Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: bakerlite on May 13, 2015, 07:05:00 am
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Hi -
Embarrassingly I cant make sense of the Primary hook ups on the fender export transformer - wall voltage is 230v here so I am hooking it up for that but I seem to be getting something wrong as the black red seems to go nowhere....
attached is my estimation of the hookup
here is a link to full schem - https://guitarrasegambiarras.files.wordpress.com/2013/11/amp.gif (https://guitarrasegambiarras.files.wordpress.com/2013/11/amp.gif)
can anyone help?!
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http://www.ampix.org/albums/userpics/10003/Blues_Junior.pdf (http://www.ampix.org/albums/userpics/10003/Blues_Junior.pdf)
thats a better document for the hookup as it gives all scenarios... 100/230/240
Maybe I am overcomplicating it and black/Red is just another no connection.
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I imagine you can figure out the primaries by doing a resistance reading on your meter
Looks like you have 5 wires total
White is common to all the hook ups
Measure from the white wire to all the others and see what they read?
100v - lowest reading
120v -
230v -
240v - highest reading
They should line up in this order by DC ohms on your meter
You can hook up the tranny on the bench and read the secondaries to see if you have it correct
You should have around
20.5 vac across the two browns
261 vac across the two reds
6.6 vac across the two greens
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the black red seems to go nowhere....
The BLK/RED always connects to P7 whether its strapped for 100V, 230V, or 240V operation. See if this pic helps. The only question I have is there will be two wires on P5. Is that possible?
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That drawing makes my head hurt Steve :icon_biggrin:
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I just noticed that the two links you posted show different schematics! Especially in the area of the PT jumpers.
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If I am not mistaken I think he is gutting the amp and will need the wire colors for a new hookup
I am thinking the main board has already been removed from the amp and so the plug in connectors are gone
I cut all those terminals off the wires when I did my conversion
I only had black and white wires since my tranny was a US model
(http://el34world.com/Hoffman/images/Img_8748_small.jpg)
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Sorry for the confusing diagrams in the OP.
I think from the service manual:
http://www.ampix.org/albums/userpics/10003/Blues_Junior.pdf (http://www.ampix.org/albums/userpics/10003/Blues_Junior.pdf)
...that i have it figured....
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...that i have it figured....
Yes
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Ok, that is different
I was looking at the image you posted at the top
You have dual primaries
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The pic in the first post shows a domestic PT on the left and an export PT on the right.
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I zoomed in and it went to the domestic
I should not get involved in post when I am doing 3 things at the same time :icon_biggrin:
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thanks you guys are the best. even Doug... :icon_biggrin:
While I am here - I know its only a small amp but I am considering using a 2 stage switch , its a nice 10A/15A carling switch.... its all pins off and then first position shorts pins 1 and 2 and then second position shorts 3 and 4 (as well as but seperate to 1 and 2)
I was thinking of wiring it like the bigger amps with 2 switches...so i would only switch one side of the PT primaries using tabs 1 and 2 (as opposed to both in Doug's layout using the standard switch) and leave the other side of the PT permantly hooked up.
And then maybe using 3 and 4 to switch "AFTER?" the rectifier and before the filter cap?
how does that sound?
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Talking about a three position progressive switch, right? And you want to have a single switch for off, standby, and operate? That'll work fine. Here's some more info on that switch...
http://el34world.com/charts/SwitchDPST.htm (http://el34world.com/charts/SwitchDPST.htm)
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exactly! thats the one.
:thumbsup:
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I stock that switch on this page
http://hoffmanamps.com/MyStore/catalog/Switches.htm (http://hoffmanamps.com/MyStore/catalog/Switches.htm)
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I stock that switch on this page
http://hoffmanamps.com/MyStore/catalog/Switches.htm (http://hoffmanamps.com/MyStore/catalog/Switches.htm)
Doug - I'm really glad you started stocking that switch (although probably a long time ago). I bought a few 4 or 5 years ago and only have 1 left. Expected to have to look elsewhere for it.
Cheers,
Chip
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Thanks Chip.
It took me a long time to find them at a decent price so I could re-sell them at a decent price
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All good with the PT hookup - fortunately I didnt get to use any of that one shot PT smoke :icon_biggrin:
7.2v
22v
268v
unloaded.
Have to say - Dougs boards are fantastic! Not to mention visualy pleasing too!
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Ok - so I am almost there - all the donkey work done - just got to drop the board components on top and I am there.
Before I do there are a few component changes I'd like to make based on experience of using this cream board BJ and known exisiting mods for it.
1) Instead of c25 as a 47uf cap use 100uf to stiffen it up a bit (to compliment using a TO22 output transformer and cannibis rex speaker)
2)Increase preamp voltage by replacing the preamp plate resistors r3 and r8 from 100k to 82k and the filter cap balance resitors r48 and r49 from 10k to 2.2k
3)replace bass cap (c6) with 0.1uf
4)replace mid cap (c7) with 0.015uf
I have heard legend of some people doing somethiing called a "twin stack mod" by attaching the centre lug of the mid to ground but I dont understand what it does (compared to the above which are simple component changes)can anyone explain what that does???
anyway... thats what I'm considering.
no alarm bells?
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I would get the amp working first and then do mods
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2)Increase preamp voltage by replacing the preamp plate resistors r3 and r8 from 100k to 82k and the filter cap balance resitors r48 and r49 from 10k to 2.2k
I'd leave the plate resistors alone and just mess with R49 if the intention is to just increase preamp B+ (for more headroom?)
If you decrease R48 you will probably increase the gain of the PI. Those EL84s are easy to drive (or overdrive). So you can probably expect earlier distortion.
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you are right... I'll do a seperate mod thread when its up and running - too eager!
regarding the stock BJ l.e.d: it picks up the power through the heaters and a 180k resistor - neither of the heater rails are + or - are they so it doesnt matter which side I stick on the resistor?
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I won't matter which side the resistor connects to, but the value of the resistor should be 150 or 180 ohms, not k-ohms.
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So just retired the iron at last!
Time for a cup of tea a markie and a schematic I can scribble all over to recheck everything and then its cookin time!
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Looks like it's coming along
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So one of my el84's seems bad outa the box- even if I swap them the 0.6mv draw follows the tube so theres no cigar tonight.
Also the heater voltage seems a bit High what do you think? anything else look alarmingly on the high side?
Ill probably switch the PT primaries from 230 operation to 240 -
wall voltage is 232 here so it may just be enough to lower the heaters.
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Well I have re wired for 240v operation and managed to get heaters down to 6.5Voltages at PT (AC) :
heaters: 6.49v bias: 19.8v HT: 244v
Ref points from hoffman blues junior conversion schematic: X=270v y=277v Z=315v B+=330v
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Is it working?
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RE: V3 cathode voltage...
I've never seen 163v on the cathode of a LTP PI. Are you sure about that reading? Do you have the correct value resistors in the cathode circuit, in particular, R21, R24, and R26?
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yeah v3 had me scratching - what a fool - incorrectly attached ground cathode circuit - and to think I had prided myself in double checking everything :(
So V3 now looking more like :
pin 1 - 201 pin 6 - 190
pin 2 - 38v pin 7 - 40v
pin 3 - 60v pin 8 - 60v
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Sluckey beat me to it
Those voltages sounded really high like the cathode resistor was wrong or some kind of grounding issue
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Is it working?
sorry I missed that post - yes it is kind of sounds ok on top 3 strings but distorts badly when i play the lower strings or dig in with the fingers .
I wonder could i have done damage (either to the meat cap c40 or tube in v3 ) running v3 at those voltages even just for the time it took to take voltages when i was doing first power ons...
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I would look for something else wrong, like the other thing was wrong :icon_biggrin:
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:l2:
fair point!
just that little cap is only rated for 50v and figured is there was 150+ hanging around i ma have torched it
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Put any cap in it's place to check it
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So i went throught with a fine tooth comb and i am confident there are no more errors, well i am gonna leave this till moday as I am using a pair of borrowed output tubes that I havent a clue the age/condition so when I have a new matched pair ill be able to rule them out as the cause
The crackling/distortion is only present if I dig in - engaging the Fat switch or having the meat control anywhere past zero seems to either cause it or make it worse
it has brief moments of sounding awesome! so hopefuly will be worth the wait if i can figure this out.
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Disconnect the NFB from the speaker jack. Get better? If so, reverse the OT primary plate leads and reconnect the NFB wire.
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Oh man that could be it. At work now so will try that tomorrow , I wouldn't mind but I read that very thing happened to someone else here and I made a mental note to think of it at the end of the build.
Must be filed a way with the rest of the mental notes like tidy my work room and what not.
thanks guys
Disconnect the NFB from the speaker jack. Get better? If so, reverse the OT primary plate leads and reconnect the NFB wire.
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I don't know if you've seen this but it is an excellent way to double check an amp build with it's schematic.
We've seen a good number of guys who couldn't find what was wrong with their build and using this method they found it.
Doug did a great job on this example;
http://el34world.com/Forum/index.php?topic=17701.0 (http://el34world.com/Forum/index.php?topic=17701.0)
Brad :icon_biggrin:
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For sure that way is great. I always print of a schematic I can scratch out with a sharpie and that's what I did. But for some reason I scratched out the ground at the same time as scratching out the cap so all in one go if you know what I mean. Anyway lesson learned...
As it turns out the hookup wire from c6 to the treble pot was damaged Internaly somewhere. After sluckey last idea didn't work for me I sat down with the.trusty chopsticks and found it pretty quick luckily.
So popped in some el84m Mil tubes in there and they are biasing up within 0.4mv (ma)
The amp sounds killer like so much better than it did before the conversion and I been on the same stage as it for 4 years so I would know! !
Very nice attack and sustain and clarity. It's still a blue's junior just a really nice one :)
The only thing now is an fx loop as reverb is essential but I promised another topic for mods.
Thank you all for steering me on. :occasion14:
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Sound clips please!!!
Congrats!
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Good deal Liam,
Glad you got it sorted out
Yes, some sound clips would be great
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Bakerlite,
Do you have room to mount an effects board somewhere in that chassis in the future? Where are you going to put the jacks? Do you have an extension cabinet jack in your rig? I am getting ready to start the same project and was considering the effects loop as well. I have a new chassis coming, I am anxious to see if I can maximize this chassis. Right now I have a fuse, line out, axillary speaker and main speaker that needs to be installed. Not sure I can get an effects loop pair of jacks in the frame. :dontknow:
That's a great feeling when a projects goes right!
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Check out this device for a self contained effects loop using a sub miniature tube
http://riotdevices.com/products/accessories/ (http://riotdevices.com/products/accessories/)
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It will take some persuading thats for sure -
I think I can squeeze fx loop plus a standard and ext speaker sockets but that will be very very tight. no way could I fit a line out too.
I would also prefer to use a heavy duty switch for ohms instead of a double switching switchcraft jack which I decided against as if that fails something could go very badly with the Output section. So for now I hae not wired the ext speaker.
I am on the lookout for a very small form factor switch for the ohms. but first priority is the fx loop as reverb is a must!
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That's what this switch is for
http://hoffmanamps.com/MyStore/catalog/Switches.htm (http://hoffmanamps.com/MyStore/catalog/Switches.htm)
(http://hoffmanamps.com/MyStore/catalog/images/IMG_2266.jpg)
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Blues junior Hoffman 1:
Blues junior Hoffman 1:
Just a snippet. Was testing a reverb pedal
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Do you have the effects loop in place or are you going direct into the main input with the reverb pedal. I haven't used an effects loop so I am not sure I need one to begin with, but if I did I would think the reverb or chorus would be processed through that loop, is that correct?
If you do have the effects loop in place does it create any noise at all?
Thanks
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Hey Pete sorry I meant to get back to you on the other thread but am away gigging this week. The guitarist is loving his new improved blues junior! Yes the verb is in the loop. I inserted it after the treble and before v2a 10k resistor. Space is very tight. It's located next to the speaker jack so this places the loop components perpendicular to the power transformer. It was noisey but after changing the b+ ground for the loop to the cathode of v2a ground it helped but not completely so I fabricated a 4" x 2.5" shield of thick metal plate and placed it over the PT and grounded it to chassis. This removes all hum and noise.
As regards the mods from billm you asked about the tone stack mods are just the 2 caps for mid and bass so yes you could apply them easily to this circuit as it's the same. As for the presence mod you have I will have to check when I get back and not using this tiny phone but maybe someone else could chime in as far as I remember there is a 7.5 resistor in dougs layout where I believe you could insert the 10k pot but I need to look at that again.
My one gripe with my build is that it does in fact need the presence mod as we are rolling off treble where ideally we'd like to be rolling off just the higher frequency so I would recommend persevering with your research on the presence mod .
As for the clean boost I don't know anything about it but if it comes with instructions just cross reference dogus layout with the schematic from fender I posted above and you should be able to find all relevant points to hook it up?
I will try and get better video showing longer and more varied playing, that was just us making sure the reverb was working in the loop before the gig.
Did you mention chassis? I badly need a blue's junior chassis If you know where to get one?!!! I have a spare bj cab I'd lI've to repurpose but it didn't come with a chassis..
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"Did you mention chassis? I badly need a blue's junior chassis If you know where to get one?!!! I have a spare bj cab I'd lI've to repurpose but it didn't come with a chassis.."
GREAT to hear I'm not the only guy looking for a blues jr chassis
Report to moderator Logged. I'm thinking there are enough people interested is a blues jr. Chassis to make it a worthwhile effort.......I hear that there are enough "dead"blues jr's that finding a chassis should be easy.......it is not.
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Sounds good
The video in link #2 says it is not listed when I hit the thumbs up button
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Clips sound great. Nice bluesy riffs too. Makes me wish I still had my Blues Junior so I could mod it.
EL34 thanks for posting that link to the almost plug and play effects loop. Tempted to add it to my 20 watt plexi. Wonder if my PT has the spare heater power capacity.
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Can anyone help chris with his 240vac hookup?
His post is here
http://el34world.com/Forum/index.php?topic=20236.0 (http://el34world.com/Forum/index.php?topic=20236.0)