Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: music321 on August 22, 2019, 05:29:50 pm
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Hi,
This is my first post. I've decided that I want to build a power amp for use with a modeler. I want to do this so that I can have the EQ effects of an output transformer, and a lot of "thump" from the amp. I don't want the power amp to "color" the tone of the preamp. I've looked around for a tube PA layout, but haven't found anything. I'm thinking of using just the power section of an old tried and true tube amp, but don't know if these power sections color the tone. My guess is that they probably don't do so to an appreciable degree.
I'd like to use a 100 watt power section, but might go with a 50, or less, instead. I looked at an old popular tube amp, and found that the 100 watt version weighs 6.6 lbs more than the 50 watt version. This brings me to my first question: How much can I expect an amp built just as a power section to weight relative to the entire amp? Building just a power section would allow me to shrink the size of the wooden cabinet and the chassis, but I'd still have the same transformers, which seem to be the main source of the weight. So, as a rough estimate, could I assume that an amp built as just a power section would weigh roughly 90% of the weight of the entire amp, or is this inaccurate? I know I won't get an exact answer, I'm just trying to get a ballpark figure. If this whole thing is over about 30 lbs, I think I'll go to a smaller amp.
When replacing power tubes, would I need to replace all of them if a push-pull amp were designed so that I could bias all the tubes independently?
Would this idea be cheaper if I were to use a tube other than the el34?
Would a power amp need as much shielding as a complete amp? It's my understanding that preamps are more sensitive to interference, and I've seen power amps sold in which the transformers are facing the same direction.
Thanks
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https://reverb.com/item/26120972-peavey-classic-series-stereo-tube-power-amp?gclid=CjwKCAjw-vjqBRA6EiwAe8TCk9_npLw7BcFmdCT4i_cvJzssHSDuNTrRFcmxplI6zCC8pEPoSPk_oBoCwS4QAvD_BwE&gclsrc=aw.ds&merchant_id=101081903&pla=1&utm_campaign=672877369&utm_medium=cpc&utm_source=google
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https://reverb.com/item/2710509-carvin-ts100-tube-power-amp (https://reverb.com/item/2710509-carvin-ts100-tube-power-amp)
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Thanks for the links. I didn't know that tube rack mounted power units were available. I see that the second unit, and possibly the first, has 12zx7 tubed in addition to the el34 tubes. Out of curiosity, why is this, since it's just a power amp?
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Power tubes in Push Pull require a Phase Inverter to drive them. The Phase Inverter is usually a small bottle double triode. The Phase Inverter is considered psrt of the Power Amp.
A "power amp" might contain additional preamp tubes.
There is no such thing as a colorless amp. E.g., every gain stage produxes a new set of harmonic overtones. No gain is perfectly linear with regard to incoming signal shape. Output trabsforners color tone.
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If you don’t want the power amp to affect the tone, why not use a suitable solid state amp?
Output transformers don’t per se work differently with tubes than transistors.
Just need one with suitable impedances, simple 1:1 windings would seem to be applicable here.
Some older solid state amps even had built in output transformers, to facilitate max power output into a variety of load impedances (Peavey?).
A whole lot simpler, lighter, cheaper than building a tube amp, even if you need to get an OT custom made.
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SS amps also color tone. There is no immunity from diversion from the input signal. IMHO as long as tone will be colored anyway, then color it tube.
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The weight difference between a standalone poweramp and a complete regular amp will be negligible.
The only difference would be a few preamp tubes and a handful of other components which probably weighs less than 1 lbs.
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A 50 watt (2 6L6) or (2 EL34) power section would be plenty. I've built a couple 100 watt amps, it's really too much. And if weight is a concern, the 50 watt transformers would be a tad lighter.
I'm not trying to scare you off, but the $$$ to build a DIY, if you even buy standard grade components (no premium stuff) , is gonna be higher than a decent used rack mount tube poweramp.
If you do decide to build, you're at the right forum. Hoffman's info pages, store, and forum are a wealth of knowledge.
Basically you would build the power section of *any (almost) reputable amp design, plexi , bassman, etc. You do need 2 12ax7s even for a poweramp. One full 12ax7 for the phase inverter, and half of another 12ax7 for input gain. You can build it so it's as tonally transparent as possible.
Also, with a DIY, you can choose from several different biasing methods, and even have different biasing easily changeable with toggle switches.
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https://www.mojotone.com/kits/BritishAmpKits_x/Mojotone-British-45-Watt-Style-Head-Amplifier-Kit_2?whence=
This amp kit would cost $700, and if you built it for a poweramp only, you would be omitting one preamp tube and 20 or 30 components. All you need is there, transformers, chassis, head box, all wiring, components, etc.
A few changes to the schematic of a jtm 45, and one heck-of-a-poweramp is born !
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Marshall 9005 50/50 stereo (which I assume you want) is going for $7-800 on fleabay. I understand the desire to build something but you will spend a lot of money.
I run my Tom Scholz Rock Modules into a SS stereo power amp and it sounds great. Heck you can buy a helluva SS Stereo Power amp for $100 on fleabay.
Not to sound cork sniffer but your typical modeler is not looking for the holy grail of tube tone. Fun project just depending on how deep the pockets...
Jim
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then color it tube.
:thumbsup:
and if you don't like the colors, use your augmented music box too change the color :icon_biggrin:
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I built a Dynaco based stereo power amp, minus the UL OT, for use with a modeler, on the forum here. Anyway, about when I was almost done I ran across a used Peavey 60 Stereo power amp. If I had found that first I either would have or should have just picked that up.
However, my project really turned out well and July 4th last year had a chance at an outdoor party-picnic, had a chance to crank it up about 70% Performed beautifully. Train Wreck OT and a 100 watt Carvin PT. Another possibility is to use powered speakers. Simplifies lots of things. They don't color the sound much if at all.
Really, you and perhaps any other tube amp aficionado will be the only ones that could hear the difference. The modelers are so close and sound so great with all the effects included, only the young and strong backed are enthusiastic about lugging hundreds of pounds of gear around anymore.
silverfox.
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Here is bar-none the best Internet resource for a DIY tube power amp...
http://ax84.com/corepoweramps.html
Pick your wattage, PP or SE, and build-away!
I've built the 20W and the 50W PP amps and they are rock-solid and great sounding designs.
The one at the very bottom of the page is for the stand-alone 50W. If you want 20W just add the input section from the 50W stand-alone as is to the 20W design. For even more flexibility, look at the October in the Associate projects section for a variable negative feedback circuit and add that to it as well.
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Old post.
The old Axe84 and later sites are not functioning anymore.
Looking to build a mono tube amp with hi/low inputs, input sensitivity pot, presence and master volume. No tone controls.
6L6/5881 power.
Thanks!