khstudio
Well-known member
What "IMPEDANCE" will a 6SL7 drive?
- or -
What Impedance does it "Like" to see?
- or -
What Impedance does it "Like" to see?
Ampeg B15-N Bass Pre-Amp section ONLY! 6SL7's
- Thread starter khstudio
- Start date
Help Support GroupDIY Audio Forum:
Junction
Well-known member
You probably should to stick to the 1M Ohm impedance it is looking into now (Volume Pot).
If you are not into modifying the amp because you want to maintain tone, then sounds like you have no choice but to add another gain stage.
You could try popping out the output tubes, temporarily connect another 0.2uF cap to pin 2 of V3 and see what level you get there. Maybe there is some gain in the phase splitter stage.
Michael
If you are not into modifying the amp because you want to maintain tone, then sounds like you have no choice but to add another gain stage.
You could try popping out the output tubes, temporarily connect another 0.2uF cap to pin 2 of V3 and see what level you get there. Maybe there is some gain in the phase splitter stage.
Michael
> I LOVE the sound of the 6SL7's.
What 6SL7s? They have a large unbypassed 2K or 5K resistor shoved up their butt, and that monster 470K resistor standing on their heads... 'bout any tube will work the "same" here.
You have gain of ~~25, loss of 10, gain of ~25, total gain near 60 at V1b plate. Inputs of 20mV to 500mV come out at 1.2V to 30V at V1b plate.
> I prefer the one I posted... with the volume control LAST.
Probably because this forces gallopingly large signal at V1b plate no matter where you turn the volume pot. It is actually critical of player, axe, style, and taste: a stong player with hot axe and clean taste will find this to be "un-clean". Obviously it suits you.
You don't need ton-of-voltage-gain after this.
You do need a gain control somewhere or a hot humbucker with a heavy pick will smoke modern chip inputs.
You do need current gain, because the 6SL7 is working near 0.25mA, the silly chip-inputs are 2.8V at 10K or 0.28mA peak.
The plate resistance of 6SL7 at 0.25mA is 85K. You want your load to be higher. 10K is not higher than 85K. Poor match, voltage sags, distortion characteristics change.
I think you want the official 1Meg pot into a cathode follower flowing 2mA-3mA. See VT40 preamp, V3. As used here, it has 180K and 270K mix resistors, which happens to work out for those reverb and EQ paths. If you go 2-channel, use your B15-N mix resistors; for one channel you may omit resistors or try 150K to fade the supersonics. Then use the VT40 V3 plan, with 0.01u 1M 1K 47K 0.47u(!) 10K(*) 100K, exact as shown.
(*) For real low loads like modern studio gear, change 10K to 2K and for Bass consider making 0.47u a 1.0u if you can afford it. Should be Film cap and 400V.
The "12DW7" fragment is much like a 12AU7, 6C4, or 6SN7. You could probably use both sections of one 6SL7 in parallel. This CF needs big current but noise and gain are non-issues, a good place to stuff any noisy tubes you get.
If you must have balanced output, use a 10K:10K transformer.
> even if the resistors were exactly matched, isn't there still a possibility that there will be hum from the filament?
If your heater wiring is lazy and lop-sided, so low-level audio points "feel" more from one side than the other, then "CENTER tap" WILL hum. A pot lets you unbalance the electrical center similar to your wiring unbalance. If your wiring is fairly neat, the "best" setting will be so near the middle that "center" will be acceptable.
Note that you have different unbalance in each stage. And the sum of hum depends on the intervening knobs. If you really need a trim, you "need" to re-trim every time you change the Bass or Vol knobs.
> explain the drawbacks in using the hum balance?
Cost 50 cents back then. Nowadays it is hard to find a suitable pot, could cost you $7.
There's a few reasons a hum-bal is worthwhile. A PHONO stage needs a much lower hum level than any gitar. We trim the whole hi-fi for low hum on Phono and hope to disregard hum in later stages. And later Ampegs used PCB construction with 6VAC on traces on the PCB... a technique which MUST hum. My VT-40 responds to its hum-pot, and is just barely better when off-center. This is actually a bad reason to use a hum pot: get the AC out of Flatland and up into twisted pair!!!! We know why Ampeg had to reduce hand labor with PCB, but you don't.
What 6SL7s? They have a large unbypassed 2K or 5K resistor shoved up their butt, and that monster 470K resistor standing on their heads... 'bout any tube will work the "same" here.
You have gain of ~~25, loss of 10, gain of ~25, total gain near 60 at V1b plate. Inputs of 20mV to 500mV come out at 1.2V to 30V at V1b plate.
> I prefer the one I posted... with the volume control LAST.
Probably because this forces gallopingly large signal at V1b plate no matter where you turn the volume pot. It is actually critical of player, axe, style, and taste: a stong player with hot axe and clean taste will find this to be "un-clean". Obviously it suits you.
You don't need ton-of-voltage-gain after this.
You do need a gain control somewhere or a hot humbucker with a heavy pick will smoke modern chip inputs.
You do need current gain, because the 6SL7 is working near 0.25mA, the silly chip-inputs are 2.8V at 10K or 0.28mA peak.
The plate resistance of 6SL7 at 0.25mA is 85K. You want your load to be higher. 10K is not higher than 85K. Poor match, voltage sags, distortion characteristics change.
I think you want the official 1Meg pot into a cathode follower flowing 2mA-3mA. See VT40 preamp, V3. As used here, it has 180K and 270K mix resistors, which happens to work out for those reverb and EQ paths. If you go 2-channel, use your B15-N mix resistors; for one channel you may omit resistors or try 150K to fade the supersonics. Then use the VT40 V3 plan, with 0.01u 1M 1K 47K 0.47u(!) 10K(*) 100K, exact as shown.
(*) For real low loads like modern studio gear, change 10K to 2K and for Bass consider making 0.47u a 1.0u if you can afford it. Should be Film cap and 400V.
The "12DW7" fragment is much like a 12AU7, 6C4, or 6SN7. You could probably use both sections of one 6SL7 in parallel. This CF needs big current but noise and gain are non-issues, a good place to stuff any noisy tubes you get.
If you must have balanced output, use a 10K:10K transformer.
> even if the resistors were exactly matched, isn't there still a possibility that there will be hum from the filament?
If your heater wiring is lazy and lop-sided, so low-level audio points "feel" more from one side than the other, then "CENTER tap" WILL hum. A pot lets you unbalance the electrical center similar to your wiring unbalance. If your wiring is fairly neat, the "best" setting will be so near the middle that "center" will be acceptable.
Note that you have different unbalance in each stage. And the sum of hum depends on the intervening knobs. If you really need a trim, you "need" to re-trim every time you change the Bass or Vol knobs.
> explain the drawbacks in using the hum balance?
Cost 50 cents back then. Nowadays it is hard to find a suitable pot, could cost you $7.
There's a few reasons a hum-bal is worthwhile. A PHONO stage needs a much lower hum level than any gitar. We trim the whole hi-fi for low hum on Phono and hope to disregard hum in later stages. And later Ampegs used PCB construction with 6VAC on traces on the PCB... a technique which MUST hum. My VT-40 responds to its hum-pot, and is just barely better when off-center. This is actually a bad reason to use a hum pot: get the AC out of Flatland and up into twisted pair!!!! We know why Ampeg had to reduce hand labor with PCB, but you don't.
pstamler
Well-known member
Agreed that the 6SL7 will not be happy working into a 10k load. Gain will drop, and distortion will be a good deal nastier than into a 100k or 1M load.
You also may have problems with driving cables longer than a couple of feet; the tube, as mentioned, has an output Z of 85k.
A cathode follower, incorporating say a 6SN7, or one of the totem-pole output stages, may be the best solution. Or hang a DI on the 6SL7's output and run it into a mic preamp.
Peace,
Paul
You also may have problems with driving cables longer than a couple of feet; the tube, as mentioned, has an output Z of 85k.
A cathode follower, incorporating say a 6SN7, or one of the totem-pole output stages, may be the best solution. Or hang a DI on the 6SL7's output and run it into a mic preamp.
Peace,
Paul
khstudio
Well-known member
PRR,
Thank man... You tell it like it is & I like it. :green:
I learn a lot from your posts... but I have a few questions:
If so... this may be the reason it's sound a little better when using the EXT out into a direct box... well not better but Punchier! But I feel I hear all the up & down gain stages when doing it like that... the bass feels more cluttered, restricted somewhat & less defined.
2 -
I don't have any 12DW7's but I do have a TON of 12AU7's (X & T's) & 12BH7's, etc...
Will anything else work as good or as easily here?
I like the simple output you showed me & I understand it & where it goes.
3 -
Your post has me rethinking this pre idea a bit & it sounds like you may even play BASS???
Do you have any recommendations that may be better but similar?
I like the EQ option & using tubes for a bass Pre/DI.
I should have said - I Love what they (or the Ampeg) do to my bass.
It's a much different sound than just using my RADIAL DI into my Neve Preamp's.
I should mention I'm using a Musicman Stingray bass & haven't tried a Fender P or J yet (getting a new P/J soon)
I like what it does to my bass but I feel it could or would be even better when set up "Properly"... which is the reason for this thread.
Thank man... You tell it like it is & I like it. :green:
I learn a lot from your posts... but I have a few questions:
Are you saying that coming directly out of the Volume pot into my LA2A's 10k isn't enough?
If so... this may be the reason it's sound a little better when using the EXT out into a direct box... well not better but Punchier! But I feel I hear all the up & down gain stages when doing it like that... the bass feels more cluttered, restricted somewhat & less defined.
2 -
I don't have any 12DW7's but I do have a TON of 12AU7's (X & T's) & 12BH7's, etc...
Will anything else work as good or as easily here?
I like the simple output you showed me & I understand it & where it goes.
3 -
Your post has me rethinking this pre idea a bit & it sounds like you may even play BASS???
Do you have any recommendations that may be better but similar?
I like the EQ option & using tubes for a bass Pre/DI.
I should have said - I Love what they (or the Ampeg) do to my bass.
It's a much different sound than just using my RADIAL DI into my Neve Preamp's.
I should mention I'm using a Musicman Stingray bass & haven't tried a Fender P or J yet (getting a new P/J soon)
I like what it does to my bass but I feel it could or would be even better when set up "Properly"... which is the reason for this thread.
khstudio
Well-known member
[quote author="pstamler"]Agreed that the 6SL7 will not be happy working into a 10k load. Gain will drop, and distortion will be a good deal nastier than into a 100k or 1M load.
You also may have problems with driving cables longer than a couple of feet; the tube, as mentioned, has an output Z of 85k.
A cathode follower, incorporating say a 6SN7, or one of the totem-pole output stages, may be the best solution. Or hang a DI on the 6SL7's output and run it into a mic preamp.
Peace,
Paul[/quote]
Hey thanks Paul :green:
You posted when I did & you answered 2 of my questions.
Interesting... that's what I was doing, running the EXT OUT into my DI then into a Mic Pre. :grin:
I have to use the Input PAD on the DI when doing this though.
(It's an Active RADIAL DI)
Do you guys think a simple Passive DI (Like a 1:10 or 1:15) Mic TX would work better for this :?:
I have some old UTC 150/600:60K :idea:
BTW... I have 6SN7's too, so...
You also may have problems with driving cables longer than a couple of feet; the tube, as mentioned, has an output Z of 85k.
A cathode follower, incorporating say a 6SN7, or one of the totem-pole output stages, may be the best solution. Or hang a DI on the 6SL7's output and run it into a mic preamp.
Peace,
Paul[/quote]
Hey thanks Paul :green:
You posted when I did & you answered 2 of my questions.
Interesting... that's what I was doing, running the EXT OUT into my DI then into a Mic Pre. :grin:
I have to use the Input PAD on the DI when doing this though.
(It's an Active RADIAL DI)
Do you guys think a simple Passive DI (Like a 1:10 or 1:15) Mic TX would work better for this :?:
I have some old UTC 150/600:60K :idea:
BTW... I have 6SN7's too, so...
khstudio
Well-known member
Hey I just tried using a passive DI with those old UTC 150/600:60K's & it works GREAT!!! :green:
No need for a PAD.
Just coming out of the EXT OUT jack (which I assume loads the output of the tube correctly?
)
into the UTC...I forgot I made a Passive DI out of one of them years ago.
That Active DI I've been using all along was TOOOOOOO much & too loud for this job... learned a lesson.
No need for a PAD.
Just coming out of the EXT OUT jack (which I assume loads the output of the tube correctly?
)into the UTC...I forgot I made a Passive DI out of one of them years ago.
That Active DI I've been using all along was TOOOOOOO much & too loud for this job... learned a lesson.
pstamler
Well-known member
[quote author="khstudio"]Hey I just tried using a passive DI with those old UTC 150/600:60K's & it works GREAT!!! :green:
No need for a PAD.
Just coming out of the EXT OUT jack (which I assume loads the output of the tube correctly? )[/quote]
Yep. Let's say you're using the 150 ohm tap: It's really seeing an input impedance on the mic preamp of, say, 1.5k. That means the 60k tap will look, to the tube, like 600k, which will make the 6SL7 very happy.
Combine that with hooking the EXT OUT jack to the top of the volume control rather than the slider, and you should have an excellent instrument preamp.
Peace,
Paul
No need for a PAD.
Just coming out of the EXT OUT jack (which I assume loads the output of the tube correctly?
)[/quote]Yep. Let's say you're using the 150 ohm tap: It's really seeing an input impedance on the mic preamp of, say, 1.5k. That means the 60k tap will look, to the tube, like 600k, which will make the 6SL7 very happy.
Combine that with hooking the EXT OUT jack to the top of the volume control rather than the slider, and you should have an excellent instrument preamp.
Peace,
Paul
khstudio
Well-known member
[quote author="pstamler"][quote author="khstudio"]Hey I just tried using a passive DI with those old UTC 150/600:60K's & it works GREAT!!! :green:
No need for a PAD.
Just coming out of the EXT OUT jack (which I assume loads the output of the tube correctly? )[/quote]
Yep. Let's say you're using the 150 ohm tap: It's really seeing an input impedance on the mic preamp of, say, 1.5k. That means the 60k tap will look, to the tube, like 600k, which will make the 6SL7 very happy.
Combine that with hooking the EXT OUT jack to the top of the volume control rather than the slider, and you should have an excellent instrument preamp.
Peace,
Paul[/quote]
I'm not coming off the top of the Volume... I just left it alone (Stock, out of the Wiper, thru the 270k's to the EXT OUT)
Both the Ampeg & my Pre seem to be at their "Sweat spot".
I also think I'm using the 600 ohm tap's... still have to open up the box to see & I'm sorry, it's not a UTC it's a TRIAD.
& YES = REVERSED. :wink:
Thanks Paul.
No need for a PAD.
Just coming out of the EXT OUT jack (which I assume loads the output of the tube correctly?
)[/quote]Yep. Let's say you're using the 150 ohm tap: It's really seeing an input impedance on the mic preamp of, say, 1.5k. That means the 60k tap will look, to the tube, like 600k, which will make the 6SL7 very happy.
Combine that with hooking the EXT OUT jack to the top of the volume control rather than the slider, and you should have an excellent instrument preamp.
Peace,
Paul[/quote]
I'm not coming off the top of the Volume... I just left it alone (Stock, out of the Wiper, thru the 270k's to the EXT OUT)
Both the Ampeg & my Pre seem to be at their "Sweat spot".
I also think I'm using the 600 ohm tap's... still have to open up the box to see & I'm sorry, it's not a UTC it's a TRIAD.
& YES = REVERSED. :wink:
Thanks Paul.
> into my LA2A's 10k
What "10K"?
> I have a few questions:
Maybe I should type slower?
> I don't have any 12DW7's
I know I covered this.
> it sounds like you may even play BASS???
You are talking about a bass amp. And a compressor. It was no deep guess that you play bass.
Do you really have a LA2a? Clone, I hope. Then the hack is trivial. Throw away the LA2a's input transformer. Jam a 6SL7 socket in there. Steal power from the LA2a. Boff three/six holes in the front (you may want a piggyback chassis for the 6SL7 and knobs).
The LA2a's opto-cell circuit needs a ~~75K impedance. Originally this was a 68K resistor plus a few stray Ks. If you don't get in this zone, the ratio will be real screwt-up.
The B15N stages have way more than enough gain. We contrive a loss network which shows ~~270K to the B15 volume pot(s) and ~~75K to the LA2a's opto-cell. Basically we take the ~~150K (270K||270K) out of the B15's mix network, drop another 150K to ground, we see ~~75K. Gain from Vol wiper to opto-cell is about 0.25.
Gain of B15 parts, Vol full-up, is around 20. Loss of 0.25 gives gain of 5. Gain of LA2a from opto to 600 ohm output is about 60. 5*60= 300. Inputs from 20mV to 500mV give output 6V to 150V, on paper. In fact the LA2a's "Gain" pot will normally be well below 1/3rd, and the B15's Vol pot can be Audio taper and set near 1/10th in normal use. Diddle the LA2a Gain and GR controls for "good output" (nice +4dBm limiting level), then bring up the B15 Vol pot for desired amount of peak-mashing.
What "10K"?
> I have a few questions:
Maybe I should type slower?
> I don't have any 12DW7's
I know I covered this.
> it sounds like you may even play BASS???
You are talking about a bass amp. And a compressor. It was no deep guess that you play bass.
Do you really have a LA2a? Clone, I hope. Then the hack is trivial. Throw away the LA2a's input transformer. Jam a 6SL7 socket in there. Steal power from the LA2a. Boff three/six holes in the front (you may want a piggyback chassis for the 6SL7 and knobs).
The LA2a's opto-cell circuit needs a ~~75K impedance. Originally this was a 68K resistor plus a few stray Ks. If you don't get in this zone, the ratio will be real screwt-up.
The B15N stages have way more than enough gain. We contrive a loss network which shows ~~270K to the B15 volume pot(s) and ~~75K to the LA2a's opto-cell. Basically we take the ~~150K (270K||270K) out of the B15's mix network, drop another 150K to ground, we see ~~75K. Gain from Vol wiper to opto-cell is about 0.25.
Gain of B15 parts, Vol full-up, is around 20. Loss of 0.25 gives gain of 5. Gain of LA2a from opto to 600 ohm output is about 60. 5*60= 300. Inputs from 20mV to 500mV give output 6V to 150V, on paper. In fact the LA2a's "Gain" pot will normally be well below 1/3rd, and the B15's Vol pot can be Audio taper and set near 1/10th in normal use. Diddle the LA2a Gain and GR controls for "good output" (nice +4dBm limiting level), then bring up the B15 Vol pot for desired amount of peak-mashing.
khstudio
Well-known member
Maybe I should type slower. :green:
I don't know how but this got WAY off topic...
All I'm trying to do is build an "Output Section" AFTER the Ampegs Pre-Amp.
Yes I built "Clone LA2A's with Jensen 10K:10K input TX's.
( With an 11k hanging across the secondary!)
I was simply using (trying!!!) the LA2A to make up the GAIN I needed & to see how it would sound & work.
You mentioned the need for higher load (85k or so) & I thought you were referring to the test setup I explained in the previous post... but I wasn't sure so I asked. :?
Are we on the same page yet???
Where? I must have missed it. :sad:
I don't know how but this got WAY off topic...
All I'm trying to do is build an "Output Section" AFTER the Ampegs Pre-Amp.
Yes I built "Clone LA2A's with Jensen 10K:10K input TX's.
( With an 11k hanging across the secondary!)
I was simply using (trying!!!) the LA2A to make up the GAIN I needed & to see how it would sound & work.
You mentioned the need for higher load (85k or so) & I thought you were referring to the test setup I explained in the previous post... but I wasn't sure so I asked. :?
Are we on the same page yet???
Where? I must have missed it. :sad:
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