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**52 Bill** · 03-08-2012, 10:40 PM

Originally posted by bsco View Post

Checked a data sheet on line and according to the specs the resistance should be around 5200 ohms for a class A design...Anybody have a rough idea of what the resistance should be reading???

5200 ohms is the load impedance not resistance of the primary winding of the output transformer.

**bsco** · 03-08-2012, 11:01 PM

Right.......I forgot........what is the best way to see if that tranny is gone south?????Bear in mind that I don't have a sub to hook up...I guess I should get one soon.....

**bsco** · 03-08-2012, 11:17 PM

Actually, I forgot to ask....If I wanted to have an output transformer to use for a sub, what exactly should I look for??? It would have to be able to handle a certain amount of power for sure so what you recomend???? Thanks in advance.
Bernie

g1 · 03-09-2012, 01:19 AM

Is this the schematic you are using, or do you have a more readable one you could upload? http://music-electronics-forum.com/a...-schematic.pdf
R5 is the dropping resistor for the screen supply. It may have fried because of the bad tube, or there may be a problem with it's filter cap C3. Also, C12 at the cathode of the power tube could have a problem causing the tube to run too hot, which in turn could burn R5.
You need to find out why the tube is now running hotter than normal.

**Jazz P Bass** · 03-09-2012, 01:27 AM

You want , preferably, a tramsformer that will fit the hole pattern that you have.
Google 'SE 5k output transformer'
You willhave to try and get the manufacturers dimensions (or a drill

)
OT8SE Tube Output Transformer 5K/7K to 3.2/8/16Ohm 8VA | eBay

**bsco** · 03-09-2012, 01:53 AM

To g-gone....How come the 6BQ5 will work but run hot and the EL84 will blow the fuse as soon as power is applied. ?? I checked C3 and it checked ok for proper capacitance and no esr and I also checked C12 as well....same thing....the only thing left there is the transformer. The EL84 tube worked fine in a Fender Blues Junior 3.....the schematic that you have is the same one that I have...it is the only one I could get...I know it's not very clear....
To Jazz P Bass.....I think I might have confused you...What I was asking was if I wanted to have a bench output transformer on hand to substitute for say a number of output transformers in different brands of amps just for a quick check, what would be a good one to use?? I read somewhere that one from a Fender Bassman 50 would be suitable...maybe I need three...one for low power, one for medium and one for high power.....What would you suggest?? Thank you both for your help.....
Bernie

**bluzmn** · 03-09-2012, 02:19 AM

C5_Circuit_Diagram_complete.[1].pdfHere's a clearer schematic.

g1 · 03-09-2012, 03:17 AM

Are you using slow-blow fuses? (T means time delay)
Assuming you are using slow-blow fuses, not sure why it sometimes blew the fuse but is working ok in the Fender, could have to do with plate voltage differences or the tube may have an intermittent fault.
Cathode biased amps are supposed to self-bias. I don't see much cathode biased or single ended stuff, but I suppose they could still allow certain tubes to run hotter than others. Some tube sellers (eurotubes for example) will let you specify whether you need a cooler or hotter running tube for specific applications. Or you could adjust the bias by changing the value of the cathode resistor.

**AtomicMassUnit** · 03-09-2012, 04:31 AM

Originally posted by bsco View Post

To g-gone....How come the 6BQ5 will work but run hot and the EL84 will blow the fuse as soon as power is applied. ?? I checked C3 and it checked ok for proper capacitance and no esr and I also checked C12 as well....same thing....the only thing left there is the transformer. The EL84 tube worked fine in a Fender Blues Junior 3.....the schematic that you have is the same one that I have...it is the only one I could get...I know it's not very clear....
To Jazz P Bass.....I think I might have confused you...What I was asking was if I wanted to have a bench output transformer on hand to substitute for say a number of output transformers in different brands of amps just for a quick check, what would be a good one to use?? I read somewhere that one from a Fender Bassman 50 would be suitable...maybe I need three...one for low power, one for medium and one for high power.....What would you suggest?? Thank you both for your help.....
Bernie

Did you try the EL84 in your blues jr after it blew the fuse in the marshall? or was that before?

**52 Bill** · 03-09-2012, 05:24 AM

I'd also suggest checking the cathode resistor and bypass cap, they might be the reason for the overheating and or shorting output tube.

**bsco** · 03-09-2012, 02:49 PM

To AtomicMassUnit....Yes I used the EL84 in the Blues Junior after it blew the fuse in the Marshall....so the tube is fine.
To 52 Bill.......I will check the cathode bias resistor and the by-pass cap again just to make sure. I also have a question...Looking through a tube data book and it specifies a certain plate resistance for a specific tube. It states that this is the resistance that is presented to the electron flow from cathode to plate under a set of operating conditions. It also give the power dissipation for a particular tube and this is the power that is generated as waste through heat??? SO if I wanted to take a resistor of a certain value of the correct wattage, would it be possible to sub this in for the tube just briefly(I have to remove the tube of course), to see if the fuse blows again and then if it does I know for sure that the optput transformer is also damaged...This is just an idea that had crossed my mind so I'd figured I ask here on the form as you guys have a lot more experience with this stuff than I do.....Thats for any input you can give me.....and thank you to the person who supplied the clearer schematic....

g1 · 03-09-2012, 06:00 PM

You haven't said whether you are using slow-blow fuses, so I assume the answer is yes?

Originally posted by bsco View Post

SO if I wanted to take a resistor of a certain value of the correct wattage, would it be possible to sub this in for the tube just briefly(I have to remove the tube of course), to see if the fuse blows again and then if it does I know for sure that the optput transformer is also damaged...

I believe Mick Bailey did something like that here: http://music-electronics-forum.com/t28025/ (post #30)
And another thread with OT test info: http://music-electronics-forum.com/t28601/

**bsco** · 03-09-2012, 06:19 PM

I'm sorry g-gone......I am pretty sure they are slow blow...they have the "T" designation on them but I'll doulbe check to be sure.....Sorry about that..... Also, thanks for the links...I'll check them out as well......I'll let you know how I make out soon....
Thanks,
Bernie

**cjm808** · 03-13-2012, 09:19 AM

Hi Bernie-how's the amp coming along? I have one in the shop which is cutting out when cranked. Think may be related to the EL84 being a bit microphonic and getting 'rattled' by the speaker (it's a combo) because it plays fine using an external cab. It almost sounds like parasitic oscillation but can't verify till get in some new tubes. The power tube is physically close to the speaker so am suspecting it may be a contributing factor. Your original issue of R5 failing is similar to this one as it had been worked on prior by someone as it was replace-don't know what actually happened in that case but it apparently also blew the HT fuse as I found one loose inside the amp. The original EL84 gets damn hot in this amp as well but all voltages and the cathode cap/resistor seem to test ok so I am banking on a wanky power tube.

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