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First DIY tube amp
First DIY tube amp
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Old 13th June 2018, 06:09 AM   #1
guitarnerd is offline guitarnerd  Taiwan
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Default First DIY tube amp

Hi folks,

I'm new to this community, so please bear with me if I made myself sound like a newbie (which I am...)

Currently working on my first DIY tube amp project, had a big picture figure out but just running into some detail questions. I have the schematic attached below, with some notes on there about how I pick the numbers, calculations, and all that stuffs

Note: The drawing app does not have a UL type output transformer, so I used two to represent it... I'm sure you'll get the idea. Second, some how it doesn't allow me to use the cathode pins of the KT88s, so I used arrows instead.

The design idea is based on the site below: Valve Amps: Output stage

I'm trying to get it to have 75 to 100W power/channel

With all that said, I questions:
1. How do I pick the primary impedance for the transformer?
2. How to calculate the voltage amplified after the power stage (or even each stage)

Also if anyone have any comments or see anything that could be potential issues in the schematic, I'm open to any of them

Thanks!
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File Type: jpg schematic.JPG (100.2 KB, 451 views)
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Old 13th June 2018, 06:27 AM   #2
jazbo8 is offline jazbo8
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First DIY tube amp
Quote:
Originally Posted by guitarnerd View Post
With all that said, I questions:
1. How do I pick the primary impedance for the transformer?
2. How to calculate the voltage amplified after the power stage (or even each stage)
1. starting with the datasheet figures for UL operation is always a good idea.
2. simple, just Vo = A x Vin, where A is the gain of the stage, which you can calculate/estimate based on the operating conditions.

There is no NFB. Is that your intention?
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Old 13th June 2018, 07:23 AM   #3
nigelwright7557 is offline nigelwright7557  United Kingdom
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Your output transformer secondary wiring needs looking at.
Currently you will get no power to the speaker as it is wired.
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Old 13th June 2018, 07:30 AM   #4
jazbo8 is offline jazbo8
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The OPT is "fake" as the OP mentioned above.
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Old 13th June 2018, 08:31 AM   #5
guitarnerd is offline guitarnerd  Taiwan
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Quote:
Originally Posted by jazbo8 View Post
1. starting with the datasheet figures for UL operation is always a good idea.
2. simple, just Vo = A x Vin, where A is the gain of the stage, which you can calculate/estimate based on the operating conditions.

There is no NFB. Is that your intention?
Right there should be a NFB, shown in the attachment
Also I just found this datasheet
https://frank.pocnet.net/sheets/084/k/KT88_GEC.pdf

In the PP, fixed bias, UL connection (which is what I have right now), it says Zout range from 6.5k to 7k ohm, I assume that means the primary impedance of the OPT. But there's also RL(a-a) range from 4k to 4.5k ohm.
And if you scroll down, there's a PP 100W schematic, and they use a 4.5k ohm transformer. So... I'm little confused over here....
Attached Images
File Type: jpg KT88 datasheet.JPG (63.3 KB, 417 views)
File Type: jpg KT88 datasheet2.JPG (40.4 KB, 392 views)
File Type: jpg schematic.JPG (105.2 KB, 396 views)
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Old 13th June 2018, 08:33 AM   #6
jazbo8 is offline jazbo8
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Zout isn't the same thing as RLa-a. You are interested in the RLa-a, so 4.5k will do. The NFB as drawn is incorrect, the signal is shunt to ground by the cathode bypass capacitor. See http://copyright.lenardaudio.com/lai...e-feedback.gif for an example for the correct connection (which still requires adjustment).
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Old 13th June 2018, 10:18 AM   #7
guitarnerd is offline guitarnerd  Taiwan
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I did find some 5k and 4.2k transformers but no exact 4.5k ones. I assume that would slightly alter the output power but shouldn't be anything serious

So for the NFB, could that be fixed by simply removing the cathode bypass cap? or is there other way of connecting the NFB loop while keeping Ck?
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Old 13th June 2018, 12:32 PM   #8
jazbo8 is offline jazbo8
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The exact RLa-a isn’t that critical. There are a couple of ways to connect the NFB network, just do a Google search or look at some well known designs, e.g., the Williamson.
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Old 13th June 2018, 01:32 PM   #9
azazello is offline azazello  Bulgaria
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You will very lucky if your schem will work. This project like first is not for You, sorry....and +U is very dangerous......Better You can start build some elementary PP project to know how adjust modes...
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Old 13th June 2018, 02:46 PM   #10
Sodacose is offline Sodacose  United States
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C2 should be connected to the junction of R3 and anode of V1. I'd also look into direct coupling V1 to V2 in order to eliminate the cap and bias divider.

You might also look into substituting a LTP input stage for the grounded cathode+cathodyne. See the well-documented AB input stages here for inspiration: 6L6GC AB2 Amp
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