Kustom defender 15h mod repair

[Sowndman]

Hello, first post.
I have a kustom defender 15 h i bought cheap off ebay that ended up near DOA. i tried the noramla chage all the tubes with the same result extremely quite output. It is an interesting design as it only has three tubes. The 12ax7 is used for two stage of gain. The el84 do there own inverting. After a $50 non fix repair bill. I am now stuck but stil plan to fix it. I thought of adding another 12ax7 for gain and invert or just for invert. I think I have identified the output transformer as being bad by running a small ac voltage thru the speaker output and measuring the voltage from the el84 output sockets. Any help would be greatly appreciated.

Does anyone here have experience with self inverting pushpull amps ?

[VMS]

Does anyone here have experience with self inverting pushpull amps ?

No, but I think this magnatone 109 is self inverting.

http://el34world.com/charts/Schematics/files/magnatone/magnatone_109_melodier.pdf

https://www.youtube.com/watch?v=tA7q0FWY4Q0

https://www.youtube.com/watch?v=VufPF3OOTd0

[PRR]

Welcome.

> running a small ac voltage thru the speaker output and measuring the voltage from the el84 output sockets.

Can you share your results? It is not clear if you got AC, didn't get AC, only on one side, or got AC of an unhappy value.

Resistance checks may be telling also.

There is nothing horribly wrong with the basic idea. It is no "18 Watt", 15 Watt badge may be generous, but it should make ample racket for a small amp. Uses classic Fender values around 12AX7. Input sensitivity appears to be very good.

[PRR]

> I think this magnatone 109 is self inverting.

Very interesting. But not the same scheme.

The 15H has an un-bypassed common cathode resistor, which shunts some of the drive to the top tube across to the bottom tube's cathode. A big variant of Fender's long-tail driver.

The Maggie 109 bypasses the common cathode, but puts a large resistor in the top screen feed. As top tube plate current varies, the screen current varies. Times the screen resistor, we get a voltage, which happens to be in the right phase to flog the grid of the bottom tube. Proper selection of Rsg makes the amplitude good for driving bottom grid. Surely worked OK for them (at a low price-point). Fairly vulnerable to tube variations (different 6V6 will have different screen current variations; a good 6L6's screen current variation is small and inconsistent).

[Sowndman]

I sent 6 volts ac thru it from a wall wart and got the same on the sockets. I am going to check it out again tonight. If 15 watts is all i'm to get I may just rebuild the whole thing. Would it be over reaching to build a marshall plexi 50 ? I am s

[PRR]

> Would it be over reaching to build a marshall plexi 50 ?

New OT, new PT, new sockets, and a new chassis to fit it all?

In short: you can't turn a lawnmower into a heavy truck. Maybe you can keep the cup-holder, about everything else is different.

Same secondary and primary sounds like a measurement mistake, not a transformer fault.

Power OFF.

Clip-lead (don't hold!) your AC voltmeter, one lead on main B+ (to OT), one lead on a plate lead.

6V _AC_ to the loudspeaker tap (beware of shorting jacks).

For a typical small OT, you should get 70V-110V on the plate side.

Disconnect 6VAC, move the plate lead meter-clip to the other plate lead, check that (should be same).

Resistance readings?

[Sowndman]

I removed the transformer for testing as it has all push on locking connectors on the leads.

Wall wart is 16 vac 300 milliamp .using 16 ohm black and orange wires secondary 1.8 volts in secondary side. Primary side blue to brown 3.7 volts. Center tap red to blue 3.3 volt and center tap red to brown .2 volts. It appears the brown primary is shorted.

I wish  i hadn't blown $50 to find out nothing. I was going to go with a beefier transformer like a 30 watt so I don't to worry about this happening again. The thing is the original transformer is a 10k primary. What will using a 8k cause ?

The power transformer is a xita electric 5611-1012 secondary 1 is  150-0-150 and secondary 2 is 13.6 internal fuse 250 volt 5amp

i'm not sure if the Wall wart is 16 vac 300 milliamp .using 16 ohm black and orange wires secondary 1.8 volts in secondary side. Primary side blue to brown 3.7 volts. Center tap red to blue 3.3 volt and center tap red to brown .2 volts. It appears the brown primary is shorted.

I have some 9 pin sockets. Just a thought to beef up the powere supply and add a tube. I wish  i hadn't blown $50 to findout nothing. I was going to go with a beefier transformer like a 30 watt so I don't to worry about this happening again. The thing is the original transformer is a 10k primary. What will using a 8k cause ?

The power transformer is a xita electric 5611-1012 secondary 1 is  150-0-150 and secondary 13.6 with an internal 250v 5 amp fuse.

Looking at the joe piazza 1987 plexi 50 schematic I thought it might be cool to build basically a combination amp. The output of the kustom would stay the same with the exception of an upgraded marshall style OT. The oreamp would be completely rewired to mimick the plexi 50 with split inputs and add one 12ax7 for more gain.

[PRR]

8K, 10K, it's all rock-n-roll (whatever).

8K is very popular (thus low-price) for 2*6V6 and 2*EL84 amps. Betcha you find one here. Good price, fast service. But you could instead haunt eBay for old 2*6V6 radios and hope their OTs are good.

[Sowndman]

I pulled the trigger and bought an OP. The ebay auction says paul ruby jtm-45 style. It has 6k and 8k primary and 4,8, and 16 ohm secondary.

I will have to see how the amps performs.


The thought in my mind still is to add a 12ax7 to have a tube inverter instead if the current self inverter. Tis would be if the power transformer could handle the extra load.

Another thought is adding another volume control and slitting the gain stages basically making it into a marshall 18 watt without the tremelo.

[Sowndman]

Thanks for the info. PL trans seem to be much cheaper than OP trans so I may look into that later. Next week when the new to me OP trans ships I will know if the amp is worth putting anymore into.

If i am out of powers for another tube. What about rewiring for EL34 ?

I assume I would have less over gain but could achieve more of the marshall sound.

As I type this and reading others who have done this to other amps it also seems to come down to installing another power supply transformer.

[Sowndman]

Well, the amp is fixed ! I took the OP out of my Bugera v55 . It could use more clean as I doubt the to stages of the 12ax7 have enough gain to max out the el84's. Time will tell as I test it out with REAL guitar players .

[PRR]

> doubt the to stages of the 12ax7 have enough gain to max out the el84's.

Disagree.

The self-inverting gimmick does need twice the drive. But the EL84 needs half the drive of most power tubes.

Two 12AX7 stages is *more* than enough for most guitar amplifiers. Fender's first 12AX7 Champ threw-away some gain (and cost) with un-bypassed cathode resistors. The AA-Champ throws in a high-loss tonestack and some NFB around the power stage, still with two AX7 stages. This 15H has little to no tonestack loss and no NFB around the power stage.

I figure input sensitivity around 10mV. Many of the classic Blackfaces needed 20mV for full roar. Yes, head-banger amps today can be slammed with under 2mV, but they are clearly in a different class than this amp.

And you can get any added sensitivity "needed" with a simple booster or mild fuzz in the guitar cord link.

I do think this isn't a "real 2*EL84 amp". The first power tube gets full drive but the second power tube gets about half drive. So ball-park this is (1/2)+(1/2 of 1/2) or 3/4 of an "18 Watt". Say 13.5 Watts. This is just over 1dB down from 18 Watts, so the difference in loudness is just perceptible. And not really worth another bottle.

I am quite curious about the voltages in this beast. If you can measure inside, live, withOUT getting killed, I'd like to know EL84 cathode voltage (5V-15V) and plate and screen voltages (300V-400V). My matchbook figures suggest they work the EL84 very hot, but maybe not.

[Sowndman]

I used it for band practice today and it works quite well.

I'm using vintage EL84's pulled from an organ left for the dumpster at goodwill

15 watt setting all the way is perfect for clean then switch to 4 watt and switch guitar to high gain and basically same volume dirty.

I will take it back apart and write voltages and determine milliamps and such. While I got the top off again, I may add a standby switch.

[Sowndman]

Everybody that plays thru this amp love the tone.  They also talk about the feel or interaction. Even with the absence of a true phase inverter circuit or a cathode follower circuit feeding the tone knob, my goto Marshall that uses no pedals loves the feel.  He told me he only used the bright input on his Marshall.

i still need to get voltages and milliamp readings to write on the schematic.

I wonder how a pair of el34's would do in this configuration? I'm assuming I
would need an extra gain stage to compensate for the lower input sensitivity of the el34's.

[PRR]

> an extra gain stage to compensate for the lower input sensitivity of the el34's.

EL34 (and most others) need about twice the drive.

An extra gain stage will give 20X to 50X the drive. FAR too much.

It becomes a very different amplifier.

EL84 are cheap. If you really thing you need more Watts, use a lot of them. Eight bottles (4 pair) wants a 2K-2.5K CT load, which is nearly a Fender "100 Watt" OT (usually 2KCT). Actual power more like 60 Watts. Power drain on B+ is nearly (without the data from the 15H) 400V 240mA, which is just in sight of a 5F6a Bassman PT. Hmmm, heater demand is way up from a 5F6a because EL84 heater is hungrier than 6L6 and you got a lot more of them. Also the 5F6a OT is somewhat over-size for this job, while the 15H's stock OT was likely under-size (but you have replaced the OT?).

I don't think there is a Joy Of Cooking type for-sure recipe to make the little amp a "the same" big amp.

On today's stages it is common to use a smaller amp at the player and mike it into the PA for band monitor and audience. 15W is probably enough to work close-miked unless your back is against the drummer.