[Logistic]

Quote:

Originally Posted by nullstring /img/forum/go_quote.gif I, for one, also plan on using this information So.. don't worry about it. I do, however, wonder if it should be put into it's own thread. Do you think the optimal capacitor values change with this tube? The tube is a different voltage, right? Also, how did you go about choosing this tube for use with the millet? I kind of wanted to have traditional looking tubes on my first tube amp. How hard would it be to adapt another tube?

You can use the 12SR7GT tubes, which are the glass version of the 12sr7, if you want glass tubes. I chose the 12sr7 because it is very cheap at vaccumtubes.net, in keeping with the starving student theme.

The capacitor values can stay the same. The voltage of the heaters change, but the resistor changes are all that is necessary. If you want, you can go lighter with the cathode bypass caps, you can probably cut it in half if you want.

As for creating a new thread, I might consolidate all the info into a new thread sometime in the future.

 

[zkool448]

Logistic, very useful and valuable info indeed IMO – just wait and see once 19J6 tubes aren't anywhere to be found

I've seen a few attempts (on paper) but I don't think I've yet seen a fully working variation of the MSSH 'in the wild' until you built yours with the 12SR7s/12SW7s -- I do think it deserves it's own thread

 

[nullstring]

Thanks Logistic.
Sounds good.

EDIT:
You even mentioned the glass tubes in your original post.
I must've overlooked it before. Sorry about that.

 

[cms5423]

So i have a general question: For the inputs of this amp, most everyone uses RCA inputs. Could I use a 3.5mm input? Or what is the benefit of using RCA jacks? Thanks guys

 

[whitelabrat]

In case anyone is interested ESRC tubes is selling 6J6's for $1 a pop. Same as 19J6 but 6.3v heater.

 

[zkool448]

A quick search through Head-Fi megathreads (luv this search feature

) suggests the 6J6s have much higher current draw and can't be used without circuit modifications, also won't be as elegant as the existing heater design.

 

[Llama16]

Would placing the reservoir caps before the power switch not eliminate the danger? Like in the Mini^3

 

[DKJones96]

6j6 tubes grow on trees! I know of at least one place in town here that has about 100 of them just sitting there. Nobody ever buys um.

What if you were to use 3 6j6 tubes per channel so the voltage requirement for the heater circuit remains the same? Current is higher so the Cisco power supply in the original BOM won't work anymore but since it's now more expensive than other, higher capacity, 48v switching units that shouldn't be a problem. You could just not use the other two tubes. They'd be there for aesthetics.

Can the FETs take that 450ma current if you can get the heat away from them?

 

[nullstring]

What do you guys think the chances of getting matching 12SR7GT/12SR7/12SW7 from vacuumtubes.net are?

I plan on buying a pair of each.

Should I buy more than just a pair?
If I request the same brand.. do you think they'd oblige?

 

[Logistic]

Quote:

Originally Posted by nullstring /img/forum/go_quote.gif What do you guys think the chances of getting matching 12SR7GT/12SR7/12SW7 from vacuumtubes.net are? I plan on buying a pair of each. Should I buy more than just a pair? If I request the same brand.. do you think they'd oblige?

the 12sr7s and 12sw7s I got from vacuumtubes.net were all military RCAs.

 

[Hayduke]

Quote:

Originally Posted by cms5423 /img/forum/go_quote.gif So i have a general question: For the inputs of this amp, most everyone uses RCA inputs. Could I use a 3.5mm input? Or what is the benefit of using RCA jacks? Thanks guys

Mine uses a 3.5mm(1/8") input jack. I feed a Bantam cable DAC into it at work.

I have yet to build anything with RCAs since I don't use them. Even my SOHA has a mini phone input (along with a built in BantamDAC).


Tom, I'm bothered to hear you fried another bantam with a MHSS. Mine has been running like a champ. I've adopted the procedure of unplugging the Bantam each night when I leave work and turn off the amp. In the morning, I turn on the amp and give it a couple minutes (sometimes as little as 1 or 2, sometimes several as I get coffee or deal with early AM crisis hehe), then I plug in the bantam. So far, so good, but you have me a little spooked. I know you've recommended modifying the circuit at the input to protect the DAC, but I haven't done anything yet. Of course I am using a 10K pot and 40k in resistors, so maybe I am already blocking some of the problem?

 

[n_maher]

Quote:

Originally Posted by nullstring /img/forum/go_quote.gif What do you guys think the chances of getting matching 12SR7GT/12SR7/12SW7 from vacuumtubes.net are? I plan on buying a pair of each. Should I buy more than just a pair? If I request the same brand.. do you think they'd oblige?

I would bother worrying about matched tubes.

 

[tomb]

Quote:

Originally Posted by Hayduke /img/forum/go_quote.gif Mine uses a 3.5mm(1/8") input jack. I feed a Bantam cable DAC into it at work. I have yet to build anything with RCAs since I don't use them. Even my SOHA has a mini phone input (along with a built in BantamDAC). Tom, I'm bothered to hear you fried another bantam with a MHSS. Mine has been running like a champ. I've adopted the procedure of unplugging the Bantam each night when I leave work and turn off the amp. In the morning, I turn on the amp and give it a couple minutes (sometimes as little as 1 or 2, sometimes several as I get coffee or deal with early AM crisis hehe), then I plug in the bantam. So far, so good, but you have me a little spooked. I know you've recommended modifying the circuit at the input to protect the DAC, but I haven't done anything yet. Of course I am using a 10K pot and 40k in resistors, so maybe I am already blocking some of the problem?

I had input resistors, too. Although, I think we were suspecting the stuttering on startup as an incorrect diagnosis. This time I fried the Bantam when the SSMH (or MHSS) had been on for quite awhile. I disconnected the RCA jacks from the SSMH and immediately got a lot of hum. I should've known something was wrong at that point, but just thought perhaps the lack of signal ground caused it. In trying to connect the Bantam, I clumsily touched the center pin of the RCA patch cable to the outer surface of the jack on the SSMH. That's all it took - that channel is dead now on the Bantam.

A few of us suspect that the Cisco's power supply ground "floats" from time-to-time. Unfortunately, it's intermittent and unpredictable. I once measured as much as 0.49V offset between the ground on the SSMH and a capacitively-coupled ground on the Bantam (the dead one, now - OK for testing

). Then it went away and I couldn't measure it again. If this is what is happening, that's why an Alien or Bantam DAC would get fried - the grounds/output on those DACs are not isolated from the amp.

I don't have a scope, but it would be interesting if someone conducted some similar tests on the Cisco power supply ground, just to be sure.

P.S. One other line of suspicion is a shorted tube. Those can easily be intermittent. I think Nate recently said his PCB prototype had experienced a lot of hum lately - it turned out that one of his tubes developed a short. Knowing that the cathode is directly grounded through the MOSFETs and the rest of the amp, this might make a lot of sense, too. Someone correct me if I'm wrong, but if the tube has an intermittent short, it may sound fine most of the time.

 

[tomb]

Quote:

Originally Posted by nullstring /img/forum/go_quote.gif What do you guys think the chances of getting matching 12SR7GT/12SR7/12SW7 from vacuumtubes.net are? I plan on buying a pair of each. Should I buy more than just a pair? If I request the same brand.. do you think they'd oblige?

Actually, I'd get them matched if you are going this route. Use of this tube series may work fine with the suggested mods, but those are only single-triode tubes. In the 19J6, there are dual triodes that are paralleled together. This tends to average out the differences in tubes and minimizes the need for matching.

With single triodes, the differences in matching from tube to tube may be quite stark, IMHO. No offense to the excellent work finding a working replacement, but I think this is one of the reasons why Dsavitsk suggested that a 12AU7 modification might work a lot better (with similar mods) for the SSMH than this series of tubes.

 

[n_maher]

Quote:

Originally Posted by tomb /img/forum/go_quote.gif I think Nate recently said his PCB prototype had experienced a lot of hum lately - it turned out that one of his tubes developed a short. Knowing that the cathode is directly grounded through the MOSFETs and the rest of the amp, this might make a lot of sense, too. Someone correct me if I'm wrong, but if the tube has an intermittent short, it may sound fine most of the time.

Unfortunately replacing the bad tube with a good one has not rectified the grounding issue that I'm having.

If I max the pot on my prototype and control the volume with my source I have no noise, anywhere below max volume and I've got bad buzz. Work continues...