kit will work in your amp. It comes with a 50uf/50v bias cap (the original blackface value). The 50v cap is more than adequate in the bais circuit (the voltage rating of the cap is the maximum voltage it can handle - the circuit dictates what it actually does handle). The capacitance is what's important here - the original 50uf is a good fit.

I was a little nervous about this suggestion given it's 50v rating in a 50 volt circuit.


Top
Profile
Post subject: Re: opinions choosing 69 Twin replacement bias cap
Posted: Sun Apr 20, 2014 4:53 am
Offline
Rock Icon
Rock Icon
User avatar

Joined: Fri Feb 05, 2010 4:31 am
Posts: 14049
Location: Province de Québec, Canada
yomammaaho wrote:
These are some good answers here. Helps me narrow my choices.
Here is an answer from an ebay seller in regards to which bias supply cap to use to replace my stock 50mf-70v cap. The choice was between a 50mf-50v.......or a 100mf-100v:
Easy choice , I write you before ; 100MFD 100 Volts

MOST silverface amps have a double bias cap of a higher value - it sounds like you have one of the transitional models. There are countless variations of the SF coircuit - many undocumented even by Fender! The standard blackface recap kit will work in your amp. It comes with a 50uf/50v bias cap (the original blackface value). The 50v cap is more than adequate in the bais circuit (the voltage rating of the cap is the maximum voltage it can handle - the circuit dictates what it actually does handle). The capacitance is what's important here - the original 50uf is a good fit.
Does not matter

I was a little nervous about this suggestion given it's 50v rating in a 50 volt circuit.
No no !


Top
Profile
Post subject: Re: opinions choosing 69 Twin replacement bias cap
Posted: Sun Apr 20, 2014 4:59 am
Offline
Amateur
Amateur

Joined: Thu Apr 11, 2013 2:55 pm
Posts: 183
If I may pose another question about this 69 TR while I'm in there.

Three of the original 470ohm carbon comp screen resistors remain untouched......measuring at approximately 920 ohms each thereabouts, while one has been replaced with a newer close tolerance non-carbon comp type measureing close to spec..

They don't appear cracked and seemed to work fine, but the reason I decided to replace the electrolytes in the 1st place was because I could swear that at higher volumes my guitar signal would intermittantly lose it's lower frequency response........kinda like engaging a fixed wah pedal.
Maybe this was my cord....or possibly something else in the amp.

So I decided to start with caps.

Should I replace the remaining 420 ohm screen resistors........keeping in mind I'm maybe overly anal about keeping things original where possible.


Top
Profile
Post subject: Re: opinions choosing 69 Twin replacement bias cap
Posted: Sun Apr 20, 2014 5:11 am
Offline
Rock Icon
Rock Icon
User avatar

Joined: Fri Feb 05, 2010 4:31 am
Posts: 14049
Location: Province de Québec, Canada
470 ohms resitors reading 920 ohms need new resistor . Carbon comp are avalaible.

Good filter caps give more power ,more punch and more low end to the amp .

Which schematic did you look for your work or you did not use one ? Any link to be sure we talk about same amp ?


Top
Profile
Post subject: Re: opinions choosing 69 Twin replacement bias cap
Posted: Sun Apr 20, 2014 5:16 am
Offline
Amateur
Amateur

Joined: Thu Apr 11, 2013 2:55 pm
Posts: 183
And yep.
The concensus is to go with the 100mf-100v electrolyte cap for the bias supply.
I have an f@t (100mf-100v) sitting there on an Antique Electronics order form waiting for advise about whether to replace the remaining off spec 470 ohm screen resistors.


Top
Profile
Post subject: Re: opinions choosing 69 Twin replacement bias cap
Posted: Sun Apr 20, 2014 5:25 am
Offline
Amateur
Amateur

Joined: Thu Apr 11, 2013 2:55 pm
Posts: 183
stratele52 wrote:
470 ohms resitors reading 920 ohms need new resistor . Carbon comp are avalaible.

Good filter caps give more power ,more punch and more low end to the amp .

Which schematic did you look for your work or you did not use one ? Any link to be sure we talk about same amp ?


No schematic at this time. Just looking at existing components likely needing replaced.
That's what I thought about the screen resistors. They must have changed value up over time.

So I'll repace them too.
Besides being original, is there any other advantage to using carbon comp resistors here?

Thanks


Top
Profile
Post subject: Re: opinions choosing 69 Twin replacement bias cap
Posted: Sun Apr 20, 2014 5:45 am
Offline
Rock Icon
Rock Icon
User avatar

Joined: Fri Feb 05, 2010 4:31 am
Posts: 14049
Location: Province de Québec, Canada
Fender screen resistor are always 470 ohms in any amp , replace them if your reading are 920 ohms .

I talk about schematic to be sure the amp is original with original value ; filter caps....


Top
Profile
Display posts from previous:  Sort by  
Post new topic Reply to topic  [ 33 posts ]  Go to page 1, 2, 3  Next
Go to page 1, 2, 3  Next

All times are UTC - 7 hours

Fender Play Winter Sale 2020

Who is online

Users browsing this forum: No registered users and 2 guests


You cannot post new topics in this forum
You cannot reply to topics in this forum
You cannot edit your posts in this forum
You cannot delete your posts in this forum

Search for:
Jump to: