my 250 watt tube amp can be a 500 watt tube amp for less than 2 bucks plus tubes - diyAudio
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Old 26th January 2011, 01:19 AM   #1
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Default my 250 watt tube amp can be a 500 watt tube amp for less than 2 bucks plus tubes

so I have a pair of Manley 250 watt amps. 10 6l6's. 250 watts each.
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Old 26th January 2011, 01:21 AM   #2
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The 250 watt model and 500 watt model are identical. same power tranny, same output tranny, same PCB's, same values of componets. difference is the bias circut and the tubes. heres the 250 watt version bias circut.
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Old 26th January 2011, 01:22 AM   #3
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here is the bias circut for the 500 watt model.
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Old 26th January 2011, 01:23 AM   #4
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so basically, add about 4k in resistors, replace the 10 6l6 tubes with 10 kt-90's and your 250 watt amp is a 500 watt amp. list price for the 250 watt version is 7k/pr, list price for the 500 watt version is 9k/pr.
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Old 26th January 2011, 01:48 AM   #5
ChrisA is offline ChrisA  United States
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Originally Posted by speakerfritz View Post
so I have a pair of Manley 250 watt amps. 10 6l6's. 250 watts each.
Until we see measurements most people here will suspect that both the 250W and 500W figures are bogus.

Put the amp on a dummy load and get a signal generaor and measure the output at several different frequencies.

A 500W output transformer is going to have to be full-on huge if it is to work well at low frequency. What you may find is that your 500W amp has limited low end
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Old 26th January 2011, 01:57 AM   #6
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I find it hard to believe that the OPT's are the same. It has been my experience that stuffing bigger tubes into an amp will not increase the power output of an amp. It will increase the potential for greater power output. To tap that potential you need to raise the B+ voltage, lower the load impedance, or both.

Look at all the work I did in the red board thread. I took an 18 WPC amp and extracted 250 WPC. In one of the experiments I had the amp running at 30 WPC. More voltage or lower load brought the red glow of death to the small tubes. I reset the load and supply to the 30 WPC values and plugged in bigger tubes. The power output was still 30 WPC. However now I could lower the load impedance from 6600 ohms to 3300 ohms and get 60 WPC.
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Old 26th January 2011, 03:58 AM   #7
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definitely all parts are identical. same part numbers on the schematic. don't forget that in the music industry tube usage is often driven more by expectations of tone than anything else. not going to debate claims by the manufacture for their various models. I'm simply explaining what I found after being curious of what it would take to make model 250 be model 500.

I am going to make the indicated circut changes and swap out the tubes. I have 25 or so brand new Penta KT-88SC's and this would be an excellent way of putting them to use.
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Old 26th January 2011, 03:33 PM   #8
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Quote:
Originally Posted by speakerfritz View Post
......... with 10 kt-90's and your 250 watt amp is a 500 watt amp. list price for the 250 watt version is 7k/pr, list price for the 500 watt version is 9k/pr.
10 off KT90 is a complete overkill. What I see in front of my eyes on the bench is what counts.
Output Voltage squared/ out load resistance and we are looking at continuous rated figures. I don't believe otherwise.

From my notes; 575V B+ with parallel p-p KT90's (4 tubes) in 43% UL just nips 200W o/p on secondary with 2K A-A.

Looking at the GEC KT88 400W amp which used 8 tubes; (I know it's a poor amp) but it is well known that KT90's can produce 20% more output power so, 10 off, in lieu of 6L6's in the the output stage apart from mis-matching is well out and wasted as is the heater currrent demand.

richy
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Old 26th January 2011, 05:38 PM   #9
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damn but it looks cool though. Like a Corvette, not like a Ferrari.
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Old 24th October 2013, 05:58 PM   #10
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Quote:
Originally Posted by ChrisA View Post
Until we see measurements most people here will suspect that both the 250W and 500W figures are bogus.

Put the amp on a dummy load and get a signal generaor and measure the output at several different frequencies.

A 500W output transformer is going to have to be full-on huge if it is to work well at low frequency. What you may find is that your 500W amp has limited low end
I realise this is an old thread but... the transformer will be big but not quite as big as you imagine...
....as the amount of iron in the transformer required for decent low end drops massively with the primary impedance and turns ratio. 10 KT90s would runa round 400Ohms Raa, the tranny will still be "well big" and may still need a car jack to move it, but not unrealistically big.
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