7-Octal: Rockster plus?

[surfsup]

I was just trying to help. Without new pics how would one be able to adequately assist? The only suggestions I would have right now:

-go over your schemo to make sure the circuit would actually work
-make sure you soldered everything where it should be soldered
-the 6SL7 is rated for 90 V g-k. At startup you have 106VDC on the grid and 0 on the cathode. That 106VDC was with the 47k rail resistor. You said you subbed in a 10k which will raise that 106VDC to something considerably higher. What's your voltage at that "D" location? Read this:
http://www.valvewizard.co.uk/dccf.html
-post updated schemo and pics

[Phil_S]

Sorry, no body language here for context. I didn't mean to be rude, just saying maybe you missed what I wrote. Thanks for the VW on the C/F. Will study it. At first glance it seems I've built the bootstrapped model, except that Rk is higher value.

New B+ voltages: C=306 D=253 E=251 F=250

You know the deal. When looking at your own work, everything looks A-OK even when its not. I've been looking...that's how I found the miswire at V1 and lucky it was an easy fix.

Something is eating signal. I was sure that correction on V1 was going to do the job. I'm thinking there is a bad solder on the cap board. There are a few turrets with more than what should probably be tried.

I'll probably post new stuff on Tuesday.

[Phil_S]

Thank you to surfsup for properly tweaking my beak. I deserved it.

I have a new schematic with voltage readings and a pic of V1 rewired.

I disconnected the 4 position switch and pulled the 6SJ7. That's off the table until the basic amp is working.

This morning, I experienced something new and not good. When probing the power tube plates for voltage, I am getting a serious spark when the probe first contacts the tube pin, and the same at the OT terminal (simply the other end of the wire.) This is true for both tubes. The meter dies on contact, but does not blow a fuse.

This happened for three different meters, including my hand held Fluke. The others are Amprobe and Tenma products, while low(er) end are not junk (at least that's my impression.) I have an old bench model Fluke that has been idle and it's batteries are not charged. I have to plug it in and charge it, but I expect the same, except maybe the meter won't go into a coma like the others did.

Pulling the power tubes, I see 527VDC at node A and the same at the plate terminals on the OT. No spark with tubes pulled. I am wondering if I need real 6L6's. I've been using a pair of Russian 6p3c and perhaps the voltage is just too high for these tubes. I did not see red plating and they seem to bias OK at around 60mA each or about 54W for the pair.

I not sure I understand surfsup's comment on Vg-k rated for 90V. Is this a typo? Vh-k is rated for 90V on a 6SL7. Assuming it's a typo, I've not previously dealt with Vh-k as an issue. Does this mean, if Vk=164V (today's reading) and heaters are at simple ground 6.3V, then Vh-k is 164-6 = 158 and this is 68V above the rating? To solve this problem, it seems like I should modify the 47k/47k voltage divider to supply about 95V to the grid. Is this right?

With regard to the Vh-k issue, I'm thinking this may stress the tube, but would it, could it be responsible for the problem I'm hearing? Or maybe with the newly discovered power tube problem, there is too much to sort out to reach a conclusion at this point?

Thanks again for any help you can offer.

[surfsup]

I am talking about the grid to cathode voltage being a difference of 169 volts at startup. Arcing could occur. Read the last few sentences of that link. As far as I know, even though the resistor/diode protection elements are not drawn in, Merlin suggests to have them with the bootstrapped CF (they are simply omitted for clarity of the drawing itself)

None of that means this is your problem though it would prob be a good idea to add it considering the difference in voltage potential at startup.

[Phil_S]

I am talking about the grid to cathode voltage being a difference of 169 volts at startup. Arcing could occur. Read the last few sentences of that link. As far as I know, even though the resistor/diode protection elements are not drawn in, Merlin suggests to have them with the bootstrapped CF (they are simply omitted for clarity of the drawing itself)

None of that means this is your problem though it would prob be a good idea to add it considering the difference in voltage potential at startup.

I missed this in Merlin's discussion of the bootstrapped c/f. It seems it certainly can't hurt to add the diode . Thanks.

I think, though, we agree this is unrelated to the problem at hand.

I've posted fresh pictures and schematic with voltages here: http://home.comcast.net/~psymonds/Rocktal7.html

Matt Morcey says (in the thread about keeping old caps) I need a grid leak resistor to ground on the control grid of the 6SJ7. When I get around to that, I'll add a 10M, but right now the 6SJ7 is off the table until I get the "normal" part of the amp running right.

I'm not sure how folks feel about the next item. Interest at TAG in this problem seems not to be here. No problem...that is how forums are sometimes. I don't want to seem unappreciative and I know that folks participate on a voluntary basis. I'm always grateful for what I receive. To expand the envelope, I've started a thread at AX84. If you are one of those people who frequents both TAG and AX84 and you've already posted here on the matter, I think it will avoid confusion if will just continue to post here and not at AX84. Of course, you're free to do as you think is best!

Phil

[martin manning]

Why such a large plate load on the input stage V2A/B? For paralleled triodes I think that should be about 75k, and the cathode resistor should be about 1k2.

[Phil_S]

Why such a large plate load on the input stage V2A/B? For paralleled triodes I think that should be about 75k, and the cathode resistor should be about 1k2.

Hi Martin. I struggle with the technical aspect of this. The front end is roughly the Matchless DC30 which uses Ra=220K and Rk=1.5K for a 12AX7.

Somehow I reasoned (probably badly) that keeping Ra=220K and raising Rk for a 6SL7 was the right thing to do, but I guess not! You've got me thinking...(questionable activity in this area)...plate resistance is 62.5K vs 44K. It probably isn't straight line math (I know, draw a load line), but taking 44000/62500 * 220K = 155K, so maybe I'll see if I've got a 150K and then I'll reduce Rk from the spec'd 1.5K to ~1K. Does that seem to make sense? Maybe I'll draw the load line when I have a few minutes and see what that suggests.

Other good news. Taking pictures seems to be a worthwhile thing to do, and posting them on the internet larger than life is even better. It forced me to look with new eyes. I rerouted the OT primary wires away from everything (d'oh!) and now the amp is working on the bench. Maybe it can get a guitar test tonight, but I've got to leave for an appointment now.

Thanks.

Phil

[martin manning]

...plate resistance is 62.5K vs 44K. It probably isn't straight line math (I know, draw a load line), but taking 44000/62500 * 220K = 155K, so maybe I'll see if I've got a 150K and then I'll reduce Rk from the spec'd 1.5K to ~1K. Does that seem to make sense?

Makes perfect sense to me!

[Phil_S]

I rerouted the OT primary wires. I can now get a plate voltage reading that is reasonable (452V). Unfortunately, there is no real sound coming through, only pops and squeals. I am thinking there is something wrong with the OT.

Maybe I can come up with a reasonably priced OT to swap. Right now, all I seem to have is a 30W Hammond and I think that it is not up to the job. This is clearly a 50W amp.

I am going to give this one a rest for a while.

Thanks, everyone, for looking in on this.

Phil

[Phil_S]

I know I said I'd give it a rest, but I thought I'd see if the OT was bad. As I went to desolder the CT B+ supply, I saw the secondary common/ground was not wired. Ta-da! It works.

So, give me the badge of shame. I can take it.

[Phil_S]

This amp hums (with or without the 6SJ7). Too much for my taste. 120Hz as far as I can tell. Pulling V1 (1st 6SL7) kills nearly all of it. When the tube is installed, the volume control controls the hum; not surprised. Schematic and pics: http://home.comcast.net/~psymonds/Rocktal7.html and one pic attached. You can see the input jacks to the left with 2x 68K into a flying green wire to the 6SL7. I realize, that PTP wiring invites a crapload of problems. I figure they can be sorted out one at time.

I've got a specific question. Do you think moving the pair fo 68K stoppers to the tube pin is worth doing, and while I'm at it, I suppose I should use some shielded cable, which means two cables.

BTW, I've got the pentode working. Sorry, web pictures are not updated yet. The pentode only has bad hum on position 3, input to the tone stack. I hadn't anticipated it, but position 4, direct to the PI allows me to turn the regular volume all the way down, so the signal chain is 6SJ7> Vol > PI and that is like having another amp in the chassis.

BTW2, all the pots were wired backwards! At least I was consistent. LOL. those are fixed except the bass pot.

[thejaf]

Devil's advocate here, but are you ever going to use the low input? I have ceased this on my projects, just one "normal" input.

[Phil_S]

I can clip the low input (no objection), but I'm not going to cut a new plate to close the hole. So, it's 6/half dozen....do you think it contributes to the hum? I am thinking it does not.

[thejaf]

6/half dozen correct, but it would simplify the input: you'd run one shielded wire and place 68K on the socket. I'd also run shielded wire on anything else tied to the grids (except grounds obviously), and pay attention to grounding scheme. Just my limited $0.02

[Phil_S]

Having slept on it, your logic prevails. Simplification is a strategy that works. Ground scheme? I dunno, I thought it was OK, but it is hard to know. The problem seems to be with the one tube. I'll work on it.