AX84 SEL KT88 build

[xtian]

Awesome! Please post photos, and I'll try to help.

[toadpipe]

Ok, photos it is. Be warned this amp is currently not pretty, I slapped it together on a board just to test it's working. It's a serious rat nest and yes, hums like mad. No worries, I'm re-cutting the board and putting it on a proper chassis next.

I used a EL34 for the power stage (I had one around so...) which means I added the adjustable bias option. A 1ohm 5watt in series with a 220ohm 5watt and ran through a 1k 5watt pot, in the place of R9. I had the pot set to about a quarter of the way up for testing (so the total resistance about 500ohm). After the amp had been on for a while I decided to start turning up the ohmage. All the way off and the amp was unhappy (R9 = to 220ohm) as I turned it up things cleared up nicely. I had my wife playing and as the pot got to be about 3/4 up (so, almost 1k total resistance) BANG!
I lost C6, it popped like a firecracker. Was kind of fun!

Long question short. For an EL34 I'm thinking I'll just dump the variable bias and make R9 a 430ohm resistor. That's what the value is for a KT88, but comparing the supply voltage with the P1x and it's R9 value, it think it should do fine.

Thoughts? I'd like to sort out my bias issue before tackling the R/L switch and the tone circuit (I only have one cap for C6 left, and mouser next day delivery is expensive).

[xtian]

Woah. There's a poster for Danger! Excitement! Anarchy!

Can't wait for the "after" photos.

[toadpipe]

Righto! So, I replaced C6 and removed the pot on R9, replacing it with a 430ohm resistor for a grand value of 651ohm. I removed my terrible tone circuit and wired directly to main volume. Plugged in, turned on and so much better without that tone stack, rhythm still sounded great, but now lead was at least making some sound. Really thin and low volume compared to rhythm and if I turned the gain up past about 7 (1-10 scale) this terrible high pitched loud squeal started up.

So, disconnected the Rhythm/Lead switch and jumped R12 to the junction of R14 and V3-6.

Plugged in, turned on and..... same as with the switch in lead. Thin sounding (tinny) low output and that squeal if the gain kicked up to 7.

So, my guess is two of the leads to V3b got switched around. I'm thinking 7 and 8? Any other possible causes?

[toadpipe]

Ok, tested the connections on the socket, not missed wired or swapped. I also switched around the tubes to see if one was bad, nope, same effect. I'm stumped.

[toadpipe]

Ok, found what I believe to have been the trouble. C11 was not actually connected to R16. Seems to me this would greatly effect the output of the 3rd stage. It's connected now, output is good, nice and high with both switch positions.

Now the problem is crazy crazy feedback if I crank either the gain or the volume too high in either rhythm or lead. I'm certain this is my lack of chassis haha!

Right then, off to etch a few new boards and do some metal work!

[xtian]

Glad you're getting things sorted. Squeal when you turn up the volume is likely parasitic oscillation. Fixing your lead dress will probably solve that.

[toadpipe]

Yep, working on making things cleaner. Here's the layout of the PCB I made for the main board. One labeled, one not 300dpi for printing.

(edit)
Forgot to mention, I added the second gain control back in. I'm finding that I like that extra layer of adjustment even with the rhythm/lead switch.

[sepulchre]

Bravo for finding the problem. Well done. Also, I applaud you adding the second gain control. I find it most useful, especially with a lead/rhythm switch.

The only problem I see is that you might want to do the math for your bias. An EL34 is a very different beast from KT88. I've got an EL34 in one amp and the other has a KT88 and the bias is very different.

[toadpipe]

The only problem I see is that you might want to do the math for your bias.

Couldn't agree more, sadly, my math sucks. After some testing it seems that the "sweet spot" I found was when R9 equaled about 670ohm. I have two 5watt resistors in series equaling 650 and am planning to order a few 680 ones after I make certain I know what it needs.

Anyone happen to know the formula I need here? Or want to double check my math?

[sepulchre]

Okay, you had the variable bias circuit installed before, right? Then you took out the pot? If you left the 1 ohm resistor in that's good. If not I would suggest you put it back in the circuit. That's how you'll measure the bias current. It should be a 1 watt / 5% resistor (the more accurate your resistor, the more accurate your reading so 1% would be even better).

The formula is: (PaMax * 0.9)/Va

PaMax is the maximum dissipation of the tube. Since you're using an EL34 that would be 25 watts. Va is the voltage on the anode, also called the plate voltage.

Note: all measurements should be made while the amp is idling, not while it's being played.

So, measure your plate voltage (pin 3). Then divide 25 * 0.9, which is 22.5 by the plate voltage. The resulting number is where you want to set the bias current.

For example, if your plate voltage is 300 then 22.5/300=0.075 or 75 mA.

You measure the bias current by setting your meter to milivolts, or mV. Measure the voltage across the 1 ohm resistor. The reading should be close to but not over the results of your math.

Hope this helps.

[sepulchre]

One other thing I thought of: Most octal power tubes like KT88 and 6L6 have Grid 3 internally connected to the Cathode. But the EL34 does not, so you need to make sure pin 1 and pin 8 are connected.

[Structo]

Or to check the bias current measure the voltage across R9 and divide by it's value.

[toadpipe]

One other thing I thought of: Most octal power tubes like KT88 and 6L6 have Grid 3 internally connected to the Cathode. But the EL34 does not, so you need to make sure pin 1 and pin 8 are connected.

Noticed that on the tube datasheets, so yes I've connected pins 1 & 8.

Okay, you had the variable bias circuit installed before, right? Then you took out the pot? If you left the 1 ohm resistor in that's good.

I had left it in yes.

So, measure your plate voltage (pin 3). Then divide 25 * 0.9, which is 22.5 by the plate voltage. The resulting number is where you want to set the bias current.

For example, if your plate voltage is 300 then 22.5/300=0.075 or 75 mA.

Plate voltage on this creature is 420.

Which gave me 22.5/420 = 0.05357ish.

The reading off the 1ohm resistor varied, but spent most of it's time at about 45mv getting as high 50 and as low as 31. So, I'm guessing I'm in the clear here with R9 being 650/680ohm.

If you know the plate voltage, the cathode cap value, and the tube shouldn't there be a formula for determining the cathode resistor value? I'd have thought there was a way to do it before testing.


Ok! I've derailed the thread now, my bad. I'm finishing up the PCB layout for the "power supply" and I'll post it up soon for them that are interested!

[toadpipe]

Ok, here's the PCB design I've done for the power supply board.