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Post subject: NAOD
Posted: Sun Nov 27, 2011 6:49 pm
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NAOD as in "New amp order day". :lol:

As most of you know, I have heavily modded my foot pedals, my guitars, my BDRI and Frontman 25 (mostly better speakers and a few minor tweaks), and completely rebuilt my Chi-Com Champion 600 to a 5E1/5F1 configuration. So now I find myself jonesing for a new challenge. What to do?

Well, with the kind permission of my lovely wife, I have ordered a Ted Weber amp kit for Christmas (Don't worry. She will get new custom made curtains and bedspread for our bedroom out of the deal. She's not THAT benevolent. :lol: ).

So, which kit did I get? Well, I recently purchased a copy of "Circuit Analysis of a Legendary Tube Amplifier - The Fender Bassman 5F6A, 3rd Edition by Richard Kuehnel. This book is fantastic if you are an engineer or just a glutton for punishment (I am both). It has math in it that I haven't seen, let alone used, in almost 40 years! :shock: So, yes, I shot my wadd and ordered the Weber 5F6A kit (Minus tubes. I want better ones then what probably come with it.). With luck, it will be here by Christmas. I will be taking off the week between Christmas and New Year's, so hopefully I will have some time to work on it (I promised my wife to do some long needed repairs around the house during that same time period. But there are 24 hours in a day and I'll get plenty of rest when I am dead, so I should be able to squeeze in a few hours each day for each of my projects.).

I have been researching schematics in making my decision and noticed some differences between the modern Bassman Reissue schematic and the vintage one. Most obvious is the use of a 12AX7 in V1 for the modern amp vs a 12AY7 in the vintage. I have a good source for NOS 12AY7s and plan to go that way. The most interesting difference is a very simple one in the tone stack. In the modern version, a 0.1 mfd cap is used as the tone cap in the bass control. The vintage amp used a 0.02 mfd for the bass control. Reading the Kuehnel book and running different cap values in the Duncan Tone Stack Calculator (http://www.duncanamps.com/tsc/index.html) shows that the low end response with the 0.1 cap drops off slowly and stays fairly flat down to very low frequencies. The vintage tone stack, on the other hand, rolls off fairly rapidly below about 60 Hz. This would account for the complaints of boomy, bloated bass when boosted in the Hot Rod Deluxe/Deville and Blues Deluxe/Deville which use the same tone stack design as the modern amp. I plan on building my kit as closely as possible to the vintage amp, taking into account modern safety requirements.

I will document the build and post a writeup with photos as things proceed. Not sure how long it will take as I plan on taking my time and doing it right the first time. Stay tuned. I am stoked. :D

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Post subject: Re: NAOD
Posted: Sun Nov 27, 2011 6:54 pm
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Are you going to mod the bias supply with a pot to allow for adjustments, Bill?

Arjay

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Post subject: Re: NAOD
Posted: Sun Nov 27, 2011 7:15 pm
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Retroverbial wrote:
Are you going to mod the bias supply with a pot to allow for adjustments, Bill?

Arjay


Most definitely. The parts aren't provided to do that and I did, however, note an error in the Weber schematic that shows that mod. The schematic says to replace the 56K bias resistor with a 50K pot. That sure won't work right as the adjustment range would be incorrect. Jeffrey Falla in "How to Hot Rod your Fender Amp" recommends replacing the 56K resistor with a 50K pot in series with a 27K resistor for a full range of adjustment. The current schematic on the Fender site is the earlier Bassman Reissue without the adjustable bias pot. I'll have to search for a later version with the pot.

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Post subject: Re: NAOD
Posted: Sun Nov 27, 2011 7:22 pm
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bluesky636 wrote:
Jeffrey Falla in "How to Hot Rod your Fender Amp" recommends replacing the 56K resistor with a 50K pot in series with a 27K resistor for a full range of adjustment.


+1!

bluesky636 wrote:
The current schematic on the Fender site is the earlier Bassman Reissue without the adjustable bias pot. I'll have to search for a later version with the pot.


Look for the 5F6-A "LTD" re-issue schem, Bill......that's the version with the revised bias circuit.

HTH

Arjay

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Post subject: Re: NAOD
Posted: Sun Nov 27, 2011 7:28 pm
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Retroverbial wrote:
Look for the 5F6-A "LTD" re-issue schem, Bill......that's the version with the revised bias circuit.

HTH

Arjay


I have been. No luck yet. Must ...... keep ...... searching. :lol:

BTW, the Kuehnel book has the best, most readable version of the vintage schematic I have found. It also has the Marshall version.

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Post subject: Re: NAOD
Posted: Mon Nov 28, 2011 1:18 pm
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Nice work Bill, sounds like a great plan and fun times ahead. 8)

Are you going to build it as a head unit or will you build a classic 4 X 10" combo?

Snowy.


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Post subject: Re: NAOD
Posted: Mon Nov 28, 2011 1:25 pm
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Have you seen this, Bill......?

https://taweber.powweb.com/store/5f6a_schem.jpg

Arjay

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Post subject: Re: NAOD
Posted: Mon Nov 28, 2011 3:40 pm
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Retroverbial wrote:
Have you seen this, Bill......?

https://taweber.powweb.com/store/5f6a_schem.jpg

Arjay


Yup. That's the schematic I was referring to in my post about the error in the bias pot. I have been downloading and researching every link I can find on the Weber 5F6A kit. I want to be well prepared for when the kit arrives. I'm a little scared about what I have gotten myself into. :oops: :lol:

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Post subject: Re: NAOD
Posted: Mon Nov 28, 2011 4:19 pm
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Great project bluesky636, nice Chrismas gift. Congrat. Many hours of fun.

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The schematic says to replace the 56K bias resistor with a 50K pot. That sure won't work right as the adjustment range would be incorrect. Jeffrey Falla in "How to Hot Rod your Fender Amp" recommends replacing the 56K resistor with a 50K pot in series with a 27K resistor for a full range of adjustment--------bluesky636


Yes a big mistake. Maybe you could have the right bias voltage. What I'm afraid is you can accidentaly turn pot wiper to ground and blow your tubes. With the 27 k this can't happenend. You'll have alway a minimum bias voltage.


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Post subject: Re: NAOD
Posted: Mon Nov 28, 2011 6:06 pm
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i'm getting a headache just trying to figure out what y'all are talking about. i guess im'm just happy if my amp turns on :lol:


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Post subject: Re: NAOD
Posted: Mon Nov 28, 2011 6:13 pm
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bluesky636 wrote:
I'm a little scared about what I have gotten myself into.


Nonsense, Bill. We have every confidence in your abilities.

8)

Arjay

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Post subject: Re: NAOD
Posted: Mon Nov 28, 2011 8:56 pm
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stratele52 wrote:
Great project bluesky636, nice Chrismas gift. Congrat. Many hours of fun.

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The schematic says to replace the 56K bias resistor with a 50K pot. That sure won't work right as the adjustment range would be incorrect. Jeffrey Falla in "How to Hot Rod your Fender Amp" recommends replacing the 56K resistor with a 50K pot in series with a 27K resistor for a full range of adjustment--------bluesky636


Yes a big mistake. Maybe you could have the right bias voltage. What I'm afraid is you can accidentaly turn pot wiper to ground and blow your tubes. With the 27 k this can't happenend. You'll have alway a minimum bias voltage.


Thanks.

Yeah, I can't figure out how they made a mistake like that.

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Post subject: Re: NAOD
Posted: Mon Nov 28, 2011 8:58 pm
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Retroverbial wrote:
bluesky636 wrote:
I'm a little scared about what I have gotten myself into.


Nonsense, Bill. We have every confidence in your abilities.

8)

Arjay


Well, it is a bit of a leap from swapping parts on a Champion 600 PCB to handwiring a honkin' Tweed Bassman clone. :lol:

I do plan on taking my time.

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Post subject: Re: NAOD
Posted: Mon Nov 28, 2011 8:59 pm
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phreddybee wrote:
i'm getting a headache just trying to figure out what y'all are talking about. i guess im'm just happy if my amp turns on :lol:


Hell, I don't always know what I am talking about. I'm happy just to get up in the morning! :lol: :lol: :lol:

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Post subject: Re: NAOD
Posted: Tue Nov 29, 2011 6:55 pm
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Deleted.

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Last edited by bluesky636 on Sun Dec 18, 2011 8:41 pm, edited 1 time in total.

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