Hot Rod Tweed Bluesmaster 4

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M Fowler
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Hot Rod Tweed Bluesmaster 4

Post by M Fowler »

A while back in 2016 I converted a Fender Bassman 20 to a Hot Rod Tweed Bluesmaster and the new owner is really pleased, so I'm happy.

The Bassman 20 had been modded and remodded so I didn't feel bad about putting new life into the chassis.

Ordered a Mojotone combo cab with 12" speaker baffle and loaded WGS Veteran 30. Used the stock transformers and can cap.
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M Fowler
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Re: Hot Rod Tweed Bluesmaster 4

Post by M Fowler »

Bias problem:

Owner wanted more wattage which means bigger PT and OT.

OT is Fender type 4k

Ordered TP26 from Allen Amps NEW! TP26 power transformer replaces the TP24. Same ratings and windings as before but with the addition of even higher voltage taps. Using a solid-state full-wave-bridge type rectifier, the TP26 will produce a B+ voltage around 370VDC at 150mADC which is great for EL84 based amps. Now with the new higher voltage leads (two red with white stripe leads) the TP26 can also produce a B+ voltage around 425VDC at 150mADC making is an excellent choice for larger octal power tubes such as 6V6s, 5881s or 6L6s. The TP26 is rated 120V to 280V/315V at 200mA and 6.3VCT at 4A with a 20V at 100mA brown to brown winding making it a great upgrade for the Blues Jr™ amp. 2" by 2-1/2" mounting centers, 4.25lbs, 3" tall above mounting surface. As used in the Allen Chihuahua, Classic 10 and Model LF amps.

I'm using the higher HT and EL34's because that gives me more tube room in a very tight area., Also, 5881 tubes might work too being physically smaller but do not have new set to try.

The PT has 20v bias tap but no matter what I do increase dropping resistor or reduce dropping resister and changing resistor on 10k bias pot can't get below 75mA.

I'm thinking I need increased negative bias voltage to lower the tube bias current down to around 35mA or even less for 420vdc loaded plate voltage.
Thinking I should tap off the HT and use 1k or 2k2 bias range resistor and 33k or 47k bias pot resistor to get to a decent tube current range of adjustment from 28-39mV.
sluckey
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Re: Hot Rod Tweed Bluesmaster 4

Post by sluckey »

I'm thinking I need increased negative bias voltage to lower the tube bias current down to around 35mA or even less for 420vdc loaded plate voltage.
Thinking I should tap off the HT and use 1k or 2k2 bias range resistor and 33k or 47k bias pot resistor to get to a decent tube current range of adjustment from 28-39mV.
I agree. But it's not a simple matter to just tap off the HT since you have a bridge rectifier. You will need a special bias circuit like some old Ampegs and JCM 900s used. Look at page 5 of this pdf for a bias circuit that will work with that FWB...

http://sluckeyamps.com/misc/Amp_Scrapbook.pdf
tubeswell
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Re: Hot Rod Tweed Bluesmaster 4

Post by tubeswell »

Merlin Blencowe also has some guff on capacitor-coupled bias supplies at the bottom of this page http://www.valvewizard.co.uk/bias.html
He who dies with the most tubes... wins
Ten Over
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Re: Hot Rod Tweed Bluesmaster 4

Post by Ten Over »

How about a voltage doubler on the 20V winding?
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M Fowler
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Re: Hot Rod Tweed Bluesmaster 4

Post by M Fowler »

Thank you all very good information to choose from.

I need to study and then carefully think about how to implement onto the existing board in the amp.
Mark
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Re: Hot Rod Tweed Bluesmaster 4

Post by Mark »

I can't say I really understand the capacitor coupled bias supply. I've used a dropping resistor and the standard circuit plenty of times.

The Fender Bassman 20's are funny little amps. The transformers in them are way too small and Fender rolled the bass off these amps to stop them distorting earlier. A bigger transformer set is a must for these amps if you really want to use them for bass. Mercury makes a set but seemed blissfully unaware of the circuit rolling off bass.
Yours Sincerely

Mark Abbott
tubeswell
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Re: Hot Rod Tweed Bluesmaster 4

Post by tubeswell »

Mark wrote: Sun Apr 02, 2017 8:12 am I can't say I really understand the capacitor coupled bias supply. ...
Did you read the article on Merlin's website that was linked above? It pretty much explains it all.
He who dies with the most tubes... wins
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M Fowler
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Re: Hot Rod Tweed Bluesmaster 4

Post by M Fowler »

The JCM900 bias circuit worked very well and I had just rebuilt a JCM900 dual reverb bias circuit a few months back so it was fresh on my mind.

I didn't get a photo of the bias unfortunately but did take some photos of the tightness associated with converting to 40 watts.
Had to use EL34's because the 6L6 glass was too large in diameter.
conversion 1.jpg
conversion 2.jpg
conversion 3.jpg
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Mark
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Re: Hot Rod Tweed Bluesmaster 4

Post by Mark »

I see you stuck with the original power transformer. I seem to remember it's about the same size as a Champ power transformer. Which worries me in a Bass amp.
Yours Sincerely

Mark Abbott
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M Fowler
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Re: Hot Rod Tweed Bluesmaster 4

Post by M Fowler »

No up further in this thread I posted about the new Allen Amps TP26 transformer I installed.

This is no longer a bass amp it is a tweed type circuit so it is not real clean more dirty clean.

I did change the V1a & b plates to 120k and 150k to clean it up since the owner plays in a gigging country band every week.
He has a Dr Z EZG50 head that I maintain for the road as well. That amp is super clean.

Mark
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