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The classis Push pull vs Single ended question
The classis Push pull vs Single ended question
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Old 25th March 2020, 07:16 PM   #11
MikyK is offline MikyK  Czech Republic
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Quote:
Originally Posted by audiowize View Post
So if you have some old radio OPT that's stacked and dipped in wax, you can pull it out of the channel frame and pull the lams out, clean all the wax off, then restack the transformer and eliminate the air gap. This is one of those things that's either incredibly easy or totally impossible depending on the transformer.

Once you have eliminated the airgap, the bass response will likely be just fine, then you can source an appropriate plate choke and off you go!
Hmm, parafeed output is a very interesting idea, but I have no idea how it will sound
I remember that Ive got pretty good bass with the same OPT, but this was with indirectly heated pentode 4654 strapped in triode...
Im a bit dissapointed.
The OPT is made for EL84 in pentode (5k, 25mA anode current, cca 250V anode voltage).
With 4654 (40-50mA and anode voltage of 270-280V) I have got pretty decent bass. It is not only about the bass whole response but also about the bass energy.
And now with 45 (lower than 250V b+ and 30-35mA current) Im not able to make good bass. I have 1k5 cathode resistor and 10uF paper in oil as bypass which should be absolutely enough (I increased it to 30uf but only very small increase in bass, almost nothing).
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Old 25th March 2020, 07:25 PM   #12
mondogenerator is offline mondogenerator  England
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I am less well versed than many here, and have only built a handful of valve circuits, but I have noticed or learned some things.

I use the bypass capacitor value to set LF corner at the input stage, each stage after the bypass is for a higher frequency - I read somewhere about this practice, but my recall is awful.

In the output stage I use something like 1k8 to 2k2 cathode resistance, and bypass by 100uF. This is probably excessive as I was aiming for 1-2 Hz. So I could probably halve that value, say 47uF, and notice little effect.

But I have been in a similar situation, and that the circuit just liked working at X voltage and not at Y voltage.

Unscientific as that is...

Some just seem to have sweet spots...like the 6P30B-R liked the B+ real high, and the Vg2 about 40V lower. The trouble then is keeping current reasonable!

In triode, I would need to be at a similar plate voltage as in pentode (300V), or higher still. 350V is the VP at cutoff maximum value for the valve. So I still havent tried it!

My point being, with higher B+, the valve may 'like' to see the Primary Z its loaded with, but at lower B+ it really wants a lower primary Z. Instead of voltage headroom being a limit, it becomes current swing/headroom.

Also, changing circuit parameters like Vg2, if in pentode mode; or switching from pentode to triode, UL or variation of loading and it's going to change how suitable your transformer is as a load.

I think that's correct (as terrible as it reads) and perhaps it will help
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Last edited by mondogenerator; 25th March 2020 at 07:43 PM.
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Old 25th March 2020, 07:30 PM   #13
audiowize is offline audiowize  United States
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Quote:
Originally Posted by mondogenerator View Post
And of course the plate choke...unless you use a CCS...which I guess, needn't be solid state; if vacuum only is your goal
The solid state and glass state options drive the B+ voltage way up. Plate chokes are a reasonable choice.
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Old 25th March 2020, 07:32 PM   #14
audiowize is offline audiowize  United States
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Quote:
Originally Posted by MikyK View Post
(I increased it to 30uf but only very small increase in bass, almost nothing).
You're definitely not helping things. Try increasing to 100uF.

Still, radio output transformers are made to power radios. Lil Jon and the Eastside Boyz weren't around in the 60s when these transformers were made. They needed to be just barely good enough for what was available at the time (in terms of source material, broadcast quality, and speaker quality), so 100Hz-8kHz was completely reasonable. After all, you didn't want to bottom out the speaker in the radio, so throwing away most of the bass was a very sensible decision.

Last edited by audiowize; 25th March 2020 at 07:35 PM.
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Old 25th March 2020, 07:49 PM   #15
mondogenerator is offline mondogenerator  England
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It sounds to me as most likely that in using a OPT for pentode, 5k may not be a good load for EL84 in triode, though its often quoted for pentode. (I think)

It probably comes down to anode resistance, Ra (Rp).

Why not try with double the load R? Or Half? See what happens.

I'm assuming you have more than 1 secondary, which you may not.
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Last edited by mondogenerator; 25th March 2020 at 07:57 PM. Reason: El84
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Old 25th March 2020, 07:55 PM   #16
k.zevenhoven is offline k.zevenhoven  Netherlands
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Default Or use the push-pull output transformers you already have.

Hi,

You wrote that you have used your Single-Ended output transformers with different triodes and that you already have a pair of push-pull output transformers with a primary of 3.5 - 4k.
Now I don't know how many and wich triodes you already have, but maybe it is possible to to make a push-pull amp with the triodes and the push-pull transformers you already have?

Things to look for this, are:
1) what are the secundairy connections on your 3.5 - 4k push-pull transformers you already have? Do these for instance have 4, 8 and 16 Ohm connections? Because that would give more opportunities.
2) what is the impedance of your loudspeakers?

Because in case your output transformers do have 4 Ohm connections and in case your loudspeakers have an impedance of 8 Ohm? In that case, you could use the 4 Ohm secundary connection of your push-pull output transformers and then the output-tubes will see an impedance of about 7 - 8k at the primary side.
In that case you could make a push-pull amp with tubes that need a push-pull transformer with a primary impedance of 7 - 8k.
That could be for instance the 45 triode or EL84 in Pentode. But if you want EL84 connected in triode you could even try these in push-pull class AB. And of course you can try several other tubes too.

Best regards,
Tubecase.
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Old 26th March 2020, 07:27 AM   #17
AmadeusMozart is offline AmadeusMozart  New Zealand
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SE vs PP articles attached.

The way I see it is the question of do you want finesse or do you want to go loud?

I've had them all and keep coming back to a small EL84 amplifier, building one at this moment, waiting for parts to arrive while in lockdown. With Russian tubes or those 6BQ5s' that have 14 Watt plate dissipation I'll get just 4 watt out of a SE using a 20% distributing load tap (aka Ultra Linear) - getting the most of the power of a pentode while retaining most of the characteristics of a triode. For me the 20% represented the best compromise between power and ultimate fidelity.
Attached Files
File Type: pdf SE-v-PP-Part1.pdf (143.7 KB, 33 views)
File Type: pdf SE-v-PP-Part2.pdf (140.6 KB, 31 views)

Last edited by AmadeusMozart; 26th March 2020 at 07:31 AM.
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Old 26th March 2020, 07:33 AM   #18
MikyK is offline MikyK  Czech Republic
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Quote:
Originally Posted by audiowize View Post
You're definitely not helping things. Try increasing to 100uF.

Still, radio output transformers are made to power radios. Lil Jon and the Eastside Boyz weren't around in the 60s when these transformers were made. They needed to be just barely good enough for what was available at the time (in terms of source material, broadcast quality, and speaker quality), so 100Hz-8kHz was completely reasonable. After all, you didn't want to bottom out the speaker in the radio, so throwing away most of the bass was a very sensible decision.

But these OPTs were quite good. The 60Hz rolloff can be truth, but I think it can be as low as 45Hz. Absolutely okay for me. And the high frequency is also good, it plays all the way to 20kHz. I dont see a problem here (althoughit can be better).
I see bigger problem that the OPT can not stand anymore than 40mA, which limits me to use only one 4P1L per channel, I cant go paralell in SE.
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Old 26th March 2020, 07:48 AM   #19
OldHector is offline OldHector
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Isn’t this one of those discussions where you need Star Trek’s Scotty to be saying (scotch accent) ‘You can not break the laws of physics, Captain!’?
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Old 26th March 2020, 10:56 AM   #20
RajkoM is offline RajkoM  Bosnia and Herzegovina
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Hi Michal.
Two questions for you:

- Do you have any experience with DIY handwinding transformers?
- do you have good nerves?

I was DIYed myself OPTs for my "second" SET with 6C33C.

C cores, G10 (PCB material) bobin, insulation is a modern paper/mylar and teflon from russian teflon capacitors and Sowter principle of winding with 4 secondarys.

Very old and bad pic, sorry.
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File Type: jpg OPTs.jpg (39.0 KB, 89 views)
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