Ace Tone Mighty Five; schematic and question :)

[Bernardduur]

So hey all! Got a Ace Tone Mighty Five in, a great little amp from really way back (the owner noted that he saw it in a magazine around '59....... no idea if it is that old!

Searched a lot on the internet on this model but no avail....... it just isn't there or the info is just wrong.

It has 3x 12AX7's, 1x 6AV6, 1x ECC85 and 2x 50HB26's

I guessed about the ECC85; it only has the pins 4 and 5 as heaters and pin 9 is tied to the kathode of the second triode; I would guess that pin 9 is a shield and therefor I guessed an ECC85 but I have no proof....... with a ECC85 the unit functions just fine!



Note with the schematic; V1a and b (wrong in schematic) share the kathode resistor, the reverb is added thru the 470k resistor and mixed (channel 1 as 2)... it is then put through 2x a single spring (parallel) and fed to channel as (which acts also as a recovery stage). The dotted lines in the first preamp are empty spots on the PCB; the dotted lines on the PSU are from later added capacitors by someone who tried to fix the unit......



The owner asked if I could replace the powertubes with 6L6 tubes; I tried this by adding octal sockets, adding 6L6 tubes, moving the screen resistor of the powertube to the anode voltage (instead of 1/2 that voltage) and biased it perfectly....... The amp sounds just wonderfull and exactly like with the original tubes.... but is there anything else I should consider? The amp is not that loud but it wasn't that also with the original tubes......

I know that my new setup is too much current for the PSU but some older amp-repair-guru told me that these Ace Tones had more current under the hood then noted........ The PT is not getting warm at all and the voltage stays fixed and perfectly with as without the powertubes.......

My main concern is that the OT is made to be used with the 50HB26's......... is there any problems by 'just' using 6L6 tubes????

[Jazz P Bass]

http://www.mif.pg.gda.pl/homepages/f.../5/50H-B26.pdf
http://www.mif.pg.gda.pl/homepages/f...93/6/6L6GC.pdf

[Bernardduur]

Great, thanks! That was what I need......

I used a couple of 5881's as they are smaller and fit in the amp

The PtP load resistance is 3.8k for the 5881 and 3.2k for the 50HB26's so the prim impedance of the OT is kept around the same ratio. Only downsides are the heaters that are now over stocked but they keep up with the abuse.

[Rychehed]

Great, thanks! That was what I need......

I used a couple of 5881's as they are smaller and fit in the amp

The PtP load resistance is 3.8k for the 5881 and 3.2k for the 50HB26's so the prim impedance of the OT is kept around the same ratio. Only downsides are the heaters that are now over stocked but they keep up with the abuse.

Hi Bernardduur, i have this same amp but i need some tech information to get it up and running again - if its possible could you help me me email is teletubbies100@hotmail.com

I have got the amp but i need to know of good replacement output tubes, i need to know what the voltage tap is all about and also what kind of wattage converter i need when i plug into my UK wall outlet...

Cheers,

Gary

[charles63]

Ace Tone Mighty-5 repair and schematic

I just repaired a Ace Tone Mighty-5 amplifier for a friend of mine and I think you might be interested. If you have one at hand, you may have to do the same repairs that I did. And it's a lot.

First of all, I found a schematic on the internet which is correct, but a bit low in resolution, so I marked it up with pen with the actual resistor and capacitor values etc. I also noted some of the voltages that I measured. I will attached the annotated schematic. It's a bit messy, but it still might help someone.

A word of warning. The filaments of the power tubes 50H-B26 are fed directly from mains! Use a safety transformer! Even in the 220V model, they are fed with an autotransformer that does not separate them from mains. Additionally, the tubes run at voltages up to 285V DC. If you are not comfortable with that, keep the case closed!

I had to replace virtually all carbon resistors below 20k, because their values were way too high. Like double the value. This is normal for old carbon resistors. The wire wound resistors are ok, no need to replace them. These carbon resistors are usually found near the kathode of the tubes. If their value is too high, the amplification will be too low.

The same is true for a lot of the oil capacitors (it says "oil" on the capacitor itself). They become leaky, which means they don't resist DC as well as they should. For example, the oil capacitor 0.05uF connected to the plate of V1A (left top of the schematic) was leaky. This causes a DC voltage of around 5V on the right side of the capacitor. This DC voltage causes a DC current through the volume potmeter of channel one. This has a number of unwanted effects. First, you can hear a lot of noise when you turn the volume knob. Second, the potmeter deteriorates because of the power it has to dissipate. So I had to replace the oil caps (also the one on the plate of V1B).

The potmeters. I first cleaned all the potmeters by injecting isopropylalcohol (IPA) through the hole behind the solder joints and then vigourously turning the knobs. But that wasn't enough for the volume pots. So I disassembled them and cleaned them again. But in the end I had to replace them completely. The new pots required a larger hole, so I had to drill out the holes. A lot of effort but in the end it helped.

The big aluminium can electrolytic capacitors were all OK. Nevertheless, I put extra capacitors in parallel because I found the amplifier makes a lot of hum. I doubled the 100uF caps. Take care of + and -! There was one small electrolytic capacitor 10uF/12V near V2B that I had to replace because it did nothing at all.

I bypassed the high pass filter on the plate of V2B by a 10nF/400V ceramic capacitor, because I thought channel 1 needed some extra bass. I'm sill not very satisfied with the sound of neither channel 1 nor channel 2, I have to put the bass pot completely open and the treble knob almost to the left to get a decent sound. And ch 1 and ch 2 behave differently. But then again, you can see from the schematic that the circuits are different.

All the components that I replaced have been marked with "vervangen" or "verv" or "vv" in the schematic.

I replaced the power cord by a power cord with protective ground. That made a total of 20 replaced components. A new personal record!