It is currently Mon Mar 16, 2020 7:13 pm

All times are UTC - 7 hours



Post new topic Reply to topic  [ 13 posts ] 
Author Message
Post subject: "Twin Amp" tech questions, please help
Posted: Mon May 11, 2015 1:24 pm
Offline
Hobbyist
Hobbyist

Joined: Sun Jul 15, 2007 10:01 pm
Posts: 10
Hello, this is my first post and first Fender amp owned. Thanks for reading! 8)
Model: PR449, DOM: 2010
Image

The tone of this amp is fantastic! Once you hear Fender twin clean, there is no going back! :wink:
Distortion channel is also really great. Lots of crunch on tap, I was shocked :shock: Fender crunch...WTF :lol:
The price is a steal for these, I paid $650 CDN used MINT, MSRP was $2,249.99 USD :shock:

I have a lot of modding experience but I am not a circuit designer or a pro amp tech :oops:
Any help would be most appreciated. My technical questions are:
I have the clean channel Bass control dimed and still find there is not enough. Therefore I would like to increase the bass level internally by changing capacitor or resistor values.

As per the schematic: (twin amp, bottom right hand side)
http://xxx2.fender.com/en-CA/support/ar ... schematics

The Bass circuit is comprised of: Capacitor C5 .1 uF 400V & Resistor R8 100K
Which should I change? Increase value or Decrease value?

Same for the Distortion channel, I have the Treble control dimed and would like more Treble.
The Treble circuit is comprised of: Capacitor C19 120 pF 500V & Resistor R33 33K
Which should I change? Increase value or Decrease value?
( a "Bright" switch would have been great here....ehem...Fender, same as the Clean channel...)

Image

BTW, I have at least 20 years of soldering experience, amplifier modding and am fully aware of the lethal voltages in this amp. I do know how to properly discharge and maintain a discharged state!

Also, here is a list of the tube positions and their functions in case anyone might be interested.
I made this list based on what I see in the schematic. Please feel free to confirm or correct me....

V1A: First stage for Clean and Dist channels
V1B: Clean Second stage feeding tone stack
V2A: Dist channel second stage
V2B: Dist channel third stage
V3A: Dist channel fourth stage feeding the tone stack
V3B: Feeds V8A (Efects loop send) and Reverb circuit
V4A: Power tube feed (Phase inverter?)
V4B: Power tube feed (Phase inverter?)
V5A: Reverb return
V5B: Reverb return to "Preamp Out"
V6A: Tremolo circuit
V6B: Tremolo circuit
V7A: Reverb transformer input
V7B: Reverb transformer input
V8A: Effects Loop send
V8B: Effects Loop return


Top
Profile
Fender Play Winter Sale 2020
Post subject: Re: "Twin Amp" tech questions, please help
Posted: Mon May 11, 2015 2:51 pm
Offline
Rock Star
Rock Star
User avatar

Joined: Sat Jul 03, 2010 2:18 pm
Posts: 6544
Before you hit the mods... have you checked the idle bias value of the 6L6GCs? Just in case you are way off nominal range, which can effect the bass.

Also, what speakers does the amp currently have in it?


Thanks!


Top
Profile
Post subject: Re: "Twin Amp" tech questions, please help
Posted: Mon May 11, 2015 7:25 pm
Offline
Professional Musician
Professional Musician

Joined: Thu Aug 07, 2014 3:25 pm
Posts: 1023
If you have the treble control turned way up, most Fenders will lose bass response.
You have to think of the treble control as a master tone control that swings from hi treble to low bass.
Back off on your treble and see if the bass comes back.

You can change the 100K resistor to 82K for more bass and mids in the same bass range.
You can change the .1uf cap to a .15 or.22 for lower lows.
If you are playing with a bass player, increasing your bass can argue with his tone and muddy up the music.

On the drive channel, the 120pf treble cap passes higher frequencies than the normal 250pf cap.
Going to a 250 pf would increase the amount of treble signal, but lower the peak frequencies.
Are you looking for higher highs or more of what you've got?

Caution, too much treble can cause overdriven tone to go fizzy. If you play with just a little crunch, you can brighten it up
C22 probably cuts a lot of treble out. You might consider pulling it, depending on your style of play.
Adding a 47pf cap across the volume control will brighten it if you play at 1/2 volume or less. It works just like a bright switch would.
Good luck....


Top
Profile
Post subject: Re: "Twin Amp" tech questions, please help
Posted: Tue May 12, 2015 11:33 am
Offline
Hobbyist
Hobbyist

Joined: Sun Jul 15, 2007 10:01 pm
Posts: 10
Thank you for the replies 8)
BMW2002Ti:
I set the Bias to 60mVDC as per factory before I even played through the amp :wink:
Amp is ALL original including the speakers.

TimsAudio:
Gain channel Bass is fine, just need more Treble (more of what I have already, Freq seems fine ATM)
Is there any way to just increase the Bass amount without affecting the Mids? Amp has tons of Mids already.
BTW, Bass players can screw off.....LMFAO. ( I WAS the Bass player for my last band....) :wink:
Not in a band ATM anyways, thanks

Can you explain what "Adding a 47pf cap across the volume control" exactly means.
From what lug to what lug on the pot?
Thanks for all your help!


Top
Profile
Post subject: Re: "Twin Amp" tech questions, please help
Posted: Tue May 12, 2015 7:21 pm
Offline
Rock Icon
Rock Icon
User avatar

Joined: Tue Jun 29, 2010 1:20 pm
Posts: 9640
Location: Indiana
axeman wrote:
...I set the Bias to 60mVDC as per factory...


Is that 60 mV at one of the test points, for two tubes? If so, 30 mV (Cathode) per tube is an idle of less than 13 watts Plate, way too cold for a 6L6.

_________________
---> "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: "Twin Amp" tech questions, please help
Posted: Wed May 13, 2015 8:56 am
Offline
Professional Musician
Professional Musician

Joined: Thu Aug 07, 2014 3:25 pm
Posts: 1023
So, where are your tone controls normally set?
Are you using a high quality input cable. That can affect brightness if its poor quality or too long.
On the gain channel, I would try to get more treble by pulling C22.

It's hard to get more bass without getting more midrange as well, but you might try this. On the clean channel, you can get more bass by changing the coupling caps C2 and C9 to .1uf.


Top
Profile
Post subject: Re: "Twin Amp" tech questions, please help
Posted: Wed May 13, 2015 3:24 pm
Offline
Hobbyist
Hobbyist

Joined: Sun Jul 15, 2007 10:01 pm
Posts: 10
Shimmilou:
Followed this exactly. From the manual:
Set BIAS: With a DC voltmeter, measure VDC between
the “GND” and “V9,V10” test points. Adjust BIAS for .06
VDC (60mVDC).
4. Set BALANCE: With a DC voltmeter, measure VDC
between the “V9,V10” and “V11,V12” test points. Adjust
BALANCE for zero (0) VDC

TimsAudio:
Can you explain what "Adding a 47pf cap across the volume control" exactly means.
From what lug to what lug on the pot?
Thanks for all your help!


Top
Profile
Post subject: Re: "Twin Amp" tech questions, please help
Posted: Wed May 13, 2015 8:21 pm
Offline
Professional Musician
Professional Musician

Joined: Thu Aug 07, 2014 3:25 pm
Posts: 1023
If you are looking at the knob with the volume pot pins pointing down, as it is mounted in the amp, the center pin and the pin to the right is where to mount a bright cap.
If you pull the control board and look at the bottom, its the center terminal and the terminal whose trace goes to the treble control center terminal.
On the clean channel schematic, it's shown as C7 and S1

This bright circuit will add treble at low volume settings that bypasses the volume control.
It has less effect as the volume control is turned up.
It's used to compensate for loss of treble response at low volumes.


Top
Profile
Post subject: Re: "Twin Amp" tech questions, please help
Posted: Wed May 13, 2015 10:38 pm
Offline
Rock Icon
Rock Icon
User avatar

Joined: Tue Jun 29, 2010 1:20 pm
Posts: 9640
Location: Indiana
shimmilou wrote:
axeman wrote:
...I set the Bias to 60mVDC as per factory...


..... 30 mV (Cathode) per tube is an idle of less than 13 watts Plate, way too cold for a 6L6.


As I thought. Fender is known to set many of their amps bias very low from the factory. You might get a noticeable improvement in sound by simply setting the bias to a more optimal setting. A minimum of 70 mV at each test point would be a good starting point for a more full sound.

Going by your Plate voltage, 465 VDC (measure to be sure), with 30 watt tubes, an idle bias of 15 to 21 watts is the optimal range, which would be approx 35 mV to 45 mV per tube, 70 mV to 90 mV at each test point.

Approx 76 mV at each test point would be my recommendation. :idea:

_________________
---> "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: "Twin Amp" tech questions, please help
Posted: Thu May 14, 2015 7:40 am
Offline
Hobbyist
Hobbyist

Joined: Sun Jul 15, 2007 10:01 pm
Posts: 10
Thank you again, both of you! Fantastic advice and lots of food for thought. Exactly what I wanted.
You two should work for Fender and "fix" many things they are doing.....;)
Like make sure they include an impedance switch with EVERY amp, just to start! DOH :?

TimsAudio, do you also agree with a bias setting of 76mVDC ?


Top
Profile
Post subject: Re: "Twin Amp" tech questions, please help
Posted: Thu May 14, 2015 7:49 am
Offline
Rock Icon
Rock Icon
User avatar

Joined: Fri Feb 05, 2010 4:31 am
Posts: 14049
Location: Province de Québec, Canada
axeman wrote:
T
Like make sure they include an impedance switch with EVERY amp, just to start! DOH :?



Impedance switch start by a more expensive output transformer , switch ......; = more expensive amp .


Top
Profile
Post subject: Re: "Twin Amp" tech questions, please help
Posted: Thu May 14, 2015 10:00 am
Offline
Professional Musician
Professional Musician

Joined: Thu Aug 07, 2014 3:25 pm
Posts: 1023
6L6 tubes should be biased from 25-35 Ma each. So 60 for 2 tubes is a reasonable setting for the casual player. For gigging or recording, pushing it to 70 or 75 does give a better tone. The trade-off is tone vs tube life. Hotter bias wears out tubes faster.

Impedance switches are a versatile feature for a head. Combos are designed for the supplied speaker, so impedance switching is usually done automatically at the extension speaker jack.
The main problem with a multi-tap OT is that it requires a larger iron core to accomodate the extra windings.
Core size affects saturation at low frequencies. Small cores limit power at low frequencies. Larger cores are harder to saturate, so the bass limiting capabilities of the OT are compromised by multi tap transformers. This leads to too much bass response and muddy tone.


Top
Profile
Post subject: Re: "Twin Amp" tech questions, please help
Posted: Thu May 14, 2015 10:28 am
Offline
Rock Icon
Rock Icon
User avatar

Joined: Tue Jun 29, 2010 1:20 pm
Posts: 9640
Location: Indiana
You must take into account the Plate voltage, and not just a current range, and 25 mV would be unreasonably cold in this case. Considering that 50% to 70% of the tubes rated wattage (15 watts to 21 watts) is the accepted range for a 6L6 idle bias, with 465 VDC on the Plate, 32 mV would be a minimum setting to achieve 15 watts of idle (64 mV for two tubes). Anything less would be too cold, possibly getting nasty crossover distortion. I routinely set my amps bias to between 55% and 60% and get 10 years and more use from my tubes. A few amps sound better at 70%, and a little bit less tube life is preferable to poor sound. In addition, since Cathode current is higher than Plate current by a few mA (mV), using Cathode measurements is a little conservative anyway. Poor sound forever, or great sound for 10 years? Tough call. :lol:

_________________
---> "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
Display posts from previous:  Sort by  
Post new topic Reply to topic  [ 13 posts ] 

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: