H. H. Scott LK-72A amp

I thought I would start a new thread just on the amp itself. The amp is mostly restored with only a few resistors to finish replacing and replacement of the output tubes once the tubes come in the mail. I thought about using a dual op amp connected to the record out jacks so I can connect this amp to my computer's sound card. Would that be a good idea? I decided against using tubes as I'm feeding a solid state sound card and an op amp is easier to connect. I will put it in a project box using an external regulated power supply. I may keep the gain at 1 so the signal level stays the same. I also need ideas on building a switch box. Here's what the box needs to do:

1. rec out to RTR in, RTR out to tape in
2. rec out to sound card in, sound card out to tape in
3. Rec out to RTR in, RTR out to sound card in, sound card out to tape in
4. rec out to sound card in, sound card out to RTR rec in, RTR out to tape in.

Basically I want it so that I can record to the RTR or computer from the amp, play back the RTR or computer, record to the computer from the RTR while playing back the computer or record to the RTR from the computer while playing back the RTR all without swapping a single cable if at all possible.

I just lost an original H. H. Scott 7591 tube. Not exacxtly sure what happened, but I saw sparks inside the tube. Now it takes about 10 volts to light the filament on my tube tester, the tube glows purple when in emission test and the grid shows high leakage. Good thing I have a set of tubes on the way. Glad it happened while I was near the amp so nothing major got burnt up.

thats gotta suck:thumbsdn: hope its nothing serious that caused the tube to go south.

Nope it was the tube . All the resistors in the bias circuit and grid resistors on the output tubes are new. I put in a spare tube and all is well. I got the Groove Tubes today in the mail. They just do barely fit. Would there be any loss in sound quality or would I have feedback problems if I put the output transformers in a separate external box connected to the amp with banana jacks and wire? I did this with a Magnavox SEP 6BQ5 stereo amp with no problems whatsoever, but since this is a higher powered amp I'm not sure if it will work. I'm not too concerned about origionality as I'm running the amp without the cabinet anyway. Of course if this amp is worth a lot of $$$ I wouldn't even think of doing that. I will be running wires back to the amp for the feedback circuit which will connect between the 16 ohm tap on the amp's speaker terminals and ground. If I do go that route I will need 20 banana jacks. Fourteen for the transformer box and six for the amp. All the jacks will be labeled so I know where each one connects and I will do one transformer at a time so that I don't get things mixed up provided this idea will work with no problems. I assume it is normal for these tubes to have a barely noticeable purplish glow? I'm not too concerned about the chassis either as some of the lettering namely where the tubes are is gone or partially gone. I do have a problem with the tube socket for the 6GH8 in the left channel. I will try bending the contacts before replacing the tube socket as the tube is a little loose and has to be set a certain way for the tube to work right. Once I replace the other half of the resistors The amp will be better. So far I have replaced the resistors in the volume + tone control circuit, the phono and tape EQ network, the power supply (except the ceramic resistors and one wirewound power resistor I haven't ordered yet), the bias circuit and the output tubes except the screen grid resistors. I did those first as I thought they were the modst critical for proper sound and operation. The others I will do as I have time. I'm still amazed at how good this amp sounds. I like how the lows are punchy how the mids are full and how the highs are smooth. This amp drives my DIY speakers very nicely :yes:. Didn't even have to adjust the L-pads on the crossovers when I switched from my solid state:thumbsdn: setup to this amp:thmbsp:. I've heard I should change a resistor in the bias supply for each tube and increase the value of the coupling caps to the output tubes. Is that really necessary or am I ok as is?

Tube Radio said:

I just lost an original H. H. Scott 7591 tube. Not exacxtly sure what happened, but I saw sparks inside the tube.

Click to expand...

Yet one more reason to lift the cathodes from ground and add a full control bias mod to this amp.

OPT's don't come cheap. Especially when you need a matched pair.

. . Falcon

Only thing about the tubes when I got them is they were supposed to all be rated at 9 (Groove tubes rated output tubes from 1-10) and two are 9 while the other two are 6. I have not noticed any problem yet. Here's a link to the thread about the tuner. http://www.audiokarma.org/forums/showthread.php?p=1830547#post1830547. I figured it would be better to separate the tuner and amp in two threads to keep the confusion to a minium.

Listening to some 60's music and this amp sounds sweet although I'm more amazed at how good my DIY speakers sound. See the thread here http://www.audiokarma.org/forums/showthread.php?t=158465

I'm going to connect my Sony TAE5450 preamp in the tape loop as it will provide the necessary switching I need for my reel to reel and computer. I will need to put a high impedance device on the record output jacks since the minium recommended load is 220K and the input impedance of the Sony is 50K. I'm thinking maybe a couple op amps set for unity gain. Should the tape input on the Scott see a high impedance or can I connect the Sony directly to it?

Eureka I've got it! See this thread for my switchbox design.

About two weeks ago I had to replace some burnt filament wiring. The amp was playing fine then I saw smoke from around one of the power tubes so I quickly turned the amp off and I guess somewhere along the line two filament terminals on a tube socket must have shorted together. I replaced all filament wiring with some stranded that was a little larger than the original solid wire which IMHO was a bit small for four power tubes and two preamp tubes. I just had something wierd happen.I was playing music through my computer and the highs were attenuated somewhat yet I could switch to the tuner and all was fine. It is now playing fine. I'm thinking it was a small glitch with my computer.