Multi-purpose tube amp (line and guitar inputs)

[smsmonster]

It's me again..

Schematic
I've used TubeCAD to calculate some values, really handy program. Entered my parameters manually and got the following:

V4A: (guitar)


V1A: (line)



Also I've decoupled V4A with 33K and 100uF. I've moved the decoupling capacitor from the wiper of P1 to the high side of P1.. I thought it would be better that because of the DC current that will flow thru it..

[lndm]

This is looking much better.

One very important thing, you need to change those plate resistors (anode resistors, Ra) like I suggested in my last post.

P.S. The plate resistor (Ra) is just a resistor whereas the plate resistance (rp) is a characteristic of the valve.

[smsmonster]

Very well, i've upped the plate resistances and fiddled around a bit with load resistors to get the gain i wanted..
See here.

Just one thing: The gain of V1A is way too high (25.66x). But because of P1 I can't change that.. What can I do about it? It needs to be 5.5x

Nearly there.. I can feel it

[lndm]

Ok.

1. I would reduce the values of R19 and R23 to give higher supply rails.

2. R4 and R5 should be between 3k and 10k each and identical.

3. Remove R24, it is a problem.

4. Place a grid resistor from V1a to ground (250k-1M).

5. Recalculate R2 and R3 (in future, it is helpful to include the voltages at the plates).

6. You say the input to V1a is 1v. I assume you mean peak to peak. In this case with approx 25 gain you have 25Vp-p. With an output stage grid to cathode voltage of -5V, you need 10Vp-p as an _absolute minimum_. IMO, 25Vp-p is only just enough in practice.

I agree that having V4a to make up the difference between 200mV and 1V is indeed overkill, but do not try to lower gain by using a tiny plate resistor. It only results in high distortion.

[smsmonster]

So I've been thinking today..

New schematic!

There is something I keep bumping into. As for V4A, the idle current, cathode resistor and gain are numbers which just don't fit together.

But that's of later concern: first off, I've adjusted V1A so it has a swing of +/-50V, which should be enough for the power stage (which is biased at -9V, so 36V-72V should do it right? It's idle current is set at 5mA as you can see.

How do I calculate the gain of V1B (cathodyne phase splitter)? And I presume it has to be correctly biased as well.. Is it particularly important that it is biased in class A? What determines the idle current and what is a realistic value? From then on I can calculate R19.

R23 is high, but OK. It has to drop 30 volts, but just at 1mA so that makes 30K right? About power supplies: Is C4 large enough? I could make it 2200uF or something, I have some really large capacitors with high voltage ratings.. (Yes those are expensive, but I can take them out written-off equipment at work (which by the way are electronic chopper installations for large DC-motors) pretty handy eh?)

V4A: ah our little problem child
What concerns me: the -Ug/Ia curve is not at all linear at 1mA (idle current). Does this introduce serious distortion? And the gain is way too high (about 6 times too high).. Can I resolve this by using a 1:6 resistor divider or am I thinking too simple?

New schematic!

[lndm]

Now that you mention your phase splitter, lets set that up.

The first thing to do is to move P1 to in between C4 and R20. This is because V1a is needed to bias V1b. Next, remove C15 and R21 and replace with a direct connection. Then you adjust the value of R2 until R4 and R5 each have about 35V across them. If you can increase your supply rail, this might be good, or leave it for now.

Hope you appreciate leaving this step till later as it may have just confused matters earlier. In this case V1b needs to sit comfortably in the middle of the two resistors with some swing available on each end. To change the characteristics of the V1b stage, just change R4,5 together.

As far as your ps caps, they look like they will work. I like to avoid large values due to the current surges that can be associated with them and because it is hard to find quality an a larger value cap IMO. If you have an issue with ripple/hum, upping C4 would be the place to start, otherwise IMO not necessary.

As far as V4a, you are correct about not running the current too low. IMO you could try a different device. Until it is built, you may not be entirely sure you will even need this stage. Even so, your idea about attenuating the signal is doable, if not the most elegant method. I guess that perfectionism is the enemy of a finished product (safety aside).

[smsmonster]

Very nice

I've adjusted things according to your last post and am hoping to visit the shop on Tuesday (shop's closed on Monday )

For now, i've cut out the guitar pre-amp bit. If it doesn't amplify enough yet, I can use it to crank up the voltage (or perhaps as a distortion stage?).

As for the Output Transformer: What is the output resistance of my power stage? In other words: what kind of input impedance do I need on my OPT?


I was thinking of this transformer.
Input: 2x2000 ohm
output: 4 or 8 ohm
Power rating: 40W
It will cost me about 52 euro's (or 115 new-zealand dollars). Is this overdone? Or is any (non high-end) transformers usable as well? (obviously not a power supply transformer) I want to use an 8ohm speaker.

I was told I should use it in 4000 : 4 mode for 8000 : 8.. Is this realistic?

I'm going to the shop on Tuesday, and start building

[lndm]

The spec sheets don't advise on PP operating points. I don't have experience with these output devices and my guess is as good as any . Get a second opinion. Enough secondary taps gives you a good range of experimentation though.

The OPT ratio can be used 4000:4 and 8000:8 no problem, it is the ratio not the absolute numbers that count. The limit typically is that the primary reflected impedance from the secondary needs to be lower than the reactance of the primary inductance. The manufacturer can advise here.

Best of luck

[smsmonster]

This is it. That's how I'm going to build it and from then on I'll see what I can improve I can calculate on and on for hours, but things will just work out differently in practice.. It's a bit trial-and-error i think

Anyway, I'll just go for the 8000 ohms guess. Or is there any way of measuring what the output impedance is (by means of a DVM or oscilloscope)?

By the way here's the datasheet for the OPT.. Sounds good so far.
http://www.amplimo.nl/download/3A524.pdf

I'd like to thank you for your time and effort.. It's been great I'll start building this week and post some pictures of the process and when I'm done.. Think it'd be great, we have some really nice measuring equipment (portable digital scopes, digital four-channel scopes with color displays nice)

Reckon I'll start with my 230V-115V isolation transformer.. Might be tricky to get my hands on one in our local store.

[lndm]

Ta

BTW did you know that you can save on parts by replacing R11,12,13,14 with a single 2250 ohm resistance from ground to all four cathodes, and in parallel a single 100uF instead of C5,6,7,8 as in the williamson amp. This is a classic amp and many have been modeled on it. Your amp bears some similarity to the Williamson. I have built one and it was worth the effort.