Amp Tech Thread / Ask a tech Q

[dunwichamps]

biasing requires plate voltage, and cathode current. Your plate voltage * cathode current <= 70% * 42W for KT88s

[VOLVO)))]

Awesome.

[Pissy]

So i have this amp:


A Harmony H303a and I want to set it up so that I use it in front of a power amp.  The amp is 8Watts RMS and sounds killer, but not very loud. 

I've seen Garnet Herzogs that basically do this and put a resistive load on the output, then peel off the signal to go into the power amp. How big is the signal that I'm looking for here, so that I'm not trying to push a banana through a drinking straw?  If I put an 8 ohm voltage divider (two 4 ohms in series) and tap it, think that'd get me in the ballpark?

[Mr. Foxen]

Check the input sensitivity of your power amp. It will be somewhere between 0.75 and 1.5v, some valve slaves want a bit more. You can figure the voltage swing from 8w into whatever impedance you have, but the 8w might be at a low THD, so best of figuring the peak the amp can put out fully distorted and aiming for that, since you don't want to clip your power amp when you slap it with full voltage.

[Pissy]

Thanks

[Hemisaurus]

You don't want a series voltage divider here, you want the 8 ohm load, or you can just leave the speaker in place, and the have a voltage divider across that load something big like a 1M resistor and a 1M pot, or a 500K if you have them handy. Bring it down to between 10K - 100K for a line level input.

The amount of current required by an input is negligible as is the effect of this large resistance across the speaker load.

[Pissy]

Ok.

I just looked at the specs for the amp I'm after, (Stewart world 1.6) and it's Input sensitivity is 1V.  I have a couple of 8 ohm heat sinkers in the garage, and I'll probably hook it up and get the scope out for the first time since I bought it.

[Hemisaurus]

Well think about it, 8W into 8ohm gives you 8V, but that's going to be RMS. Oddly Stewart don't mention the input impedance of the amp atall. Assuming a perfect input impedance a 1M pot in series with a 1M resistor, you'll get a 7.99997 ohm load for the amp. The pot will allow you to get a max of 4V out, with the pot fully cranked, and the amp seeing a 7.99994 ohm load.

Even at a line level 10K input using 10K pot and 10K resistor, your Harmony still sees 7.994 ohms of load, and you still get 4V max out.

In an idealised world a 10K resistor in series with a 68K resistor would get you to 1/8th of the max output, approx 1V, but I'd use the pot myself so you can play with the output level to get what you like, and adjust for the loading of the power amp input.

[Hemisaurus]

Wasn't in the manual, but was on the website

Code Select Expand

Input Impedance (Balanced/Unbalanced)  20k Ohms/10k Ohms

so 10K was a fair assumption.

Absolute minimal connection would be to hook a 68K to one terminal of the speaker in the Harmony, hook the other end of the 68K to the amp + (on barrier strip) and the ground to the other terminal of the speaker in the Harmony.

[Pissy]

Got it, I had seen the input impedance as well.

[Hemisaurus]

I'd still probably go the pot in a voltage divider route, just for flexibility. If you have serious noise/hum issues, you could also get a little isolating xfmr, I know Weber and Jensen both make something that would do the job. This would also make the signal properly balanced, if you had a long cable run.

[Pissy]

I probably will. Here is how Garnet did it. This is two different schematics, the top one suggesting pretty much the same thing you are I think.

http://www.webphix.com/schematic%20heaven/www.schematicheaven.com/bargainbin/garnet_g12h_h-zog.pdf

[Hemisaurus]

Yeah pretty much.

[OUTLANDAH]

Anyone have a basic 4-track usb mixer? In dire need.

[Hemisaurus]

Wrong Thread

[Metal and Beer]

Yeah man, repost in the other stickied thread if you want, and I'll shitcan the post here (I tried to move it for ya, couldn't see how w/o dissecting the thread, maybe Jake0))) knows how?)

[VOLVO)))]

Just repost it, I'll clean up afterwards.

[mortlock]

so my mod synth doesnt seem to work [ i just got this]..i have no idea how to get it to make sounds..all i can get it to do is make a hissing sound across 4 different modes..the volume affects that but nothing else is responsive..ive plugged the patch cords in so many different ways that i would have accidently made this thing make noise somehow..im thinking it doesnt work.??

[Hemisaurus]

Took me a while to get the ARP to make noise.

It's kind of an awkward question to answer without knowing the make and model, you should be able to make the oscillator (not the LFO) make some kind of sound, by patching it straight to the output, skipping the filters and waveshapers. There's probably primers out there, on how to make a Moogsynthitron Model #4 make noise.

Modes?

[mortlock]

i say modes because i dont know the terminology..ill try and post a few pics..hang on

[mortlock]

[mortlock]

wn rn gn dn..they're the modes i was refering to.

[Pissy]

Here's the harmony guts.



But something has me scratching my head.

Here's the schematic:


Here's the input jacks. J1 is on the right.



See how J3 connects to that resistor there, the schematic has it bypassed from J3. But the sound is quite noticeably higher gain in J3 as the schematic would suggest. Looking at this, J2 and j3 are in parallel.

I'm pretty giddy about this things simplicity.

Sorry morty, didn't mean to cockblock.

[mortlock]

its cool..im thinking im fucked anyways..

[moose23]

its cool..im thinking im fucked anyways..

Can you get any sound from the saw out? That's the obvious oscillator to me.