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Post subject: Dripedge Bassman circuit and breakup
Posted: Sun Jan 26, 2020 2:20 pm
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Hey guys!
I have recently purchased a Dripedge Bassman. I had a tech retube it and change 2 filter caps to TAD, because they were out of spec. My other Bassman which is Blackface was retubed with same set of JJ tubes and also had some filter caps replaced with TAD, and restored by the same tech. I've been comparing them lately and it is obvious that the Silverface starts to break up earlier and has more overall break up. (Silverface as much breakup at around vol5 setting as Blackface at max volume setting).
My tech said that both were AB165 circuits and no modifications. He is very passionate about his job and I have great respect for him. I have trouble reading circuits so if any of you could help out to see if something might have been overlooked I would be very thankful. I will surely ask him about this and maybe we can compare those to amps back to back but I was curious what you have to say about this. Thanks in advance. Also this is my first post, so hi I'm Marek I guess :)

https://drive.google.com/file/d/1PSoQ-4ZGEurAqzs8V44YIuMG1JhhLHrF/view?usp=drivesd

https://drive.google.com/file/d/1PSoQ-4ZGEurAqzs8V44YIuMG1JhhLHrF/view?usp=drivesd

https://drive.google.com/file/d/1P_IKEwHc8cxLjZMHrCKcMth_4Z-N0DO9/view?usp=drivesdk


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Post subject: Re: Dripedge Bassman circuit and breakup
Posted: Sun Jan 26, 2020 3:58 pm
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For one thing your 470Ω grid-stopper resistors on the power-tube sockets are mis-matched -- the larger non-spec component looks to be rated for two watts (it should be one watt, just like the other).

And no way would I ever use a barrel nut to join two high-voltage wires inside an amp chassis!

Find a new tech.

Arjay

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"Here's why reliability is job one: A great sounding amp that breaks down goes from being a favorite piece of gear to a useless piece of crap in less time than it takes to read this sentence." -- BRUCE ZINKY


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Post subject: Re: Dripedge Bassman circuit and breakup
Posted: Sun Jan 26, 2020 10:23 pm
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Fender Bassman don't sound the same over the model years. It's not a problem you can really fix. The biggest difference I have found is the output transformers are built differently. The SF put out more power and Bass than the BF. Compare the dimensions and resistances ot the output transformers. I would expect the SF to be larger, with lower primary resistances. A replacement Fender OT would be closer to the old BF, but still not the same.


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Post subject: Re: Dripedge Bassman circuit and breakup
Posted: Sun Jan 26, 2020 11:11 pm
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"Silverface as much breakup at around vol5 setting as Blackface at max volume setting."


Tim, prolly something more than transformer winding changes going on, to have this kind of difference. No? Wonder why the tech didn't change ALL of the electrolytic caps, under the doghouse cover? That's where I'd check. Then, the bypass and bias supply caps. My SF Bassman is LOUD and clean to the house plaster falling off.

:lol:


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Post subject: Re: Dripedge Bassman circuit and breakup
Posted: Mon Jan 27, 2020 2:00 am
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Retroverbial wrote:
For one thing your 470Ω grid-stopper resistors on the power-tube sockets are mis-matched -- the larger non-spec component looks to be rated for two watts (it should be one watt, just like the other).

And no way would I ever use a barrel nut to join two high-voltage wires inside an amp chassis!

Find a new tech.

Arjay


Thank you for your insight Arjay!

This is a photos of before the amp has been serviced. My tech fixed the barrel joint. I'm sorry for the photos they were misleading - my bad!

Doesn't the voltage value specify the maximum voltage that can pass through the resistor? Can anyone confirm this is out of spec and can actually make a significant difference?


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Post subject: Re: Dripedge Bassman circuit and breakup
Posted: Mon Jan 27, 2020 2:06 am
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TimsAudio wrote:
Fender Bassman don't sound the same over the model years. It's not a problem you can really fix. The biggest difference I have found is the output transformers are built differently. The SF put out more power and Bass than the BF. Compare the dimensions and resistances ot the output transformers. I would expect the SF to be larger, with lower primary resistances. A replacement Fender OT would be closer to the old BF, but still not the same.


Hello Tim,
that is really interesting. So you're saying that these transformers were built differently within the same era? This is not the ultra linear SF tranny. It runs on 110V so I guess it must be leftover from the BF era.


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Post subject: Re: Dripedge Bassman circuit and breakup
Posted: Mon Jan 27, 2020 2:11 am
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BMW2002Ti wrote:
"Silverface as much breakup at around vol5 setting as Blackface at max volume setting."


Tim, prolly something more than transformer winding changes going on, to have this kind of difference. No? Wonder why the tech didn't change ALL of the electrolytic caps, under the doghouse cover? That's where I'd check. Then, the bypass and bias supply caps. My SF Bassman is LOUD and clean to the house plaster falling off.

:lol:


Hey,
his reasoning was that he wanted to change as little as possible to keep the amp authentic. Only 2 caps were leaking and out of spec and advised me to leave the rest in until there's a problem with them.

Is your SF also dripedge or later model?


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Post subject: Re: Dripedge Bassman circuit and breakup
Posted: Mon Jan 27, 2020 6:35 am
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Drip edge. The cab is DIY.

Image

Image


Last edited by BMW2002Ti on Mon Feb 03, 2020 8:48 pm, edited 1 time in total.

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Post subject: Re: Dripedge Bassman circuit and breakup
Posted: Mon Jan 27, 2020 12:06 pm
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Wow I always though that the dripedge models were a kind of transition models between BF and SF and that the purpose was to use available parts from BF era (that's what many folks suggest on the internet). Maybe they are just totally different? Have you compared your Dripedge to any BF?


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Post subject: Re: Dripedge Bassman circuit and breakup
Posted: Mon Jan 27, 2020 12:21 pm
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markozord wrote:
Wow I always though that the dripedge models were a kind of transition models between BF and SF and that the purpose was to use available parts from BF era (that's what many folks suggest on the internet). Maybe they are just totally different? Have you compared your Dripedge to any BF?


To an extent, that's true. Initially, all of the drip-edge models used the same circuits as their blackface predecessors, eg: Bassmans were still AB165, Pro Reverbs were still AA165, Super Reverbs were still AB763, etc. It wasn't until the middle of the following year (1968) that CBS-designed revisions began to replace Leo's original lay-outs, with both tone and performance suffering to varying degrees.

Arjay

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Post subject: Re: Dripedge Bassman circuit and breakup
Posted: Mon Jan 27, 2020 8:47 pm
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Converting the AB165 to the earlier AA864 is not a simple thing. Fender Guru goes through some of the negative feedback gyrations. The bias supply needs to be converted from BALANCE pot to BIAS pot. There are plate voltage changes to the phase inverter (and maybe the gain/input stages as well). Plus. changes to the tail resistors of the long-tailed PI. My Bassman was an earlier AA165. Which I converted to AA864.

Here are Fender Guru's tidbits.

http://fenderguru.com/amps/bassman
I.) Combining the AB165 and AA864


I will try to explain the following schematics for these circuits. The red circles in the schematic below illustrates these changes. From right to left:

1.) Switch the polarity of the two wires coming from the output transformer going to the speaker terminals. You need to do this only if you change the negative feedback loop to AA864 specs.

2.) Remove the 220k feedback resistors from each of the plates on the two 6L6 tubes. The amp will give earlier breakup and distortion. More volume at a given volume knob setting.

3.) Replace the 0.022uF coupling caps in front of the 6L6 to 0.1uF.

4.) Modify the negative feedback loop from the AB165-design to the AA864/standard Fender NFB (negative feedback) design. Remove the 0.1uF cap and 47KOhm resistor in the feedback loop and replace with an 820-ohm resistor (which you can make switchable by using the ground switch).
Personally, we have experienced good results with a 1.5K which reduces the negative feedback and increases distortion and treble. The .1uF cap you are about to remove may be used later as coupling cap between phase inverter and power tubes. The negative feedback loop ends up at the left side of the 22 K-ohm resistor which is a new soldering point.

5.) You need to insert a 100-ohm resistor between the left side of 22 K-ohm resistor to ground (the new entry point of the neg feedback loop in previous step).

6.) Between the second preamp tube (V2) and the phase inverter circuitry, replace the 0.1uF coupling cap with a 500 pF.


II.) Negative feedback loop

You can tweak the negative feedback loop to alter the treble cut and distortion profile in your amp. The purpose of the NFB loop is to clean up the tone and cancel out the mid/higher frequencies and upper harmonics (distortion), at the entry point of the phase inverter. Which is just in front of the power tubes. The NFB theory takes the signal from the speaker output (secondary of OT). Passes it through a resistor and mixes it in at the entry point of the phase inverter. The speaker signal is out of phase (180 degrees) with the signal at the entry point of the phase inverter and should cancel out non-signal information.

Disconnect the NFB loop and you’ll notice that the volume increases. The tone gets much more aggressive. Unfortunately, there is more white noise too. Canceling this noise is part of the function of the NFB loop. Without the NFB loop the amp’s sweet spot should be at a lower volume knob setting. Not only is there a volume shift, the amp’s clean headroom is reduced slightly. Most importantly, the tone gets rougher and rawer with more mids
and higher frequencies, aka presence. If you think your tone is too bright or harsh or you’re seeking a mellow and nice clean tone, you probably want to keep the NFB loop. This mod is for those who want more bite and a tone that really cuts through in the mix.

You may choose to implement the mod in several ways and in various combinations with the ground switch or a foot pedal. A foot pedal works like a boost/FAT pedal. If you are struggling to find a transparent boost pedal that keeps the natural Fender tone, you should try this out. You will still have that beautiful Fender tone, with a bit more volume, distortion, and noise.

No negative feedback at all. Disconnect the feedback loop (a wire) and tape insulate it. You get the most effect of this mod by disconnecting the NFB loop entirely. You may experience that the tone gets harsh, depending on guitar, speakers and EQ settings of course. If so, read more about the cap in the next bullet point.

1. Keep the feedback loop and install a .01 µF in series with the NFB resistor. The cap blocks the lower and mid frequencies. Depending on the cap value treble frequencies will still feedback to the phase inverter. Which cancels out the treble noise in the main signal path. The tone gets less harsh. You should experiment with different cap values. Start with 0.01µF.

2. Increase the feedback loop resistance value. A good starting point is twice the original resistor value. This reduces the effect of the feedback loop. The amp will break up earlier than when the NFB is engaged. If you make the NFB switchable there will be less volume difference if you use a high resistor value. (Using a .01 µF cap is also recommended to minimize the difference a little bit between NFB on and off).

3. Use the existing ground switch or make a foot pedal to make the NFB switchable. You can remove the death cap and free up the ground switch by removing existing wires to it (if you’re wondering what the ground switch does, you don’t need it). Then, wire the ground switch in series with the NFB resistor. If you’re using a cap in the NFB loop this goes across the switch allowing treble to bypass independent of the switch. When the ground switch has disconnected the NFB loop the higher frequencies are still fed back through the cap. In practice you solder each the two cap legs to at each side of the switch between speaker terminal and NFB resistor.


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Post subject: Re: Dripedge Bassman circuit and breakup
Posted: Mon Jan 27, 2020 9:26 pm
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Corresponding schematic with Fender Guru's changes.

Image


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Post subject: Re: Dripedge Bassman circuit and breakup
Posted: Sun Feb 02, 2020 4:39 am
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Oh I don't think I'll be wanting to change specs to previous circuits anytime soon. I mean I'm glad it sounds different than my Blackface. Thanks though, that might change in the future :)


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Post subject: Re: Dripedge Bassman circuit and breakup
Posted: Sun Feb 02, 2020 8:12 am
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Regardless of current condition, you should want to preserve the irreplaceable components. Its a fine looking chassis minus the wire nuts, wax paper caps.
I think most everyone here would recommend replacing ALL the electrolytics.
As far as the levels of breakup and or loudness differences, there are plenty of factors that keep each amp slightly different from one another. RC components drift w/ age and wear, electrolytics become diminished and less responsive to charge, tube differences/hours of usage, voltages aren't identical and so on as others have alluded to.
It would be nice to see a few of the new updated photos.


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Post subject: Re: Dripedge Bassman circuit and breakup
Posted: Fri Feb 07, 2020 1:40 pm
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BMW2002Ti wrote:
Drip edge. The cab is DIY.

Image


VERY nice head and cabinet. I prefer the silverface drip edge look to blackface.


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