# Hiwatt (mojotone) DR504 zero sound



## SteveAppleton77 (Jun 26, 2020)

So I built myself a Hiwatt dr504. It’s a mojo kit, except for the power transformer that’s from tube town in Germany (Supply chain issues)
It’s worked and sounded great up until yesterday. Plugged in this morning and zero sound. No hiss, hum or static. Nothing. No matter how high the volumes are. 
It worked fine yesterday so I assumd logically I’ve unplugged something by accident. Made sure of course the cab was plugged in and then plugged the guitar straight into the amp. Nothing.
Ok so power off and plugged the cab into another amp and it makes sound. Ugh so speakers and cable are fine.
Sigh, so I start thinking OT blown. I always jump to worst case in my head.
Removed the amp from the cab to inspect. No visible signs. Plugged in and powered up again. All tubes warming up.
All tubes were new with the build three months ago. This thing has seen consistent light use. Ie I don’t crank it. It wasn’t built for its overdrive, but for its beautiful cleans.
So.. I will find a known good 12ax7 and swap it in each spot.

Is there a way to determine if the OT is cooked?

As I said there’s no visible signs that somethings fried. 
The OT is a mojo (Heyboer).
Preamp tubes are new mullards
Power tubes are tung-sol
Any other directions to look?

Thanks for any advice

oh, I know the faceplates wrong. There’s a story behind that. And it was free


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## SteveAppleton77 (Jun 26, 2020)

Forgot say, I’ve checked/tested the fuses. 
I have swapped three of the preamp tubes with a known good one. Ran out of time for the last one


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## Paul Running (Apr 12, 2020)

SteveAppleton77 said:


> Is there a way to determine if the OT is cooked?


Continuity check with an ohm meter is a quick and easy method.


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## Alan Small (Dec 30, 2019)

She is a beauty...very nice work and parts choices.
I would not suspect a tranny until all else has been eliminated and the standby switch has been checked too🤪 would certainly check b+ at the plates of the power tubes if you have not yet...if present then try new power tubes.


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## Paul Running (Apr 12, 2020)

SteveAppleton77 said:


> Plugged in this morning and zero sound. No hiss, hum or static.


Dead quiet would indicate power supply or output section...even if there was no input to the output section, there would be some hum...I have never heard a dead quiet power supply.


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## Latole (Aug 11, 2020)

99%is a power supply issue

1-Check B+ voltageS and report
2- Don't waiste you time with OT for now


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## Latole (Aug 11, 2020)

By the way, how this bias pot is fixed on the chassis ? Not only with glue ?


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## WCGill (Mar 27, 2009)

Unlikely to be the OPT. Start with some voltage readings, esp. the output section.


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## SteveAppleton77 (Jun 26, 2020)

You guys are awesome!
quick response. Will do some voltage checks this weekend.
bias pot is held in place with thick rubber two way tape.


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## SteveAppleton77 (Jun 26, 2020)

So I did some probing around. Don’t worry I am being careful. Not my first time. 
My plate voltage on the power tubes is way too high. 516 vdc.
Plate voltage in the preamps is almost nonexistent. 1.2 ish vdc. 
original B+ was not that high. The PT is putting out 370 vac to the diodes. I did find an suspicious point. 
the 100 ohm 5 watt resistor that follows the standby switch has about 506 vdc going in but very little coming out. 
burned out resistor? No exterior signs, but perhaps there isn’t always? According to the schematic and logic, there’s suppose to be rectified 460ish vdc going in and a smig less coming out.
As it feeds the large cap cans that feed the preamp section seems like it would make sense that it’s choked off everything. 
Is my line of thinking right?


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## SteveAppleton77 (Jun 26, 2020)

Oh and I am getting audible pops and crackle when probing the power tube plates. So that’s optimistic


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## Paul Running (Apr 12, 2020)

SteveAppleton77 said:


> No exterior signs, but perhaps there isn’t always? According to the schematic and logic, there’s suppose to be rectified 460ish vdc going in and a smig less coming out.


Check the schematic below. So you are measuring near normal voltage at point XE (red-dot) and very little voltage at XD (orange-dot)...correct?


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## Paul Running (Apr 12, 2020)

SteveAppleton77 said:


> My plate voltage on the power tubes is way too high. 516 vdc.


Is the amp powered by a MOJO785 power transformer?...sub trannies may not measure the same.


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## Latole (Aug 11, 2020)

"Plate voltage in the preamps is almost nonexistent. 1.2 ish vdc."
-SA

Issue is there ; Power supply don't feed preamp tubes; Find why

Less preamp load on PS = more dc volts on power tubes


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## SteveAppleton77 (Jun 26, 2020)

Paul Running said:


> Check the schematic below. So you are measuring near normal voltage at point XE (red-dot) and very little voltage at XD (orange-dot)...correct?
> 
> View attachment 443585


Yes that’s correct. About 506vdc at xe and I think it was about 1.2 ish vdc at xd. I would have to re measure for exact.


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## SteveAppleton77 (Jun 26, 2020)

Paul Running said:


> Is the amp powered by a MOJO785 power transformer?...sub trannies may not measure the same.


No mojotone was out of PTs at the time. It’s a tube town.


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## Paul Running (Apr 12, 2020)

SteveAppleton77 said:


> No mojotone was out of PTs at the time. It’s a tube town.


Okay so, that TFM is rated 350VAC @ 115VAC mains. Your mains could be 120+VAC which means that you should multiply by a factor of 120/115 = 1.04. So peak DCV will be 1.04(350VAC X 1.414) = 515VDC.
Just to be safe, after you get the amp serviceable, you should check the bias current to the output tubes.


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## Paul Running (Apr 12, 2020)

SteveAppleton77 said:


> burned out resistor? No exterior signs, but perhaps there isn’t always?


Correct...resistors do open with no visible signs....I would double check with an ohm meter.


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## Paul Running (Apr 12, 2020)

See the image below; is there a wire or jumper underside, connecting from terminal with red-dot







?


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## SteveAppleton77 (Jun 26, 2020)

Yes, that leads to pin 6 (unused for el34) of the power tube socket. 
My daughters are up. When I get a sec I’ll hit that resistor with a multimeter. 
does it need to be removed fromthe circuit to test?


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## SteveAppleton77 (Jun 26, 2020)

Latole said:


> "Plate voltage in the preamps is almost nonexistent. 1.2 ish vdc."
> -SA
> 
> Issue is there ; Power supply don't feed preamp tubes; Find why
> ...


Agreed. I think my issue may be at the 100 ohm resistor. It may burnt and choking off the voltage to the preamp section. I think. 
Was hoping it hasn’t damaged the tubes or anything else. I have a set of el34 on their way. Even if I do t need them, I figure a back set is always good.


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## SteveAppleton77 (Jun 26, 2020)

Paul Running said:


> Okay so, that TFM is rated 350VAC @ 115VAC mains. Your mains could be 120+VAC which means that you should multiply by a factor of 120/115 = 1.04. So peak DCV will be 1.04(350VAC X 1.414) = 515VDC.
> Just to be safe, after you get the amp serviceable, you should check the bias current to the output tubes.


Ya agreed. Thought I had it set, but I suppose that’s all unsure now.


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## Paul Running (Apr 12, 2020)

No, to the removal of the resistor.


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## Paul Running (Apr 12, 2020)

SteveAppleton77 said:


> It may burnt and choking off the voltage to the preamp section.


It's also supplying screen-voltage to the output tubes...for me screen voltage is an important factor for pentode operation...that could explain the popping noise, if that resistor is intermittent...screen-voltage should be solid for maximum output power.
I'm not trying to steer you to any modifications however, if that was my amp, I would replace that 100Ω resistor with a quality choke.


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## SteveAppleton77 (Jun 26, 2020)

Ok got a chance. It’s reading infinity.
Is replacing it with a choke as simple as using a 100 ohm choke?


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## SteveAppleton77 (Jun 26, 2020)

My knowledge on chokes it limited.


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## Lincoln (Jun 2, 2008)

I'm watching this one with great interest. I've been collecting the parts for a Hiwatt Special 50 build. But I waited for Mojotone to get the transformers back in stock again, so I haven't even started the build yet. I just got the transformers within the last month finally. I bought the chassis and face plate in June. Nice looking build Steve.


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## SteveAppleton77 (Jun 26, 2020)

Thanks Lincoln!! Now if I can just correct whatever I did. 
I guess the question I should be asking is why did the resistor blow in the first place. 
could faulty power tubes do it?


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## Paul Running (Apr 12, 2020)

SteveAppleton77 said:


> Is replacing it with a choke as simple as using a 100 ohm choke?


No however, I would rate it not difficult even for a first time assembler. If you are lucky, the holes for the power TFM will line up for an under-side installation as indicated below:








Lots of real estate on that chassis so you have several options for location...I would try to isolate from sensitive components, especially any high gain areas.


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## Paul Running (Apr 12, 2020)

SteveAppleton77 said:


> My knowledge on chokes it limited.


It's just something to consider...there must be a reason why Mr. Reeves never used chokes in any of his amps.
Attached is a copy of a manual from an Electronics series...this was my first readings in electronics and I found it helpful. My dad had the complete series in print. This copy was taken from the web. Anyways, starting at 1-68, there is a description of the filter choke.


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## SteveAppleton77 (Jun 26, 2020)

Man Paul thanks for all the help so far!!!
Any thoughts on why a resistor like this would fail? 
I can get a new one but don’t wanna just keep repeating things


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## Alan Small (Dec 30, 2019)

SteveAppleton77 said:


> Man Paul thanks for all the help so far!!!
> Any thoughts on why a resistor like this would fail?
> I can get a new one but don’t wanna just keep repeating things


A fellow GC member had a Benson push pull 6V6 model that had a failed B+ supply resistor in a similar position: it was overloaded by a faulty power tube...I suggest that after you replace the resistor you power up without the old tubes installed: check b+ all around and install a known good set of new power tubes and check the bias.


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## SteveAppleton77 (Jun 26, 2020)

Wise advise. I have a new set of JJs on the way. 
thought I had this sorted out the first time. My original el34 were new but had done a short stint in a jtm45 converted for el34s. Perhaps something happened.


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## Alan Small (Dec 30, 2019)

New tubes, NOS or new production, are not always going to work or last as long as we would like...
...they get dropped at the warehouse or in the ups truck; they have a weak solder or crimped part; any number of other causes of failure....poor contact in the socket(new or old) can cause all sorts of ghost issues...

If only tubes were as inexpensive as filament light bulbs


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## Paul Running (Apr 12, 2020)

SteveAppleton77 said:


> Man Paul thanks for all the help so far!!!
> Any thoughts on why a resistor like this would fail?
> I can get a new one but don’t wanna just keep repeating things


You could perform some forensic work on it. Try to establish if it is truly a hard-failure...it could be mostly intermittent.
Try grasping the body of the resistor and gently apply torsional force back and forth on each lead to see if there is any loose-action...I would do this with an analog ohm meter and watch for any meter-pegging. I have had some leads on ceramic type resistors experience a semi loose connection at the body to lead union...it could be caused by over-stress. These resistors are relatively heavy for their size and if not supported properly which most are not, will eventually submit to failure...the way some amps are assembled would never be accepted by some standards, most notably Mil-spec.
The resistor is more than capable of handling the power rating and if it wasn't there would be visible thermal damage, if you have the hours on it which I believe you would...it's a nice amp, EL34, my favourite pentode.


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## 2N1305 (Nov 2, 2009)

Hi,

I've been loosely following this thread, and since the discovery of the open resistor (the 5W 100 ohm), I wanted to ring in on this. I would insert an ammeter in there to see just how much current is flowing through it once you put in a new one. I do not know how much current there typically should be, but according to the schematic Mr Running provided, there is 2V across it during idle operation. That means 20mA. If you decide to follow this suggestion (it would involve a few alligator clips), and see more than 20mA, there is likely still an issue and your new resistor will suffer the same fate.

I do not mean to rain on your potentially new found repair parade, but I can offer a few places I'd suspect:
1- anything (downstream, obviously) shunting that node to gound, such as capacitors (they can be defective or very leaky)
2- Wrong value resistors (a 220 in place of 220k can make words of difference)
3- bad tube
4- incorrect connection or component lead touching a spot it shouldn't

Maybe I wrote this down for nothing and you've since repaired the amplifier and have no issues with it. I hope for that.


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## Rski (Dec 28, 2013)

The standby switch should read almost zero across the terminals, when closed.

Possibly cold solder joint could be some where?

Measure the audio transformer when voltage discharges where they attach to the power tube pins to XE .. the reading will be something like 60 ohms .. give or take 20 ohms ... when probing and the speaker is plugged in, you should hear a soft sound.. this indicated the audio transformer is fine .. ohms law will determine your bias voltage flowing through your power tubes, once powered up and the meter probes are on XE on either leg Soldered on power tube ..ie: 60 ohms with 1.5 VDC (1.5 / 60 = 0.025) 25 MA .. example. 

To test the power amp .. turn Master volume half way .. touch with screw driver, a hum can easily be heard .. this means the V3 to speaker is working .. something isn’t workingV1 or V2


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## SteveAppleton77 (Jun 26, 2020)

Well parts finally arrived. 
actually the tube store and mouser actually ship fast. New power resistor and tubes and it’s alive again!
Next setting bias and going over the circuit and making sure all voltages are where they should be. Still unsure why it happened. 
2n1305 I’m feeling the same way. I can live with the idea it was a bad tube, but I think further testing investigation is in order. Something started the problem and it’ll likely just happen again if not corrected. I order three resistors as I suspected I may blow another while looking for the problem.


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## Alan Small (Dec 30, 2019)

SteveAppleton77 said:


> Well parts finally arrived.
> actually the tube store and mouser actually ship fast. New power resistor and tubes and it’s alive again!
> Next setting bias and going over the circuit and making sure all voltages are where they should be. Still unsure why it happened.
> 2n1305 I’m feeling the same way. I can live with the idea it was a bad tube, but I think further testing investigation is in order. Something started the problem and it’ll likely just happen again if not corrected. I order three resistors as I suspected I may blow another while looking for the problem.


Good job....you may discover your bias is within a proper range and all is good in the world except that one faulty tube(now relegated to the recycling bin) which is more common than one expects.


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## Paul Running (Apr 12, 2020)

SteveAppleton77 said:


> I think further testing investigation is in order


Clip the test leads of your voltmeter across the 100Ω resistor (between TPs XD and XE). According to your schematic the voltage drop should measure 2VDC during static conditions. The calculated current flow will be 2V/100Ω = 20mA.
Keep monitoring that voltage drop across the resistor and begin to moderately load the amp (plug your guitar in and strum away). If you use an analog meter you should see the voltage drop increase, due to screen-current increase. At the operating voltages of your amp, maximum screen-current for typical EL34 will be 2 X 20mA = 40mA.
Preamp current will not vary much so, the voltage drop could peak to 60mA X 100Ω = 6V, I would use an analog meter to measure this as it provides real-time viewing plus a better visual of fluctuations.
Worst case scenario, the power dissipated by the resistor will be 6V X 60mA = 360mW...the resistor is rated 5W and well with the safe area of operation.


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## SteveAppleton77 (Jun 26, 2020)

Well I’ve got the bias set. I’m measuring about 2-2.4 across the resistor. The voltage seems to vary a few .1 volts. So I’ve got about 20-24mA across the resistor at idle. 
I’ll give what you’ve suggested a try Paul. Probably tomorrow night. 
It’s back to sounding good again anyhow.


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