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Kofi Annan in: "Tube Amp for Multi-Way Speakers"
Kofi Annan in: "Tube Amp for Multi-Way Speakers"
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Old 11th August 2019, 07:49 PM   #21
rongon is offline rongon  United States
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Kofi Annan in: "Tube Amp for Multi-Way Speakers"
The EICO HF-87 was basically a stereo version of the Mullard 5-20 but with a 6SN7 for the LTP and a half a 12AX7 as the input/voltage amp.

If the 18k 1W tail resistor (from joined cathodes of 6SN7GTB to ground) is changed to a constant current sink (CCS), the plate resistors of the 6SN7 can both be 33k ohms (instead of one being 33k and the other 28.75k).

I'm sure there are lots of other tweaks possible too.
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Old 11th August 2019, 08:38 PM   #22
kevinkr is offline kevinkr  United States
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Kofi Annan in: "Tube Amp for Multi-Way Speakers"
I still think 45's suggestion of paralleled El84 has merit, very easy to drive and at least 30W in PPP pentode.

I don't understand the negative comments about current hogging, individual auto-bias is easily arranged and will prevent serious problems, yeah they won't match perfectly but I doubt that is a major concern in this application. I have built some PPP amps in the past for clients and they're all still running today +20 years later. A little bit of thought and the problem is easily solved.

You can drive a quartet of them with split load (cathodyne) comprising a 12AT7 shared between channels and a single 12AX7A also shared for the first stage voltage gain.

Full wave bridge or doubler, single choke, transformer with a single HV winding and couple of filament windings and you are good to go.

Simple, inexpensive and with a little thought will work well. 10 inexpensive tubes for 30Wpc stereo, not bad.

Or horrors do the same thing with a pair of 6L6GC per channel for 6...
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Old 12th August 2019, 10:31 AM   #23
45 is offline 45  Italy
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Quote:
Originally Posted by Eli Duttman View Post
Because of current hogging, forget PPP EL84/6BQ5s in a self biased amp.
That's really trivial to sort out: back bias with individual pot in the worst case. So no need for additional secondaries.

Anyway I didn't have any problem and used one pot for each side of the PP. Having 8 tubes is easier to match them.

I used the Silvanya 10GK6 which is 10v version of 6GK6, an up-rated EL84 (different pinout which allows for 330V plate and G2 voltage). This is still available in good numbers at a fraction of the price (I paid $3 each). Better quality than current production JJ et similar....

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Originally Posted by Kofi Annan View Post
Any designs you would recommend?
The amp I built with the 10GK6 was my personal take on existing amp designed and published on Italian review in the late 90ies. I have that number and could scan the circuit. The output stage is PPP of EL84's running in (almost) class A for 20W power. Back bias from the main supply and Vg2 regulated by means of mosfet + jfet as error amp.

I added the cathode feedback and run in class AB.

Here the bias can be tweaked in 2-3 way depending on what one wants. Can be as the original (i.s. only passive RC filtering) which will cause some bias shift like in cathode -biased circuit (not a bad choice a priori if one gets a soft clipping) or regulated. You might even start with a rough regulation using zeners and then improve that once you get it going. Regulation does not need to be complicated. Have a look to the Red Light District amplifier in this forum for example.

From the design point of view I only changed the front-end. I used a board that I already had in my box consisting of a Williamson style front-end with 6CG7's.

In general what do you need to change? Once you have the cathode feedback at 10% it just means that 10% of the output voltage at the primary of the output transformer comes from the cathodes of the EL84's and so, because there is no gain there (actually a bit less than 1), it's the driver that has to deliver it.

30W into 4K means that each side of the PP is outputting 245V peak in total. 10% of that is 24.5V. Make 25V peak. Add to this the normal swing for the plate output 12-13V peak and that makes no more than 40V peak. Basically any frontend that can drive EL34's properly will do.

P.S.
What the difference respect to using more standard ground fbk with EL34's?

The difference is that, if the transformer is good, you don't need to add other fbk and amp is easier to make stable but more importantly the reduction in distortion is more effective for the same amount of fbk.

For the PPP with EL84's into 4K for 30W and 10% cathode fbk the actual fbk in dB's is just under 10 db but the reduction in distortion is about 19dB!
You don't get that with loop feedback.

The reason for this is that you first make the tubes linear and then combine them. They really behave as very linear triodes with the efficiency of pentodes.
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Old 12th August 2019, 04:04 PM   #24
Kofi Annan is offline Kofi Annan  United States
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Kevin / 45,

Thanks for responding!

45-- I'll sketch something up with your recommendations in mind. I've never implemented cathode feedback before. Not sure that I can get a transformer with cathode feedback windings, so wouldn't I have to tie the feedback loop to an unused tap on the transformer secondary? Is that how you implemented yours?

Kofi
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Old 12th August 2019, 05:41 PM   #25
45 is offline 45  Italy
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I put a link to a good and reasonably priced transformer in my first post. It's well worth the price.

You can use the secondary for CFB but usually the amount of fb is less and the secondary has to be such that you can get a symmetric configuration.

I normally design and get done the transformers (especially in cases like this) but also have commercial products.

Regarding the tubes...you can find those 6GK6, 10GK6 and 16GK6 at Vacuumtubesinc at moderate price. They have got plenty so you can buy 2-3 times the amount for the same price of one set of properly selected EL84's and give your friend 1 or 2 sets for replacements. They will outlast him I guess...

The 6GK6 also has 13.2W max plate dissipation rating vs 12W of the normal EL84. It's a bit like the Russian military 6P14P-ER but different pinout.
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Old 12th August 2019, 07:21 PM   #26
Lingwendil is offline Lingwendil  United States
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Maybe try the EL86/6CW5 if going for EL84 types? 20 watts is easy, and they need less impedance than the EL84 does (3-5K a-a is good) making a nice transformer easy to design/source. They sound great, same pinout, and look the same as EL84. They are cheap too.


Also, we will be expecting airplane or at least helicopter rides for the advice, 'tis only fair
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Old 14th August 2019, 12:11 AM   #27
Kofi Annan is offline Kofi Annan  United States
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Hey! Sorry I vanished for minute. My kid actually spoke to me for about ten seconds and I couldn't get over it...

I have attached an LT Spice sketch of the EL84PPP using a 12AX7A into a 12AT7 cathodyne as Kevin suggested. Open loop it models about 50W peak output. Not bad.

In LT Spice it's running into a 5K primary. Once I fugure out how to lower the model to K and allow for cathode feedback we should get close to reality (note: LT Spice really does not like modeling NFB. Not. One. Bit. Acts like it's calculating PI to the last digit while developing cold fusion).

Any values look incorrect here?

I'll respond regarding the other posts in a bit-- just wanted to get this one out there for review.

Kofi
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Old 14th August 2019, 12:55 PM   #28
Lingwendil is offline Lingwendil  United States
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That looks lovely. My first impression is that C5, 6, and 7 are way bigger than needed. The biasing scheme for the concertina is interesting, any reason for this over self bias?
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Old 14th August 2019, 11:53 PM   #29
Kofi Annan is offline Kofi Annan  United States
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Quote:
Originally Posted by Lingwendil View Post
That looks lovely. My first impression is that C5, 6, and 7 are way bigger than needed. The biasing scheme for the concertina is interesting, any reason for this over self bias?
Yep. I used the fixed bias because I was initially unable to get LT Spice to deliver a uniform phase split from the self-biased cathodyne on account of I blew it and forgot about the bootstrapping resistor.

I reconfigured for self bias and reduced the coupling caps to .33uF on all stages (see below). Will that suffice?

More to follow...

Kofi...

Oh, and would a CCS in the voltage gain stage help here? It would be cool to have CCS on the power tubes, but I really don't wanna make EIGHT little CCSSEESSESS.

Kofi
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Old 16th August 2019, 12:08 AM   #30
kevinkr is offline kevinkr  United States
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Kofi Annan in: "Tube Amp for Multi-Way Speakers"
Definitely moving in the right direction, I would reduce those 0.33uF coupling caps to about 0.1uF to the grids of the EL84.

I don't think C1 - C4 need to be larger than 47uF - 100uF each.

I believe there is sufficient gain that when you close the loop you will not need more than what you already have - therefore the CCS is really not necessary.

Simplicity and components unlikely to fail are what you want here.

This is a lot like the original Eico HF30 design (had a pair long ago) and should be very reliable.
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