120W PPP ULL Bass amp, first desgin, please shoot - diyAudio
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Old 18th March 2013, 03:40 PM   #1
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Default 120W PPP ULL Bass amp, first desgin, please shoot

I've completed my first schematic for a 120W PPP UL bass amp I'm looking to build. Everybody on this forum was very helpful with all my questions, so offcourse I'm curious about all opinions and insights concerning this schematic.
input and PI are ECC99 and powervalves are KT88's. Some values like the zobel networks on the OT are to be determined when build.

powerampstage.png

Last edited by funk1980; 18th March 2013 at 06:29 PM.
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Old 20th March 2013, 07:47 PM   #2
Struth is offline Struth  Canada
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Hi Guys

Assuming this is for bass guitar, there are simplifications that can be made and some values that definitely need changing.

The cathode follower stage can be removed . If you want to retain it, consider making it self-biased. High-current followers with grids tied to positive voltages easily latch up at turn on.

The duplication of the bias set networks is unnecessary as all four pots can be paralleled and share the same range resistor. The X-end of the pot should be to ground to indicate maximum current, maximum heat, maximum bias. This follows the universal bias definition given in TUT2.

The diodes in the bias circuit are not needed. The filters from the pot wipers to ground, although technically justifiable for hifi, are also not truly needed.

The zobels for the OT can be eliminated if you make the necessary screen-stop value change to 1k-5W per tube.

The minimum safe screen-stop value in MI is 1k-5W per tube, as we've always said in every TUT volume and everywhere else. TUT2 details why. The engineering ideal is zero screen/grid-stop values, but real world layouts demand some compensation. You see small incremental stop-R values like 100 and 220 in hifi but these values provide no protection whatsoever in MI.

See TUT3 for the Custom Special chapter outlining a 160W bass amp that uses any large-bottle tube type or combination - four tubes for 160W. Uses off-the-shelf Hammond OT and PT (1650T, 278CX - 378CX for int'l).

Splitter can be any 12A_7, although the T gives best clean output. All of these are rated for 540V open-circuit, so are fine in the typical 120-160W amp environs.

For the feedback loop, use higher values for better note articulation. For example, Fender often uses 820 + 100 ohms giving a gain of 9.2 and a very sterile tone. Increasing to 8k2 + 1k opens up the sound and the gain is still the same. There is also less issue with OT parasitics with the higher circuit values. TUTs explain why.

Have fun
Kevin O'Connor
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Old 21st March 2013, 01:03 AM   #3
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Kevin! Thanks for the reply. You're correct, it's for bass guitar.

I will up the value of the screen grid stoppers. A bonus If I can do without the zobel networks. And an awesome tip for the feedback loop values. I would have never thought of that.

I agree the cathode followers are a bit overkill, and realise even a triplet of KT88's only require around 2mA with the 100k per tube grid leaks in AB1 mode. Is there a reason you prefer the 12A*7 over the ECC99??

I will look into the bias networks. I don't know what you mean exactly with the x-end of a pot, but less components is always good. However, the 22u cap and diode are for protection against short time power failures and immediate switch off and on again. The diodes force the caps to discharge through the 100k, so a bias voltage will remain after switch off for a fair amount of time.

I'd love to get my hands on TUT2 and TUT3 (the whole series for that matter), but shipping costs and time to Holland are killing me. Are there any plans to release them as ebooks?? Or is there a supplier in Europa perhaps?

Last edited by funk1980; 21st March 2013 at 01:13 AM.
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Old 21st March 2013, 04:55 PM   #4
Struth is offline Struth  Canada
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Hi Guys

We always design circuits to use widely available tube types that are current production. The 12A_7 family fits that requirement. Tubes like ECC99 are current production but not that widely available. A design that truly requires that tube will have to be redesigned later if the tube is not available. The 12A_7 family will be the very last to be put out production when such a day comes.

In your circuit, there is no "requirement" that the splitter be an ECC99 and in fact the 12AT7 will likely give better results. The T has proven itself in thousands of amps in exactly this circuit. An X can be dropped in for a different tone - its higher mu does not translate into more drive, as its total performance rather limits how clean it will play at the signal amplitudes required here.

The diodes and caps really should go - the diodes at the very least.

Most bias supplies are built as "after thoughts" and are quite lightly filtered and lack current reserve. For the most part, this is actually beneficial as it allows the bias voltage to track the loading on the plate supply, keeping the tubes optimally biased. Making the bias too stiff relative to the screen supply is a bad thing as the tubes will be underbiased (cold) if Vs drops and -Vb does not. Having a direct connection between the bias output caps and the bias set-networks allows the latter to drain the charge down when required - it will rise back up as quickly as is needed.

You pay for shipping regardless of whether there is a local supplier or not. They had to get the books shipped. Physical books are much more portable and useful to people doing real physical work.

Have fun
Kevin O'Connor
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Old 21st March 2013, 10:58 PM   #5
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Thanks again, it's really appriciated. I will take all advice into considiration.

I have to disagree about the ebook thing though. My iPad much more portable than any amount of physical books, with search function, custom indexing, editable text highlighting and also functions as calculator, notebook and rudimentary function generator when out on the job.
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