Blues Jr: Handwired with octal conversion...with photo

[jecars]

Winter project: Blues Junior with Hoffman turret board and modified tube board for octal sockets & Tung-Sol 5881 reissue.  Sounds great and kit went together without a hitch.

Objective was everyday-player amp at ~95bB sound level at home.

Seeking any forum thoughts/tips/questions.

[shooter]

Seeking any forum thoughts

play it like you stole it

[sluckey]

Very nice!

I'm curious about your bias circuit. I would have thought that PT with a 1/2 wave rectifier would not give enough voltage for 5881s. How much negative voltage do you have on pin 5 of the 5881s?

[jecars]

Regarding bias for 5881:

* Stock BJr PT brown wire tap is 20V AC. The Hoffman board kit gets about 27V DC as max C- available. 
* For 23 Watt Tung-Sol 5881 and 320V B+, 70% idle current is 50mA.
* I set the bias to read 55mV across R98/99 (1 ohm) which connects to pin 8 (cathode for octal socket).
* C- reads -24V and I still have about 15% remaining on trim pot before hitting the max.

By the way, using the Interactive Valve Data Sheet tool on http://www.bmamps.com/Tech_tds.html, I also get -24V as the proper "analytical" bias for 5881 with BJr PT. I have had good luck matching this tool with reality.

[jecars]

More about 5881 compared to EL84.

Using 1kHz input signal (free app on my smart phone), I get 24 watts output on dummy load with 5881. Doing the same test with EL84, I got 20 watts. As expected, no real power output change...4 watts is not perceptible...less than 1 dB difference. For me, I like the distortion character of 5881 better than EL84.  Also, the 5881 has significantly bigger plate which gives better bass response and authority...which I dig.

[kagliostro]

Nice Job 

Just curious, all other parts (PT - OT) are stock and you changed only the power tubes on the kit ?

Franco

[jecars]

Power Transformer (PT) and Output Transformer (OT) are both stock based on my testing.  FYI, I bought the Blues Jr used on Craigslist, so I confirmed as much as I could.

For PT, converting to 5881 increases total heater current draw by 280 mA because the 5881 heater draws 900 mA compared to 760 mA for EL84, multiplied by two. I did the 5881 conversion first (using the stock PCB) about six months ago.  No problems, PT temperature never got above 80F according to my non-contact IR thermometer (awesome technology for just $17 on Amazon!)

For OT, my testing indicates 6.6K Ohm impedance (w/ 8 Ohm speaker) and winding ratio of 28:1. This matches stock values, near as I could research.

[jecars]

While I'm thinking about it, the only non-stock part is the main power switch.

It is a Carling Off-On-On.  I wired it so that first "on" position (photo shows) is Standby...PT primary is connected but only 6.3 VAC (green) and 20 VAC (brown) are hot to board. 

Moving to the second "on" position connects the 250 VAC and activates B+, X, Y & Z.

[kagliostro]

Many thanks for answering

only one thing

EL84 heater consumption is 760mA each

and

5881 heater consumption is 900mA each

your consumption w 5881 is 280mA more than using EL84

Franco

[jecars]

^^^ I stand corrected. kagliostro is correct, EL84 heater draw is 0.76 A, not 0.6 A that I wrote.  I also had a units typo in my post, so I edited to fix both errors.

[BetterOffShred]

Sounds pretty cool man.  Does it jam hard?  I'd love to hear some sound clips of it if possible.   I dig big plates too!
-Brett

[MFowler]

Great looking build.


Just a note: Belden tube sockets 9 pin fit the stock BJ socket holes without having to use a board. But I do like Doug's tube socket board makes it very stable and good looking conversion.

[jecars]

Thanks all for the positive feedback on my build.

Regarding Hoffman tube board, one consideration that tipped in favor of Hoffman was that 5881 tube could still fit behind stock back cover / metal shield. 

Octal sockets that mount directly to the chassis will cause tubes to stick down about 0.3" further overall.  In my case, the 5881 would have then clashed with the aforementioned metal shield, requiring that it be removed.  The result would be increased damage vulnerability when I stuff the power cord, guitar cable, my lunch, etc, in the back of the amp (bad habit, but I'm too old to change).

[gtrplayr1976]

Nice !  I'm in the middle of a conversion for a friend. I'm hoping this will make it cleaner. I have upgraded the OT to a TO22 from Allen amps ,and may implement some component value changes that Lyle Caldwell suggested for the stock Blues Jr boards if this doesn't get it clean enough.