# Practice Amp



## fig (May 19, 2021)

This is not recent, but I've mentioned it a few times...

It's a "Tube Screaming Preamp (B. Robertson)" into a "100 watt" one-channel power amp. From my understanding, these are used a lot to power car subwoofers and such. Being that it is a _only_ power amp there are no external controls, it is simply a gain stage for the signal [there is an onboard gain trimmer on this one. Others I've played with don't have them]. That's where the TSPA comes in. The signal goes into the clipping stage first, and is boosted at the same time, controlled by a gain pot. The signal feeds through a presence pot and straight into the TMB finishing with a volume pot.At least, that's the circuit through my eyes. Then through the power amp. The output is an isolated banana plug.-type jack.

It is a great little practice amp. That particular power amp runs very quiet at 9v despite the obvious lack of wire routing (someday). I was using the same one that is listed as "arriving soon" at this site, and it would handle 20vdc. I powered it with a Dewalt 20v Max battery. I had to use a voltage regular to step the power down to the preamp (9v only). It ran noisy at 9v and was EXTREMELY sensitive. I fried a half-dozen or so before switching to this one.

It was fun to build, and is a hoot to play through. It easily drives an 8ohm 12" Jensen 35w


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## Cucurbitam0schata (Jul 9, 2021)

Wow - I missed this post when it came out! This is super cool, @fig 

What type of speaker/cab are you sending your amplified signal to?


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## fig (Jul 9, 2021)

I've used it with an 1x8 tweed combo which later became the home of the little tube amp I built. Right now, I use it with a hand-made 1x12 extra deep, finger-jointed cab with that Jensen. I've yet to tongue oil it, though I did put the grill-cloth on. Eventually it will be used with the Marshall 74x TMB build, that's been packed away until we move. It has a matching head cab and cloth. I am excited to get it built!

Back to the question....I may build a 1x8 combo cab for it. I built a few 78L09 daughterboards so I can easily pop in in that enclosure and power it with the drill battery (just because it's awesome and portable!)

I will add this to the cab, it's a slip-in battery adapter to power devices. I rewired a 9vdc jack in place of the one shown, and use a PSU pigtail to the amp. It also has a handy USB charging outlet so you can charge your phone while jamming (though of course this adds to battery drain).


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## peccary (Jul 9, 2021)

fig said:


> I've used it with an 1x8 tweed combo which later became the home of the little tube amp I built. Right now, I use it with a hand-made 1x12 extra deep, finger-jointed cab with that Jensen. I've yet to tongue oil it, though I did put the grill-cloth on. Eventually it will be used with the Marshall 74x TMB build, that's been packed away until we move. It has a matching head cab and cloth. I am excited to get it built!
> 
> Back to the question....I may build a 1x8 combo cab for it. I built a few 78L09 daughterboards so I can easily pop in in that enclosure and power it with the drill battery (just because it's awesome and portable!)
> 
> I will add this to the cab, it's a slip-in battery adapter to power devices. I rewired a 9vdc jack in place of the one shown, and use a PSU pigtail to the amp. It also has a handy USB charging outlet so you can charge your phone while jamming (though of course this adds to battery drain).




Did you follow any design when making the cabinet? I've got a little something in the works right now and I'm planning on making a little 1x10 or 1x12 cabinet for it. 

I'm looking forward to seeing your completed combo, that sounds really cool.


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## fig (Jul 9, 2021)

peccary said:


> Did you follow any design when making the cabinet? I've got a little something in the works right now and I'm planning on making a little 1x10 or 1x12 cabinet for it.
> 
> I'm looking forward to seeing your completed combo, that sounds really cool.



The tweed 1x8 is a mojotone basement bargain clearance. For the 1x12, yeah the design was, I called another Tim in NH and said "_Tim, this is Tim. I need a cab...blah yadda...thanks!"_ He does swell work and my router and jigs are also packed up. Now the next 1x8 I will do myself, but it will have to be custom to fit the amp enclosure. I'll take some pics of the cab I had the other fig...er Tim make.

Just got text while typing, the Mesa Booger is on it's way back. They traced it to a tube so they put a whole new regiment in. It's a great amp, but I have a _need for tweed_.


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## Feral Feline (Jul 10, 2021)

Time to play some Moving Blues, Fig; you need a bass player for that?


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## fig (Jul 10, 2021)

Feral Feline said:


> Time to play some Moving Blues, Fig; you need a bass player for that?


Yes, and a guitar player as well. I'll play the cowbell.


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## fig (Jul 10, 2021)

Feral Feline said:


> Time to play some Moving Blues, Fig; you need a bass player for that?


I'd love to hear that BassBass sometime. The tone is remarkable.


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## Bricksnbeatles (Jul 10, 2021)

That looks pretty awesome! I have an enclosure on the way for a cool vintagey solid state amp project I’m planning, and that power amp might be a cool alternative to the Sapphire amp I was planning on making as long as I can come up with a really vibey, lofi-ish preamp


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## fig (Jul 10, 2021)

Bricksnbeatles said:


> That looks pretty awesome! I have an enclosure on the way for a cool vintagey solid state amp project I’m planning, and that power amp might be a cool alternative to the Sapphire amp I was planning on making as long as I can come up with a really vibey, lofi-ish preamp


There are a lot of nice ones...you could pick a separate boost, dirt, and active EQ board if you can't settle on a complete preamp package.


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## Bricksnbeatles (Jul 10, 2021)

fig said:


> There are a lot of nice ones...you could pick a separate boost, dirt, and active EQ board if you can't settle on a complete preamp package.


Yeah, definitely something I’ve been considering. I just need to figure out what kind of sound I’m even going for— needs to fit the aesthetic of the cool enclosure I got, since that’s the only reason I’m going to make one like that. Might even go for a Vox UL730 normal-channel preamp with a built in 1-knob mk1.5 Tonebender (I have an idea on how to keep it just above unity at all degrees of the Fuzz knob’s rotation) if I’m able to fit that many controls on the front panel


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## fig (Jul 10, 2021)

Bricksnbeatles said:


> fit the aesthetic of the cool enclosure I got, since that’s the only reason I’m going to make one like that.


Now I'm REALLY wanting to see the enclosure!


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## fig (Jul 10, 2021)

Hey, this is timely...just opened the mail and I may have a contender to consider?


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## Bricksnbeatles (Jul 16, 2021)

fig said:


> Now I'm REALLY wanting to see the enclosure!


Unfortunately you may not get to— after having no tracking updates in over two weeks, it was just confirmed to have been lost in the mail. This is one of three different packages that USPS has lost in transit in the past week and a half for me. The other two being a rangemaster enclosure and a vintage Frank Zappa cover issue of Guitar Player Magazine. Gotta love the postal service 🙄


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## Feral Feline (Jul 16, 2021)

THAT SUCKS!

Wherever your mail is being routed through collectively, there is an unscrupulous guitar-playing pedal-builder postal-worker who's into Zappa. I wonder if he/she even realises the stuff he/she pilfered was going to the same address...

Send a Zappa-pedal to yourself, with a GPS-tracking device hidden inside it, nail the sucker.


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## Paradox916 (Jul 16, 2021)

fig said:


> Hey, this is timely...just opened the mail and I may have a contender to consider?


What is that?, where did you get it? and I want one!


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## Bricksnbeatles (Jul 16, 2021)

Paradox916 said:


> What is that?, where did you get it? and I want one!


Looks like an L5 preamp board from Aion. I’ve been wanting to get one of those for a while now— really cool stuff.


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## Stickman393 (Jul 16, 2021)

Bricksnbeatles said:


> Unfortunately you may not get to— after having no tracking updates in over two weeks, it was just confirmed to have been lost in the mail. This is one of three different packages that USPS has lost in transit in the past week and a half for me. The other two being a rangemaster enclosure and a vintage Frank Zappa cover issue of Guitar Player Magazine. Gotta love the postal service 🙄



Woof.  That sucks.  

I've always had great luck with the USPS...occasionally something will get hung up...sometimes for like a couple of months... especially during the holidays.  It's rare occasion that one's been lost.

I actively avoid FedEx...yeesh.  Seems like they treat ground service as an afterthought (not far from the truth...considering their business model) AND...just a personal gripe...Their drivers leave packages in front of my garage...which is in a common parking area that divides three  4-unit condo buildings on the opposite side of my building from my front door.  I'll usually grab packages for all 11 of my immediate neighbors and bring em around to their front doors when I see the truck swing by.

Just like...prime territory for porch pirates.  UPS, though?  Solid.

Now let me further hijack this thread with my dissertation on DHL vs OnTrac...*blinks*

I need to build one of these.  Gotta get something on my bench for troubleshooting with an audio probe...


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## SillyOctpuss (Jul 16, 2021)

fig said:


> Hey, this is timely...just opened the mail and I may have a contender to consider?


I've built one of those @fig and it's really super.  It was a fun build too


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## Bricksnbeatles (Jul 16, 2021)

Stickman393 said:


> Woof.  That sucks.
> 
> I've always had great luck with the USPS...occasionally something will get hung up...sometimes for like a couple of months... especially during the holidays.  It's rare occasion that one's been lost.
> 
> ...


Ah yeah, fedex is the worst, but I’ve been having pretty bad luck with the USPS too— this is the first time I’ve actually had them lose multiple packages in a short timeframe, but I’ve had plenty of records broken by them due to mind boggling delivery choices (my personal favorite is when my mailman put a record mailer, which had a few LPs inside so it was a bit heavy, on top of a pile of other packages that were delivered, leaning against the garage door. Obviously, the only way to know the packages were delivered there was to go outside, and the way to do that is… through the garage door. So of course, when I opened the garage door, I was greeted by a record mailer jumping to its death from four feet above— there were no survivors. 😂


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## Paradox916 (Jul 16, 2021)

@Bricksnbeatles just looked it up... I like it!  Definitely on the to do list.


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## fig (Jul 16, 2021)

Paradox916 said:


> What is that?, where did you get it? and I want one!


It's a...oh wait, too late.


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## Mentaltossflycoon (Jul 27, 2021)

Bricksnbeatles said:


> Ah yeah, fedex is the worst, but I’ve been having pretty bad luck with the USPS too— this is the first time I’ve actually had them lose multiple packages in a short timeframe, but I’ve had plenty of records broken by them due to mind boggling delivery choices (my personal favorite is when my mailman put a record mailer, which had a few LPs inside so it was a bit heavy, on top of a pile of other packages that were delivered, leaning against the garage door. Obviously, the only way to know the packages were delivered there was to go outside, and the way to do that is… through the garage door. So of course, when I opened the garage door, I was greeted by a record mailer jumping to its death from four feet above— there were no survivors. 😂


As a regular online vinyl consumer this made me rage shudder.


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## Bricksnbeatles (Jul 27, 2021)

Mentaltossflycoon said:


> As a regular online vinyl consumer this made me rage shudder.


Luckily, despite getting plenty of records in the mail this past week (RSD selections that I missed out on, as well as those that weren’t available in the US), I’ve only had one casualty thus far, and it wasn’t even the USPS’ fault— it was a reissue of REM’s original Radio Free Europe single that was just released, and Amazon sent it in a large box with no packing material, so the actual single came out of the sleeve and was bouncing around in the box, getting badly scuffed in the process. Luckily that was just a regular release, and not one of the very limited releases I had ordered, such as the RSD release of Roland Kirk at Ronnie Scott’s, or Procol Harum’s Grand Hotel reissue.


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## Bricksnbeatles (Jul 30, 2021)

fig said:


> Now I'm REALLY wanting to see the enclosure!


Well, despite the first enclosure being lost, found, and subsequently obliterated by the USPS, I was able to find a suitable replacement, and if I do say so myself, it’s just as cool.
As this one is a bit bigger than the initial enclosure I planned on using (but still quite compact) I’m thinking I might go with two inputs, a built in one-knob fuzz (ala Kustom), preamp gain, active bandaxall tone stack (of course with center detent pots to make a flat response easy), and a master volume, with a big old carling power switch (for that satisfying clunk that a mini toggle just can’t replicate), and a fat old red amp jewel. External bias control for the fuzz on the back panel for the sake of convenience, as well a DI output, and the obligatory speaker out and 2.1mm DC jack


https://imgur.com/ti6T5wa


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## fig (Jul 30, 2021)

That is definitely a worthy enclosure. Good choice!


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## Bricksnbeatles (Jul 30, 2021)

Now that I think about it, I could even easily fit an Accutronics AMC2BF spring tank in the enclosure as well. Hmm… 🤔


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## fig (Jul 30, 2021)

Lot's of room, what are you using as PSU?


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## Bricksnbeatles (Jul 30, 2021)

fig said:


> Lot's of room, what are you using as PSU?


Haven’t decided on what I’ll be doing as a power amp yet, so the power requirements are still to be determined. 
also I’ve already nixed the reverb idea because I remembered I have another cool enclosure that I think would be big enough to potentially use for a stereo standalone spring reverb unit to run synths and drum machines through, as well as to use as outboard recording gear, and I only have two of the reverb pans on hand.


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## fig (Jul 30, 2021)

I can send you a couple of power amps if you'd like. If I did another, I think I'd DIY it anyway (just to do it) . They work really well and might give you an idea of what to expect.


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