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Oddwatt Push Pull Tube Amplifier Projects
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Author:  Emsworth [ 19 Apr 2018, 11:42 ]
Post subject:  Re: Oddwatt Push Pull Tube Amplifier Projects

Bruce,

Thanks very much for the prompt response.

Quote:
The one you used is not as robust and should use about 200-215 v.

I don't need the maximum possible gain and I'm in favour of long, trouble free operation. Would you expect 200V rather than 215 to make an worthwhile difference? (In which case I need a higher value resistance.)

Quote:
Putting the 47K resistor between the last filter and the capacitor that is right at the tube anode would be my choice.

Just to clarify, would that be between the 'output voltage' pin of the LR8N and the PIO capacitor?

Quote:
If there is some sort of noise then probably it is related to a circuit grounding or layout.

Well, you won't be the least surprised to hear that moving the input ground wire about 1" further away from the power supply resulted in silence - no unwanted noise at full gain!

Thank you very much for this design and all the help you and other forum members give to us newcomers. I just have to do the power supply for the left hand channel and there should be stereo! Can't wait.

Author:  gofar99 [ 19 Apr 2018, 16:04 ]
Post subject:  Re: Oddwatt Push Pull Tube Amplifier Projects

Hi, The difference between 200 and 225 will have no effect on tube life. In this design I estimate tube life to be more than 10,000 hours. Many of us will never live long enough to need to replace the driver tubes. BTW the power tubes should last at least 2000 hours. I have some with triple that now and they still test excellent. For home use audio gear I see no reason to design something that needs frequent tube replacement. I actually consider it poor design. The difference in anode voltage has nearly no effect on the stage gain. The early amps used the lower voltage. I only went to the higher ones when it is regulated. It actually doesn't seem to make any difference. The regulators add some extra filtering and amps that use them ought to be a little quieter. I can only measure about 1-2 db though. They are very quiet without them (over -85 db usually). In theory it would allow for more head room in that stage. Probably a moot point as the max the output stage needs is equal to +/- the cathode voltage (varies from 25-38 depending on the tube type and brand). With a 200 volt anode supply the stage can deliver at least +/- 65 and typically +/-80. With a 300 volt anode voltage this increase to +/- 100 to 120. Overkill in either case. My test bench measurements on the two voltages do not indicate anything significant in response or distortion.

The resistor would go between the tube anode (where there should be a poly cap to the ground) and the source of the voltage going to that point. That would be the output of the LR8 which should have its own capacitor.

I am not surprised that just moving the wire improved the S/N. It points to the toughest part of diy projects. The schematic does not really tell you how to built most things well. The skill and background experience that allows a diyer to know how to route wires, locate components and such is something that is difficult to tell to another diyer. Identifying what matters and what doesn't can drive newer builders nuts. I have sen really great builds that sound horrible because of noise and hum issues.

Good listening
Bruce

Author:  Emsworth [ 20 Apr 2018, 02:24 ]
Post subject:  Re: Oddwatt Push Pull Tube Amplifier Projects

Bruce,
Thank you for the reassuring and informative reply. I had around an hour's listening last night and couldn't be more pleased with your design. If I get enough free time over the weekend, we should move on to stereo!
best wishes,
Denis

Author:  gofar99 [ 20 Apr 2018, 08:51 ]
Post subject:  Re: Oddwatt Push Pull Tube Amplifier Projects

Hi Denis, :thumbsup:

Good listening
Bruce

Author:  Trackhappy [ 25 Apr 2018, 20:04 ]
Post subject:  Re: Oddwatt Push Pull Tube Amplifier Projects

Excitement!!! My Edcors have hit Australia (Presumably now in Customs). Ordered the chassis, valve bases and main caps (from Evatco).
Been searching in Australia for the 25 ohm pots. Finally found them:
https://www.wiltronics.com.au/product/1 ... wound-pot/
Most other sources had ridiculous postage twice the price of the pots themselves. Getting close to fun time!

Author:  Danny [ 26 Apr 2018, 12:28 ]
Post subject:  Re: Oddwatt Push Pull Tube Amplifier Projects

Hi all,
I am in the process (it has been a slow process due to life...) of building two Oddwatt amps. Now I have decided to add an active subwoofer output to each amp (for when I listen to hard rock). From previous comments I see that I can do this by inserting a 4.7K ohms resistor between the 220K resistor at the output of the driver tubes and ground and by tapping at the junction of the 2 resistors. Questions:

1) Is this still the recommended design?
2) Would this affect the quality of the signal going to the power tubes?
3) What happens if the subwoofer is disconnected or turned-off? Would I need to terminate the output in this case?

Any help is appreciated. Thanks,

Danny

Author:  ILoveHiFi [ 27 Apr 2018, 15:59 ]
Post subject:  Re: Oddwatt Push Pull Tube Amplifier Projects

If you want hard rock, then bypassing the 680ohm resistor with a 36v or so capacitor would work. 680r via cathode to ground-input valve
This means you get more gain and more distortion.

I'd say the 4.7k is silly.
If you insert a 4.7k, in parallel with the 220k to ground, all that means your loading up your valves hard, and your cut off frequency is much higher, this means you don't have any more bass
cut off is now approx 0.33uf and 4k7
https://www.electronicproducts.com/RC_F ... lator.aspx

I would recomend a 47uf to bypass the 680r cutt off freq is 5hz where hearing is 20hz lowest

Author:  gofar99 [ 27 Apr 2018, 16:34 ]
Post subject:  Re: Oddwatt Push Pull Tube Amplifier Projects

Hi, Using the 4.7K (in series) to tap off for a sub is correct. I have determined that the signal level is compatible with most sub amps and tracks well with changes in the volume setting of the amp. The connection will not compromise the sound of the amp and there is no need to cap it off if you don't use it. I would not bypass the cathode resistor as there is no advantage. It will increase gain a bit as well as distortion but not the output power. Since the design is for hi fi use neither of these are an advantage. I suggest that you use the newest version of the design as it is significantly better. The one posted will certainly work, but being 11 years old ....the amps have evolved since then. I attached the latest non-dual mono block version schematic. As you can see there are a number of changes. All of them have been covered at one time or another in the various threads. (check the Poddwatt thread particularly) The most significant ones are: changing the output transformers to ones custom designed for Oddwatt Audio for the amp (the originals are fine, just a bit larger than required, cost is about the same), changing the grid circuits on the output tubes (the positive voltage raises it a bit and puts the tubes in a more linear range when driven hard), adding some (really very little and frequency selective) negative feed back to guard against possible instability at around 40-70K HZ.

BTW, you can not get more power out of an amp designed according to either the early or later schematics. It is a forced class A operation. This was an intentional design feature (to stay in class A) for maximizing sound quality. My thoughts are that if you need more power then you need to use different tubes as in the bigger Oddwatts. The dissipation of the 6BQ5/EL84 is the limiting factor in the case of the Poddwatt series amps. Since the design is scale-able, I have previously posted larger versions all the way to KT120 amps for folks that need more power. Still in each case tube dissipation limits the output power. Class A is not efficient but sounds very nice.

Good listening
Bruce

Author:  Danny [ 27 Apr 2018, 19:12 ]
Post subject:  Re: Oddwatt Push Pull Tube Amplifier Projects

Bruce,

Sorry, I should have been clearer. I am building a pair of OddBlocks using 5751s & KT88s. February 22, 2012 schematics. I assume that I still can use the sub woofer output with the 4.7K resistor.

Thanks,

Author:  gofar99 [ 27 Apr 2018, 20:16 ]
Post subject:  Re: Oddwatt Push Pull Tube Amplifier Projects

Hi, No problem, you are correct.

Good listening
Bruce

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