Music Angel Amplifiers

[Edriz]

well, I have 2 more of the bias adjustment sockets on the way, once they get here Ill have 4 and will be much easier to try to figure out what pot does which tubes. I tried adjusting bias but couldnt seem to get tubes 1&2 and 3&4 to balance. I might be using the wrong pot.

 

[OXAUDIO]

I'm not sure what you mean by 'Bias adjustment Sockets' Be that as it may, as others have suggested, two of the preset pots may be 'hum bucking' controls in the heater circuits-the idea is to adjust these for minimum hum. It should be fairly easy to determine whether such adjusters exist, since they will be connected in parallel to the heater winding on the mains transformer[and with the valve(tube)heaters themselves.] The remaining pots. will be for setting the O/P tubes' bias. There will almost certainly be some interaction between them, but, by connecting a DC Voltmeter across the cathode bias resistor of each O/P valve in turn it should be possible to identify which is which, and set the bias accurately.

 

[Edriz]

the sockest I speak of plug into the tube socket, tube plugs into the adaper socket, the adapter socket has a wire that connects to a switch box capable of handling 4 adapter sockest and the switch box hooks to a DMM. Makes it much easier to adjust bias withought haveing to poke around with probes. If this link works, it's the best way to show you what I mean by "adapter sockets"

Quad Tube Amp Bias tester 6L6 El34 KT88 6V6 6BQ5 7591 | eBay

 

[OXAUDIO]

Edriz, I hadn't thought about that. Seems a very good idea. I would guess it simply consists of four sockets wired thru a selector switch, plus the necessary probe leads. On the rare occasions when I've needed to check bias settings, I've simply connected a voltmeter across each cathode resistor in turn, but I seldom service/repair amps of this type, so have no real need for such a device.

 

[Edriz]

Yes, thats what it is. I wasnt suggesting you get one. I have one. The only way to bias the Music Angel is through the bottom. I got this so I can hook all 4 tubes thru this and to be able to adjust bias without messing with probes. I have very shaky hands. Makes it easier to adjust all 4 and having to do nothing more then switch the switch box from one tube to the next

 

[OXAUDIO]

Several years on, I was recently asked to repair a Marshall Guitar Amplifier, and obtained one of those Bias Adjustment Sockets from a firm in Kidlington, Oxfordshire, England, although the one I bought is only wired for EL84s. Makes it so easy to connect a milli-ammeter in series with the anode (plate) circuit to check, and if necessary, adjust the current being drawn. It would not be difficult to build version(s) for other output valves(tubes) and/or other valve (tube) sockets, though these are no doubt available commercially.

Bias Probes are available from several eBay sellers. Various versions, from simple one valve (Tube) types to quite sophisticated models incorporating built-in meters. As a correction to my earlier post, the probe I purchased (which was from an e-bay seller) is for a single KT66 (or equivalent)valve, not an EL84 as I erroneously stated in the above post. The amp. in question was a JTM45, but the probe can obviously be used with any amp. employing KT66 tubes, oncluding some models of Music Angel amplifier.