Picked this up today

I doubt anybody could get 38 watts out of a PAIR of EL84s for any length of time. All these oddball tubes were pretty common up to the late'60's--you wouldn't have believed the racks of tubes they had at the old electronic store here. They supplied the TV repair shops, so they had to stock everything.
 
dervish riff said:
I doubt anybody could get 38 watts out of a PAIR of EL84s for any length of time. All these oddball tubes were pretty common up to the late'60's--you wouldn't have believed the racks of tubes they had at the old electronic store here. They supplied the TV repair shops, so they had to stock everything.
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Agreed, 18/20W is probably the most we should try for but then 38W from a pair of valves with a Pa of 13W is asking for trouble IMO!

The general rule of thumb for fixed biased amps is 2x rated Pa. Yes, you CAN get 100watts from a pair of EL34s but I don't know of anyone that does! 50-60W is the norm. (but then I know of a "60W" amp that will easily top 80W if pushed!)

Dave.
 
dragonworks said:
that is 38 watts peak, it is 27 Watts RMS for this amp.
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Well "rms" watts is a mathematical absurdity (yes! I KNOW most of them still do it!) so the "average sine power" would actually be about 1/2 peak. Let's say 20watts for jazz?

But the actual power output of a valve amp is very difficult to specify. Watts into X R at Y distortion is the only fair(ish) way and even then the incoming mains voltage must be specified.

The power of a valve amp is rarely told in the name. The "GangBang 25" MIGHT be over-stressed 84's or well treated 34s or 6L6s! Then in another guise (after benchtests showed it could do it, just.) it might be EXACTLY the same amp now badged the "Bloodlust 35"!

Dave.
 
I am not into it that much and am not looking to be an amp guru. I just like fooling around with electronics and it takes me five days to find and fix something that would take someone else an hour.
 
dragonworks said:
I am not into it that much and am not looking to be an amp guru. I just like fooling around with electronics and it takes me five days to find and fix something that would take someone else an hour.
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Well! The NEXT time you have a problem, post it here on DAY ONE!(heh!) 'Swhat we's here for. Or you can PM me if you wish.

Dave.
 
I am always buying broken amps and trying to fix them. I have a Masco Concert master, it is just an amp, no preamp dual tube rectifier, 5V4Gs, two 6L6 in push pull, two 6SJ7s, one for a pre, one for a phase inverter. It originally came with a preamp but I don't have it. The first time I turned it on the filter caps just melted. I replaced them and one large 20 watt power resistor, then had to replace some coupling caps and some decoupling caps. Now I have a clean signal coming through, I can see it clean on my scope but I don't have much power. I put a joe meeks preamp in front of it. It seems all my voltages to plates, grids, and bias are correct, I have several different tubes and have swapped them all out in different combinations. When taking a voltage reading on one of the 6L6s now I get motorboating, if not, the amp is quiet. I am wondering if there could be a problem in the phase inverter and the signal is getting cacnceled out or only half the signal is being sent to the push pull? I have a dual trace scope. Can I hook it up to each side of the phase inverter and see if they are 180 out of phase? This amp is rated at twenty watts RMS. I am not that up on the scope operation. I have HP old school good AF signal generator. I have the sams photo fact for it but can't post it, I sent it to a site that has it but can't remember which site and can't find it.
 
Put the reverb tank it and it is working but noisy as hell. The connections going to it are pretty bad, I will replace them and the tubes for the reverb and see where we are at. It may be just the paper clip I soldered in there.
 
well the motorboating is from your meter loading the circuit.

it might be the joe meek might not have enough drive.
using a .1uf at 400V cap hooked in series with the positive output of the AF gen. set the signal to two volts peak to peak.ground the AF generator to the chassis. now use that to inject your signal. this is assuming you changed the cathode bypass cap in the first stage. now with the power off take an ohm meter test the input attenuator and turn it to full scale so it should read 500K on the wiper to ground. there should be ideally zero ohms from the "flat " input to the wiper. if that is ok. power on the unit and inject audio in the first stage. it should give you an output about 20Vp-p going to the phase splitter with 2vp-p going in.

the reason to put a cap in series with your af generator it will isolate it, thus preventing a ground loop that typically knocks out AF generators
 
drtechno said:
well the motorboating is from your meter loading the circuit.

it might be the joe meek might not have enough drive.
using a .1uf at 400V cap hooked in series with the positive output of the AF gen. set the signal to two volts peak to peak.ground the AF generator to the chassis. now use that to inject your signal. this is assuming you changed the cathode bypass cap in the first stage. now with the power off take an ohm meter test the input attenuator and turn it to full scale so it should read 500K on the wiper to ground. there should be ideally zero ohms from the "flat " input to the wiper. if that is ok. power on the unit and inject audio in the first stage. it should give you an output about 20Vp-p going to the phase splitter with 2vp-p going in.

the reason to put a cap in series with your af generator it will isolate it, thus preventing a ground loop that typically knocks out AF generators
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thanx for the suggestions.
will get around to doing this, this amp, the masco is on the back burner for now, have to try and get the magnatone up to snuff. Have ordered all new tubes for it an some more caps. May have to order a reverb tank, not sure if what I did to try and fix it would actually work or cause problems? One of my problems is that I am not familiar with the scope operation. I am not sure how to measure voltage or frequency with it yet. I can inject the signal and see if there is distortion and amplification, but not exactly sure how to set it, I am pretty new at this.
My scope and generator.
 

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dragonworks said:
thanx for the suggestions.
will get around to doing this, this amp, the masco is on the back burner for now, have to try and get the magnatone up to snuff. Have ordered all new tubes for it an some more caps. May have to order a reverb tank, not sure if what I did to try and fix it would actually work or cause problems? One of my problems is that I am not familiar with the scope operation. I am not sure how to measure voltage or frequency with it yet. I can inject the signal and see if there is distortion and amplification, but not exactly sure how to set it, I am pretty new at this.
My scope and generator.
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I can't see the model number of that scope. Does it give a screen readout of frequency and level? If not you need a millivoltmeter and an analogue display is far easier for audio work. Added to this is the fact that most digital AC meters can't be trusted for accuracy past about 2kHz and sometimes even lower!

Noted too is the absence of a meter on the sig genny? Vital this. You must keep the input level constant with frequency otherwise all measurements go to H in a a handcart.

Fortunately it is fairly easy and quite cheap to build very good AC meters. The scale can be an old VU meter (the bigger the better) or a standard 1mA fsd meter scaled 0-100 and 0-30. You don't need a vast number of ranges or great sensitivity for general work on amps and tape machines and there are plenty of circuits on the web based on the humble TL072.

Dave.
 
the scope is a techtronix 1741A, the only reference it has on it is 1v DC for calibration. I don't believe it gives out a screen readout of cps and level. I have a very good simpson analog VTVM. As I said, I am new to this and am not that familiar with the controls. The AF generator is a HP 200CD
 
I am bringing the scope manual into work to someone who will explain where and how to set what, and what I should be seeing.
 
I spent too much time explaining the oscope. the thing timed out.

look on the web for some stuff. but here is some operating practices I encourage:

1. never have the probe connected when changing coupling. GND is usually between and this will short out the circuit.

2. always make shure your on x1 or x10 and if you don't know what your anticipated waveform would be, put it on x10

3. use an Isolation transformer connected as the power source for the device your testing.

A division(DIV) is one block of the grid.

here is the manual for your scope:

cp.literature.agilent.com/litweb/pdf/01741-90911.pdf‎
 
bought and put in new tubes, replaced some more of the filter caps but still have a decent buzz, not bad enough where you can hear it while playing, but annoying. There is a ground switch but that seems to do nothing. Will have to start checking bias etc. When I first turned her on she buzzed you right out of the room, it was damn loud, so, I have made progress. Still have to get either the reverb tank working or buy another.
 
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