Voice Of Music 1450 Tube Amp

The 3300 ohm resistor starts burning up when I power this amp up.  Can someone please help me determine the cause?

30 uf filter short or tube short screen to whatever, to find which, reolace resistor, pull tube, power up, watch for smoke or put finger on resistor if calloused,

Thanks CJ! Looks like I need a new 6X4 rectifier tube,,,

musika said:

Thanks CJ! Looks like I need a new 6X4 rectifier tube,,,

Click to expand...

I think CJ meant for you to pull V2 output, not the rectifier.

Gene

Leave in the 6X4 and 6EU7 and pull EL84 ? 

musika said:

Leave in the 6X4 and 6EU7 and pull EL84 ?

Click to expand...

Yes. If the resistor still blows then it is most likely the smoothing cap after it that has shorted. If it no longer smokes then the EL84 or its socket is faulty.

Cheers

Ian

Ah... Thanks for the specifics Gene and Ian.  I had pulled all tubes on the prior test.

musika said:

Ah... Thanks for the specifics Gene and Ian.  I had pulled all tubes on the prior test.

Click to expand...

Then it's 99% chances the capacitor needs replacing; quite common with these old caps. Replacing the whole can is quite expensive. Just disconnect the leg that's faulty and wire a single cap. The problem is the other sections of this multiple cap are likely to be EOL. Most of the times, elcaps dry out and loose capacitance, resulting in audible hum. Failing short is less usual. I would suggest you check them all. Detecting a cap that has lost capacitance is not too difficult. Just connect temporarily a capacitor of same (or close) nominal value and listen. If hum decreases, the cap needs replacing. Most of the times, you can simply connect the additional cap permanently, without altering the original wiring.

I've looked into replacing the 40/20/20/30 multi can with individual axial electrolytic capacitors and leaving the can in place.  I was planning on using 450v rating for all.  Space may be a problem so I am looking for small footprint caps.

I have replaced the 40/20/30/20 ohm 450v electroyltics in this amplifier and replaced surrounding resistors with 1 watt versions (1k is 5W).  Since it is the first vacuum tube amp I have repaired, I want to make certain I test the repairs appropriately.  Can someone please give me advice here i.e. should I start up with new electrolytic capacitors without tubes to start?  Tubes in this amp are EL84, 6X4, and 6EU7.

you can gut the can, and with heat shrink on the leads, and the right size caps, might just be able to build a new one.

I have done that before with great results.

Here is an example of what I am talking about...

http://www.picturetrail.com/sfx/album/view/22501148

Thanks for the link....

I replaced the caps with these from Studio Electronics...  but now I need to test.  Used phenolic tabs to tie each side and it worked out great. 

https://www.studioelectronics.biz/sunshop/index.php?l=product_detail&p=1577

I am asking for advice on how best start testing after the recap.

Best way would be a solid state replacement for the 6X4 and a Variac.  Bring it up slow and watch the current draw.  Without those, just stick in the 6X4 and hit the On switch.  Watch/smell for smoke.  If all looks good, pop in the other tubes and try again.

unplug amp
got a 10 ohm 1/4 watt resistor? 50 or 100 works too, 1/8 watt even better, solder in line with B+ as fuse
if really paranoid, use 100K to limit current in case of rev polarity

check cap polarity

with amp still unplugged, set pwr switch to on, hook up DC volt meter from ground to B+

plug in 6x4 but nothing else,

plug in amp and watch dc voltage, s/b positive as tube warms up, unplug amp as soon as it hits 50 to 100 volts, this might happen quickly if it is a directly heated cathode,

if anything funky starts up, pull plug, keep hand on cord during this test,

watch voltage after unplugging cord, it should drift downward, the rate depending on if you have bleeder resistors or not, if it drops like a rock, you have a short somewhere

repeat to 200 v, 300 400, plug in tubes,

play guitar amp til neighbors call cops, do this three times a day or as needed  (which always means more);D

I'm replacing the 2 prong power cord with a 3 prong with fuse.  Calculated 500mA 250V slow blow fuse for this unit.  It is 45 Watts power consumption.  Is this the correct fuse?  Can someone please help me verify?

The fuse primarily protects the line-cord. 0.5A seems safe for any line cord.

This is a Champ (small guitar amp). Near-enough. What do you 230V folks usually fuse a Champ at?

PRR said:

The fuse primarily protects the line-cord. 0.5A seems safe for any line cord.

This is a Champ (small guitar amp). Near-enough. What do you 230V folks usually fuse a Champ at?

Click to expand...

I have found that very often (even those made in Europe) 230V xfmrs have excessive surge current and a larger rating than "one half the 120V rating" may be necessary. I would think that's because 50Hz xfmrs should be 1.2 times bigger than 60 Hz, but aftermarket winders have to retain the same dimensions as the originals.
Less of a problem when the amp has been designed from the start for export.

240V here in NZ. I used a 1A slow blow fuse in my Champ build. I think the amp  uses a Champ specced Hammond power transformer with international primary options.

I love that little guy.

Nicholas.

120 VAC 60 Hz:   

Vibro Champ Blackface is 1 A slo blo

Tweed Champ 2 Amp fast blow

here is a pic of the Super Reverb fuse, 3 A  120, 1.5 A  220-240

Transformer size needs 1.2 increase in core area for 50 Hz

5 pts extra credit: why does that not correspond to a transformer that is 1.2 times bigger?

I should have mentioned that I had calculated (45VA/120V) * 1.5 - 0.5625. 

I'm not sure about the 45VA but maybe I should have more information on the power transformer.

This has been discussed before on this forum. 

https://groupdiy.com/index.php?topic=58300.0