L.S.
Since I found out my SE EL84 amp could be driven by my preamp I got rid of the driver AND the coupling capacitor. There's no way back..
I read an article by Satoru Kobayashi who designed a tube amp using a OPA 445 PA as driver and phase splitter. Even though it's blasphemy in the tube section I'm really intrigued by this idea. Especially because there's no need for a coupling capacitor! Such amps are quite rare and I've never heard one, has anyone of you?
Since I found out my SE EL84 amp could be driven by my preamp I got rid of the driver AND the coupling capacitor. There's no way back..
I read an article by Satoru Kobayashi who designed a tube amp using a OPA 445 PA as driver and phase splitter. Even though it's blasphemy in the tube section I'm really intrigued by this idea. Especially because there's no need for a coupling capacitor! Such amps are quite rare and I've never heard one, has anyone of you?
Electra print has two schematics (and the needed iron) for a class A2 amplifier (PP and SE).
Electra-Print.com Push-Pull A2 6A3/300B
A couple of months/years ago there was some discussion around the LME49811 and LME49830 IC's: opamps that can be fed from +100V/-100V and therefore produce about 60V RMS supplying some mA of current. Never seen a tube amplifier with one of these though.
http://www.national.com/ds/LM/LME49811.pdf
LME49830 - Mono High Fidelity 200 Volt MOSFET Power Amplifier Input Stage with Mute
Erik
Electra-Print.com Push-Pull A2 6A3/300B
A couple of months/years ago there was some discussion around the LME49811 and LME49830 IC's: opamps that can be fed from +100V/-100V and therefore produce about 60V RMS supplying some mA of current. Never seen a tube amplifier with one of these though.
http://www.national.com/ds/LM/LME49811.pdf
LME49830 - Mono High Fidelity 200 Volt MOSFET Power Amplifier Input Stage with Mute
Erik
Haven't tried this as I prefer integrated amps, especially when the whole amp has only three stages to start with
.Then you avoid a whole bunch of stuff and cables, boxes etc!
Jack at Electra Print told me that using a mosfet to drive a DHT into A2 was a stupid idea. I did it anyway and it works great. Now he is pushing the idea of using a chip amp through a custom Electra Print transformer to drive DHT's. I haven't tried it yet, but there was at least one thread here where someone did it without an EP transformer.
I have seen a magazine article somewhere (Glass Audio / Audio Xpress?) that used one of these chips to drive power tubes (KT88's maybe). It was at least a year ago.
I haven't tried an opamp driver yet, but I have made opamp/tube CCS's. I also used opamps and a microcontroller for auto bias circuits, and a dsPIC chip to create a class H tube amp.
As far as I'm concerned, if it uses mostly tubes in the signal path, it is a tube amp. Sometimes some silicon in supporting roles just makes sense.
Especially when you have something to sell.
I respect Jack. He is a talkative person with strong opinions. He told me that PowerDrive was a stupid idea, but had no problems selling me the OPT's for the first prototype of what is now the Tubelab SE. That is still the best sounding amp that I have, all 2 WPC of it.
I called back to find out about OPT's for my 845SE, but they were far beyond my price range. We discussed PowerDrive again and it was still stupid despite the fact that it works and sounds great. I found some cheap OPT's on Ebay and built that one too. It is also a great sounding amp that makes 40 WPC, but dumps almost 500 watts of heat into the room so I don't use it much.
I'm happy with the tube outputs of my Hammond H182 organ to the speakers, but the reverb input to the EF86 tube cannot maintain contact with the grids of the tubes in front of the loud woofer. See http://www.diyaudio.com/forums/tube....html?highlight=millivolt+signals+tube+socket In that thread I was advised to replace the tube socket, or put in an out of stock solder in tube. I have bought a gold socket, but Hammond has 6 grounds soldered to the mounting ring of the socket, which mounting ring is not an available modern part.I've spent $55 on tooling for brass stock and .890" punch (nominal .875") to make a grounding ring, but now need a $195 nibbler tool to make the square grounding tabs on the grounding ring. Alternatively, I've been trying to replace the EF86 pentode with a 650 v rated nfet, since 250 VDC is the power supply available.(FCPF11N60 fairchild). The input from the spring reverb is about 10 mv and capacitor coupled. I rectified the heater voltage to the tube, ran to the top of a pot with the bottom to ground with the wiper connected to the gate of the fet parallel with the capacitor input signal. No sound- consumes power only. The fet is typically supposed to turn on at 5.5 v. 2nd try, in parallel with the pot to rectified AC, I connected a 1meg resistor from the gate of the fet also to B+, and since the fet has no internal gate zener and a 30v gate maximum, attached a 15 v zener to ground also to the gate. The zener popped off the solder joint on installation, no sound, I presume the gate of the fet is blown. When I touch the probe of a DMM to either gate or drain resistor, there is a huge pop in the speaker, so that part of the hookup is okay. There the experiment is stopped until the fall rains hit again. High voltage op amps are very expensive, and I have lots of high voltage in this organ but no +-15 VDC. All the transistor circuits in the organ are done with VCC of 100 V dropped down from 350 V by resistors, pretty strange, but it was 1968.
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I'm working on an amp that uses op-amps to drive tubes. It uses the differential output from a DAC, then an opamp lowpass filter, LDR volume control, and an op-amp with a gain of about 7 to drive a pair of EL-84s in PP. The voltages seem to be about the right level. I'm still putting the amp together, but I probably won't make any progress for a while since I'm moving soon.
Broskie has a few schematics for this on his site, buried somewhere.
Broskie has a few schematics for this on his site, buried somewhere.
No I haven't, but would be interested to, mainly to hear the diff between a SS and tube PI/driver.
I was looking into this a few weeks ago but felt the +/- 15V supply for conventional opamps would be too limiting even for an EL84 PP. I didn't even think there might exist some high voltage audio opamps. I guess the reason I was looking into it was that I've read a lot of debate on the merits and disadvantages of both diff amp and split load PI's. I was thinking a SS PI might be a good comparison (or some sort of comparison) to learn how "bad" or how "good" a tube PI/driver stage for an EL84 amp is, how it sounds I mean.
It just happens there are stocks of this IC in my country (free shipping), and I have a suitable experimental amp with nice OPT's that such a circuit could be tried in. I've never had a problem with opamps in the signal chain, but I certainly prefer a tube amp to a SS any day, I don't think that will ever change.
I was looking at this for a PI, and a PCB (P87B) here. Still need some gain in front of it, either tube or opamp, unless someone can safely design some gain into the input buffer of the P87B cct.
OK, you sparked the old light bulb, I remember the speakers, and it was the Burning Man festival which is an entirely different thing from Burning Amp.
http://www.diyaudio.com/forums/tubes-valves/158769-amplifier-burning-man.html?highlight=burning+man
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