Coincidentally, I have just started testing a new amplifier.
This is balanced like all my stuff.
It uses a 13D3 input diff pair. This is unusual in that I am flying in the face of theory, because whilst it has a conventional cascode SS current sink at the bottom, it has active loads on the anodes. Most people say that this will not work as the sources and sink will fight each other.
Well it certainly works at DC; it is passing more or less the design current.
The next stage, is a pair of D3As, as pentode cathode followers, which will be DC coupled to the output valves, a quartet of 6CK4s.
I am having a bit of trouble testing because obvioully I have a -ve supply and am having difficulty with getting the voltages right in testing without the output stage. Really I need two adjustable power supplies; and I have only one variac!
More later.
7N7
This is balanced like all my stuff.
It uses a 13D3 input diff pair. This is unusual in that I am flying in the face of theory, because whilst it has a conventional cascode SS current sink at the bottom, it has active loads on the anodes. Most people say that this will not work as the sources and sink will fight each other.
Well it certainly works at DC; it is passing more or less the design current.
The next stage, is a pair of D3As, as pentode cathode followers, which will be DC coupled to the output valves, a quartet of 6CK4s.
I am having a bit of trouble testing because obvioully I have a -ve supply and am having difficulty with getting the voltages right in testing without the output stage. Really I need two adjustable power supplies; and I have only one variac!
More later.
7N7
dhaen said:
I have no website, so it would have to be a file.
Shall we wait until the bloody thing works?
When I have finally got the g2 cathode follower voltage correct, I can thne have a look for oscillation in the driver section (there usually is). When I have cured that, I can try the output stage and do some AC testing. When all is in order, I'll draw it up and happily send the circuit diagram. I have to say that as usual, I have had help with this designm so I will have to ask for permission.The amplifier is intended to be a temporary replacement for my other amplifiers while I modify them. My aim was to use only parts that I had in stock, and so far, so good!
7N7
dhaen said:7N7,
Ok we'll wait if we must.
Anyway, I'm glad you are "balanced". I'm quite "balanced" myself.
Cheers,
I am glad to see that "balanced" appeared in inverted commas. How can anyone who turns a reasonable flat into a complete tip, because of a hobby, can be described as "balanced" beggars belief.
The reason for the balance means I can a) use more valves - very important that b) spend more money (a disease you know) C) have the power amplifiers closer to the 'speakers and d) well there was a plan for a neat layout with all the sources up at one end and a nice collection of stuff at the other. This means about 17 feet of cable which means balanced and cathode followers - more valves.
I will say that I try to avoid transformers and capacitors; and anyway, glass is better looking than iron.
I hope that the amplifier will be operational before very long so you should not have long to wait (providing I can obtain the necessary permission.
7N7
13D3/D3A/6CK4 amplifier
Hello all.
Well I have sorted out the DC in this thing all is in ordnung.
However I have some AC problems that I am about to start addressing.
I have been given permission by the "guru" to post the circuit diagram, so once it's working, I'll draw it up and you can have a look
7N7
Hello all.
Well I have sorted out the DC in this thing all is in ordnung.
However I have some AC problems that I am about to start addressing.
I have been given permission by the "guru" to post the circuit diagram, so once it's working, I'll draw it up and you can have a look
7N7
Still not there
Update.
I have at least located the problem; curing it is proving elusive.
The problem relates to the g2 supply to the pentode cathode followers. At -12dB input all is well. At -10dB input the whole thing goes beserk; instead of +96V at g2, suddenly I get +200V, which rather changes things.....
More investigation required
7N7
Update.
I have at least located the problem; curing it is proving elusive.
The problem relates to the g2 supply to the pentode cathode followers. At -12dB input all is well. At -10dB input the whole thing goes beserk; instead of +96V at g2, suddenly I get +200V, which rather changes things.....
More investigation required
7N7
dhaen said:Hi,
Then listen at low volume..
Come on, wheres the schematic then?
Cheers,
As I said, there will be a circuit diagram when I have the thing working.
Only then can I draw it up, since component values and voltages are bound to change.
And you wouldn't want a circuit diagram without voltages would you?
7N7
Update 2
This amplifier has been something of a struggle.
I have not been able to solve the problem with the pentodes, so for now they are triode-strapped and running at 13mA each.
Initially results were rather poor: the cathode feedback prevented the amplifier from producing more than about 1W, so I removed it. This got me up to about 7W, unfortunately -5.5dB at 10kHz: OK for a very early cassette player or an AM radio I suppose.
The fault lay with the output transformer. I have one other I could try: now this one is a rather crummy old thing, but I can now get 12W (target) with only 295V HT.
Initial tests suggest that distortion is quite low at this level (0.5%) but this wants checking again - if only I could depend on the mains!
Next move is to remove/change all the grid stoppers etc., I included when I detected oscillation, mount the transformer and get those pentodes working.
7N7
This amplifier has been something of a struggle.
I have not been able to solve the problem with the pentodes, so for now they are triode-strapped and running at 13mA each.
Initially results were rather poor: the cathode feedback prevented the amplifier from producing more than about 1W, so I removed it. This got me up to about 7W, unfortunately -5.5dB at 10kHz: OK for a very early cassette player or an AM radio I suppose.
The fault lay with the output transformer. I have one other I could try: now this one is a rather crummy old thing, but I can now get 12W (target) with only 295V HT.
Initial tests suggest that distortion is quite low at this level (0.5%) but this wants checking again - if only I could depend on the mains!
Next move is to remove/change all the grid stoppers etc., I included when I detected oscillation, mount the transformer and get those pentodes working.
7N7
13D3/D3A/6CK4 ANother Update
I finally found out what was wrong with the pentode cathode followers; that person deserves to be sacked; so I sacked myself for a stupid mistake.
I eventually got 12W out of it, but THD was disappointing - that sacked person also made a mistake in calibration. THD at 12W is 3% but mostly 2nd & 3rd harmonic. The higher harmoincs were very low.
So I put it all back together and now the mains transformer has thrown in the towel.
A friend is bring another transformer which will necessitate re-jigging..
And there will have to be an outboard heater transformer.
I may try an alternative output stage: a single 6528 double triode in push-pull.
However, several things have been established: you can make a diff pair with active loads and a CCS underneath - and it works beautifully as designed. The D3A makes a superb cathode follower; in fact the driver stage is a success.
I have been let down by the output transformer and now by the mains transformer
Oh well no-one ever said it was easy - onwards and upwards
7N7
I finally found out what was wrong with the pentode cathode followers; that person deserves to be sacked; so I sacked myself for a stupid mistake.
I eventually got 12W out of it, but THD was disappointing - that sacked person also made a mistake in calibration. THD at 12W is 3% but mostly 2nd & 3rd harmonic. The higher harmoincs were very low.
So I put it all back together and now the mains transformer has thrown in the towel.
A friend is bring another transformer which will necessitate re-jigging..
And there will have to be an outboard heater transformer.
I may try an alternative output stage: a single 6528 double triode in push-pull.
However, several things have been established: you can make a diff pair with active loads and a CCS underneath - and it works beautifully as designed. The D3A makes a superb cathode follower; in fact the driver stage is a success.
I have been let down by the output transformer and now by the mains transformer
Oh well no-one ever said it was easy - onwards and upwards
7N7
Some decent progress to report.
The new transformer arrived yesterday, and very fine it is. It has 2 x 126v outputs @250mA each plus a 208v output at 160mA and a 204v output at 50mA.
So I built two bridges and by combining the 208v and one of the 126s, together with a 4uF cap folowed by the 20H choke and a 220uF capacitor, I can now have 320V at 200mA.
The pentode cathode followers are now working as intended.
I have replaced the 4 6CK4s with a 6528A. This is a big regulator double triode. It is rated at 30W per section, has graphite anodes. Gm is 35 and mu 9. It requires 6.3v @ 5A to light it up.
I am running it p-p into 4k a-a. Bias is circa -37V and with +5dB input I am getting 16.8W into 4.7 ohms just before clipping.
The cheapo antique output transformer offers a choice of primaries: 4k, 6k, 8k.
Tomorrow, I shall do some distortion testing and try alternative loads.
Once all is finalised, I'll measure the voltages, draw it and post the circuit diagram.
7N7
The new transformer arrived yesterday, and very fine it is. It has 2 x 126v outputs @250mA each plus a 208v output at 160mA and a 204v output at 50mA.
So I built two bridges and by combining the 208v and one of the 126s, together with a 4uF cap folowed by the 20H choke and a 220uF capacitor, I can now have 320V at 200mA.
The pentode cathode followers are now working as intended.
I have replaced the 4 6CK4s with a 6528A. This is a big regulator double triode. It is rated at 30W per section, has graphite anodes. Gm is 35 and mu 9. It requires 6.3v @ 5A to light it up.
I am running it p-p into 4k a-a. Bias is circa -37V and with +5dB input I am getting 16.8W into 4.7 ohms just before clipping.
The cheapo antique output transformer offers a choice of primaries: 4k, 6k, 8k.
Tomorrow, I shall do some distortion testing and try alternative loads.
Once all is finalised, I'll measure the voltages, draw it and post the circuit diagram.
7N7
Re: 6528A
fdegrove said:Hi,
Well done.
That is another valve that is hardly used these days.
Along the lines of the 6336A but with higher mu if I remember correctly.
Happy building,
Thanks.
6528A is similar in appearance to 6336A and indeed has the sme Pa rating.
As you say, mu is higher (9 instead of 2) and gm is much higher at 35mA/V max. This means an ra of about 245 ohms.
The price as usual for all this is Ca-g, which is claimed to be 23.8pF (but is probably more). This means that a cathode follower is de rigeur to drive it.
7N7