Another Ohms Question?.

bluesjunior

Member
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5,907
About a year ago I built a bandmaster sized tweed covered pine cabinet for my 1998 green board Fender BJr'. I added 2 x 8" 4ohm Eminence 820H speakers wired in series giving an 8ohm output and sitting above the original 8ohm 12" Eminence Special Design speaker in a triangle. As the amp has a single 8ohm output jack I gave them each a jack and have used them seperately since then, either the single 12" or the 2 x 8".

I play mostly at home nowadays in my attic converted workshop/music room and over the last year found that I much prefer the sound of the 2 x 8" over the single 12" but would still like to hear them together. So last week as a test I bought a so called Y cable, ie 1 x male 1/4" jack with two seperate 1/4" female sockets and plugged the two speaker systems into it to hear what it sounded like. I really liked the sound actually, not so trebly as the single 12" and not so mid rangy as the 2 x 8" just a nice combination of the two. As far as I can figure out, doing this is giving me a combined speaker ohms output of 4ohms which I was told before was not a very good idea.

My setup is the BJr' with the Master volume at 12 and the vol/gain at 3. From the guitar I run into a wah pedal to a tremelo pedal to a delay pedal then into a Boss FBM1 bassman pedal then into an Audio Storm VBM volume control box. I have the BJr' set up for a nice clean sound and set the FBM1 for mild crunch which I can increase as needed via the 25db Dennis Cornell mid boost system in my strat and use the VBM as a master volume and I am very happy with this as I get to my ears an acceptable compromise between volume and tone at so called "bedroom" level".

Now I found out that I could get an upgraded Hammond 1760F output transformer for the BJr' that would give me 4/8/16ohm options with the addition of a rotary switch and if no other option was open then I would likely go for it but it means laying out more cash and then if I couldn't do it myself even more for a tech to do it. I thought I would ask you experienced techs here at the TGP, before I make a final decision on this and taking into consideration that as I play mostly at home at low volume what are the short or long term chances or even dangers of me blowing up or destroying the amp playing it through the Y cable like I just did?.
 

pdf64

Member
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7,280
I strongly advise against a mismatch on such an amp, with very hot idling pentodes in fixed bias. As with signal, a ‘low’ mismatch would increase plate dissipation significantly, with various consequences. But if you really are keeping the volume domestic friendly, eg others in the house can watch tv without needing headphones or crazy loud volume to avoid the guitar annoying them, then it won’t really matter. As a mismatch at idle or low power output levels would be very unlikely to cause a problem.
Y cable splitters can be nasty though, they can be prone to intermittent connections, which are the worst case scenario on the output of a tube amp.
 

Tone_Terrific

Silver Supporting Member
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32,714
I strongly advise against a mismatch on such an amp, with very hot idling pentodes in fixed bias. As with signal, a ‘low’ mismatch would increase plate dissipation significantly, with various consequences.
A mismatch won't affect idling current, that's all DC.
I kind of doubt the the BJ power supply can deliver enough current to take the tubes to dangerous dissipation levels, but don't know.
 

HotBluePlates

Member
Messages
10,295
A mismatch won't affect idling current, that's all DC.
I kind of doubt the the BJ power supply can deliver enough current to take the tubes to dangerous dissipation levels, but don't know.
@pdf64 isn't talking so much about idle current, as "hot-running amps." And if you lower the primary load on those and play it loudly, it could overheat the output tubes. Basically, an argument for buying the transformer with the appropriate taps if the amp will be cranked a lot, and perhaps not worrying about it if the amp will mainly be played quietly.
 

Johndh

Member
Messages
400
You could add a 4ohm power resistor in series, to bring the combination back up to 8ohms. Safe to try, if may make a small change to tone, but very little, and be slightly quieter.
 

bluesjunior

Member
Messages
5,907
Thank you all very much for the replies. As I really do play at low volume I will keep on using the Y cable for now and see where it takes me. What would be the warning signs to look out for if it did start going wrong?.
 

HotBluePlates

Member
Messages
10,295
... What would be the warning signs to look out for if it did start going wrong?.
If the sound starts cutting out to either speaker, you would suspect an intermittent cable connection. That may be unlikely if you're not really moving the amp around or flexing that cable.
 

HeavyCream

Silver Supporting Member
Messages
3,543
Have you considered trying a different 12” speaker? Seems like a lot of trouble to go through. From your post, it sounds like you might dig a Greenback.
 

VICOwner

Gold Supporting Member
Messages
1,847
The transformer is about $40 and a NKK rotary switch is about $10. You’ll thank yourself if you swap the transformer. The stock transformer is pretty weak compared to the Hammond.
 

bluesjunior

Member
Messages
5,907
The transformer is about $40 and a NKK rotary switch is about $10. You’ll thank yourself if you swap the transformer. The stock transformer is pretty weak compared to the Hammond.
Hi VICOwner, which NKK switch would you recommend?.
 

bluesjunior

Member
Messages
5,907
The one in your link is correct. I was wrong.
Thanks for the info, Just had an email saying both parts will be delivered tomorrow. Looking at the diagrams the wiring of the primary side in the 1760F OT seems pretty straightforward but on the secondary the present OT only has the black and the 8ohm green connections while the 1760F has the extra 4ohm yellow and 16ohm white connection wires. I have done a lot of googling today trying to find a wiring diagram for the NKK HS13Y-D 3-way switch but still haven't found one. Do you know of such a drawing or could you explain to me how to do this?. I can see that I connect the Black/yellow/green/white in that order to the switch and then a wire from the black terminal on the switch to the point on the amp where it is attached at present. Do I then need to add a wire from each of the other colours to the point on the amp where the green 8ohm wire is attached at present or is this wrong?. I attach a pdf diagram of each transformer.
https://www.hammfg.com/files/parts/pdf/1760F.pdf
https://www.hammfg.com/files/parts/pdf/1750F.pdf
 




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