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Old 17th August 2013, 05:56 PM   #1
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Default help! Variable Frequency/Varible Q Boost/Cut tone circuit for tube bass amp.

This has been talked a little bit around here before, but I could really use some help on this. I'm thinking of building an all tube bass amp for home recording, and thinking of loosely modeling it after the SVT. Planning on using the bandaxall EQ for low and high shelving. I want to keep the layout of the amp as simple as possible, but also allow for mids to be pushed or scooped, and adjust the frequency where it's pushed or scooped, and the bandwidth of that frequency.

I'd also like to keep the EQ parametric. I don't want to mess with anything graphic. Been reading up on gyators, but I'd like to keep the amp free of op-amps. Using an inductor sounds more appealing to me. But I'm a bit lost at the moment. I'd like to have a mid control that boosts at 1kHz, over a wide bandwith, 2 or maybe even 4 octaves.

Any help to point me in the right direction would be a great help.
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Old 17th August 2013, 06:14 PM   #2
kevinkr is offline kevinkr  United States
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All musical instrument amp threads belong in Instruments and Amps per forum directives. (See forum subheaders) Will move to I & A.

In any event I think you are much more likely to get the answer you need there.
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Old 17th August 2013, 07:13 PM   #3
Minion is offline Minion  Canada
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You can't boost with a passive tone control/EQ , you can only cut , boosting a frequency requires amplification of that frequency which can't generally be done passively so if you want to boost the mids by say 12 DB you are pretty much relegated to going active ....

The other option is to go passive and cut the highs and lows which boosts the mids in relation to the highs/lows .....

What I did with a Tube guitar amp I built last year was use a standard 3 band tone control then put in a FX loop so I can run the preamp output into a 15 band graphic EQ (and other FX gear) .....

Cheers
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Old 17th August 2013, 07:50 PM   #4
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One way I could see you do it is by the use of one tube for bass, mid and highs. Frequency depended feedback controls the boost or cut. Rotary switches could be used for the mid frequency and bandwidth. Check figure 18 at this link: click. You'll probably be able to figure out the switchable caps.
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Old 18th August 2013, 12:24 AM   #5
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Sorry about posting in the wrong location.

funk, thanks so much! This will be a big help!
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Old 21st August 2013, 12:11 AM   #6
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If you're serious, you'll need a real killer output transformer. The last of the Trace Elliot V-6 and V-8 output transformers are on sale. They used 6 or 8 KT88 respectively, at high voltage pushing them really hard. But now you can use 6 or 8 KT120 for a lot better reliability than either an SVT or a V-8, better bass, and more power. The only catch is that you have to find your own bell-end covers as the transformers arrive with an open-frame taped winding. I used the bell-end covers from a 225-watt Peavey VB-2, which fit, but the V-6 and V-8 output transformers have iron laminations twice as thick.

I like the SVT layout with power amp below and preamp up top, as long as you cool it well. It puts the weight low and keeps the heat out of the power amp chassis. But then it really needs a chimney / separator between the power amp and preamp if you want to make one that's convection-cooled with air grates front and back instead of fans for recording and home use. In fact, I'd build a separate power amp and separate preamp on separate 19" chassis, and build a dual case that put the power amps below and controls above but in its own area with some insulation barrier. That way you could swap for an Alembic tube preamp or a Trace Elliot tube preamp. And if your project got sidetracked halfway thru you could still complete the power or pre section.

BTW look carefully at the tube compressor in the better Trace Elliot tube preamps, really handy for getting the optimum tube bass sound and a lot cheaper than adding a tube compressor afterward. I think the new Bassman 300 also has a compressor. The old SVT also has plenty of gain! I worked for a band in the early '70's that used SVTs for both guitars and the bass, and they were pretty darn amazing.

Do you really need 300 watts for recording at home? You might consider picking up a used Harman-Kardon Citation II with its really excellent output transformers and converting it to KT120's like Carver did, then adding a preamp with a variable compressor and variable gain, or even relays for switching between multiple preamps. That would give you the option of using stereo effects between the preamp and power amp.

Of course my dreams are not yours...dream on. All I mean to say is that for a tube bass guitar amp, you might start with the output transformer and the output tubes.

Last edited by cyclecamper; 21st August 2013 at 12:26 AM.
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Old 21st August 2013, 02:22 AM   #7
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Cycle, thanks for your informative input! One of the biggest problems with tube bass amps is that they aren't all that reliable, and they're heavy. Having internal fan cooling can also cause issues for home recording, or any recording environment. But what occurs to me is that with more and more guitarists playing or recording at home with tube amps that are 1 watt to 20 watts, I imagine one could build a tube bass amp that doesn't weigh a ton, is pretty reliable, and is loud enough to compete against 20 watt guitar amps, but quiet enough for getting a good sound for home recording.
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Old 21st August 2013, 04:55 AM   #8
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I thought you said modeled on an SVT, which has 6 output tubes and 300 watts output.

Indeed, that's for filling a large stage outdoors without instruments thru the monitors, not for recording at home.

Seriously, consider a stereo tube power amp and separate preamps then. Though more and more people run a smallish amp into an absorptive isolation speaker box with a microphone, then use transistor amps as necessary for monitoring and coverage.

Bass power amps just have bigger output transformers and bigger interstage coupling caps, as compared to guitar power amps. A hi-fi stereo power amp (not ultralinear) without too much feedback works surprisingly well.

There's also things like the Peavey Classic 120 rack-mount mono power amp, with a bunch of 6L6s in it. Decent bass guitar power amp for really cheap.

I'll hold any comment or advice until you reveal more about what you're lusting for.
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Old 21st August 2013, 08:41 AM   #9
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A 60 to 100W max amp with two or four 4 KT88's respectivly + hammond OPT (1650N or 1650R) would be plenty loud for home and/or studio purpose. My 120W PPP amp can rumble my livingroom to destruction. Preamp with headphone would be even better for the neighbours
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Old 21st August 2013, 05:57 PM   #10
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Funk, that's a lot of what I'm thinking. I'm not really wanting to build a clone, so much as just get a tube bass amp sound that's ideal for recording. Lower wattage has another advantage in that it makes the amp cheaper to build, it makes it lighter, more reliable, you can even omit fan cooling. They're pricey for power tubes, but I've been reading some very good things about KT120's. I'm thinking a tube bass amp in the 50 watt to the 150 watt range would be ideal.

I'm very interested in the Trace Elliot V Type compression circuit, but I haven't had much success finding a readable schematic. If someone has one and could post it, I will be eternally grateful. I'm considering using an optical tube compression circuit, much like what was used on some Teletronix circuits from days of yore. I have no idea if this is similar to the Trace circuit.

Back in the late 90's I bought a solid state Trace. I picked it over a V-Type. At the time I thought, "what is this junky V-Type amp?!? It doesn't even have a graphic EQ!" Man, am I ever kicking myself for that now. Now that I'm older and wiser, I've noticed that every single tube bass amp I've ever heard, be it an SVT, or a Marshall VBA, or the new Fender Bassmans, or those Trace V amps, all of them sound ungodly good, and leaps and bounds better than any solid state bass amp I've ever heard. Sadly, they either cost a fortune or are difficult to find.
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