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Old 30th November 2012, 07:08 PM   #111
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Wolfpunk,

Are you paralelling the primary and secondary of your 1650Ws?

Is that for hifi, or instrument amplification?

Thanks!

Blair
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Old 30th November 2012, 07:27 PM   #112
ChrisA is offline ChrisA  United States
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Quote:
Originally Posted by cotdt View Post
What about paralleling 150 6C33 output tubes and running them OTL?
How many tubes would it take to drive an 8 ohm load to 500W with the OTL design? OK maybe go with a 16 ohm load

I think the only way to go other than making a custom and very expensive transformer is to use many standard output transformers in parallel. Then the question comes up as to how to combine their outputs. The usual solution is to add the output in the air. Back in the days where all there were was tubes that was how "everyone" built kilowatt sound systems. The prime example was the very large Hamon organs installed into large churches. This instruments use 100% tubes and would literally shake the walls of a large building. Typically the amps were instaled inside cabinets. Three amps and several speakers in each cab. Then they would add cabinets until they had the desired sound volume level. The amps were not large but all those drivers would creat some impressive sound.
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Old 30th November 2012, 07:30 PM   #113
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Yes I'm paralleling both and it will be both instrument and hifi. It has line inputs as well as instrument inputs but I designed it primarily for bass guitar amplification which needs a good low frequency response. It has a stereo preamp. One channel with a baxandall style tone stack, the other with a fender bassman style tone stack. The line ins bypass the tone stacks and either preamp channel can be assigned to either power amp and line out via the front panel or foot switch.
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Old 30th November 2012, 07:52 PM   #114
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Quote:
Originally Posted by wolfepunk View Post
That's a lot of tubes! Otl would be cool, it would shave 115 sounds off my amp but would be stuck with a fixed output impedance and also stuck with funky, non production tubes.
To produce that kind of power would not take that many 6C33s if you ran OTL. Also, the 'fixed output impedance' thing is not a concern.

We built a 500-watt OTL using 6AS7s: Atma-Sphere music systems, inc. That took only 42 tubes per channel, you could make 900 watts with 90 6AS7Gs. The 6AS7G is still being made.
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Old 30th November 2012, 08:08 PM   #115
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Thanks Wolfpunk!

I'm just curious as to whether anyone else has used this method and measured the distortion vs. using a single transformer. I have a current project I may use this method for.

Blair
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Old 30th November 2012, 09:15 PM   #116
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Quote:
You will like having 150 output tubes per side.
Think about this for just one second. Each 6C33 draws 6.5 amps of heater current. 150 of them is about 1000 AMPS. That's 6 KILOWATTS just to light the tubes. Game over!

Quote:
I'm just curious as to whether anyone else has used this method and measured the distortion vs. using a single transformer.
I have experimented with this method. I have a large quantity of guitar amp OPT's that are good for anywhere from 25 to 100 watts depending on the driving impedance, load and required low frequency response. I have a succesful EL84 based amp that makes 60 WPC using a common driver for two push pull pairs of EL84's per channel running from 430 volts B+.

Simply paralleling two OPT's results in the effective primary inductance dropping in half. This is OK if the number of tubes is also doubled.

As previously stated I ran a seperate pair of output tubes for each transformer with no connection between the primaries of each OPT. All tubes ran from a common driver with individual bias adjustments. The best results occur with the secondaries wired in parallel, but this often results in a low output impedance if several sets of tubes/OPT's are paralleled.

Wiring the secondaries in series can result in fireworks if one set of tubes is weak and not sharing the load duties. The output current from the other OPT's is forced through the secondary of the weak set, and which can result in a large voltage drop across the secondary, and a huge voltage induced in the primary. Cranking a guitar through a setup like this at levels well into clipping can and WILL invite a visit from the fire gods.

Further experiments are planned in the future, but my tube time has been rather limited now, so I can't say when. If I get anything working soon, and this thread is still active, I'll try to remember to post it here. Otherwise watch this one:

Mono push-pull driver PCB

Previous big powered stuff (525 watts paralleled mono) was here:

Posted new P-P power amp design
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Old 30th November 2012, 11:23 PM   #117
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Quote:
Originally Posted by cotdt View Post
What about paralleling 150 6C33 output tubes and running them OTL?
Ouch! Heater power alone would be in the region of 5kW!!!
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Old 30th November 2012, 11:36 PM   #118
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Let me calculate please... I have 10 fingers on hands.... No more, sorry,,, Let's calculate using 10 fingers... Each tube wants 5K. The typical boutique fancy horn has 8 Ohm in the middle of it's waist. How many of tubes would I need? It is doable. But who wants to pay for such an experiment?
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Old 1st December 2012, 01:00 AM   #119
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Quote:
Let me calculate please......How many of tubes would I need?
The OP originally asked about making a 500 WPC tube amp. Reasonable cost and performance goals were assumed. Later, reality convinced him to look at 200 WPC, then 120 WPC. I don't know what happened to him, but he hasn't been seen on this forum in about 8 months.

It is common knowledge that the transformers in a tube amp represent a big portion of the total cost. In a BIG tube amp that portion could be like 90%. I don't claim to be an OTL expert, but I think a 500 WPC OTL would be big, and produce considerable heat. So for reasonable cost, performance, and thermal load (I live in South Florida it's hot enough), I will assume the use of one or more OPT's per channel.

It has been suggested that multiple small OPT's could be combined to produce a big amp. I have experimented with this, but on a small power scale. Let's scale things up and see if we make big power or summons the fire gods! How big?????

The OP wanted 500 WPC, then 200, then 120. Well I did a 125 WPC amp, and several builders succesfully cloned it. Using Ebay sourced power transformers, and Edcor OPT's the total iron cost is just over $200. Using Edcor OPT's and Antek toroids for power makes that $300, add $150 for everything else.

So lets think BIGGER. The new target was 200 WPC, and the "goal" is 500 WPC! OK, how do we get there.....in other words, how many tubes and transformers do we need to wire together. Lets not calculate please.....lets just wire things up and turn up the juice! OK, lets guess how many to wire up, and what to connect them to.

Recently I have been connecting things up to one of Pete's driver boards. I built that board so that it could drive just about anything, and have recently tested it with a few tubes. The real surprise came when I connected up some ultra cheap 13GB5's and one of the guitar amp OPT's that I used with the EL84's. I could squeeze 150 watts from a pair on 600 volts. See post #66 here:

Mono push-pull driver PCB

So, I propose a simple experiment, wire up 4 pairs to 4 OPT's, get the fire extinguisher ready, and turn the power supply UP!

First I will make measurements on one set at power levels up to 125 watts, then add 3 more sets, and test again. I can connect a real Plitron for comparison, but the comparison wouldn't be fair....the Plitron cost $400 and the guitar amp OPT's were $20 each!

Sound like a plan????????
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Old 1st December 2012, 01:09 AM   #120
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Sounds like a great plan, yes.

It also sounds very expensive to get where you want to be.

BTW, I built Pete's amp for 100W based on your work, thanks very much.

Last edited by scott17; 1st December 2012 at 01:12 AM.
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