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Vacuum tube based 2.1 Home Theatre Amplifier

Wharfedale Evo 4.2

radium7

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Basically I’m not interested in using tubes for movie sound, my opinion modern movie sound have digital base, and they are fast which require speed circuits. Solid state amps are best for such sound than tubes. Besides that for a 5.1 system 6 OPTs, and 6 individual channels needed. Hard to find tube based low pass filter circuit. Lot of money and more complex.....so i avoided.

But I like cinema scope movies (Hollywood) and have noticed the difference in sound quality of those movies which is more live. Recently I got chance to meet a technician who is in cine-field for a long period. He gave lot of information about cinema scope sound and DTS. He explained how both are very close in technical aspect and differs in quality. He asked me to try tube amps for old movies. After that meeting I plan to built a fully tube based 2.1 amp. After searching a lot I found 2 circuits first is 5W+5W+8W output and second is 14W+14w+18W amp, both circuits are RCA amplifier circuits. 3 channel amp, 2 for right and left channel 1 for woofer (have tube based low pass filter no solid states)

I decided to build the first model

Tubes used: 6FQ7-2, EL84 -3, GZ34-1
 

radium7

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placed order for tube sockets and control
 
Last edited:

reubensm

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What nice work, 2 queries: (1) for cinemascope, 3 equivalent channels were used, right? left, right and center?
Also a regarding the chassis paint job, what process is used? enquiring due to the heat factor.

edit - actually 3 queries, the metal work, done locally?
 

radium7

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What nice work, 2 queries: (1) for cinemascope, 3 equivalent channels were used, right? left, right and center?
Also a regarding the chassis paint job, what process is used? enquiring due to the heat factor.

edit - actually 3 queries, the metal work, done locally?
This amp has 3 channels left, right and woofer(sub woofer) . cinemascope is a complex system with four track sound system. sound recording quality is very high and imaging is more live. In CDs they are in stereo format so this amp is best for those type of movie sound. important thing is the woofer section has low pass filter circuit which is designed with vacuum tube and not solid state.
chassis made locally in Madurai,Tamilnadu and painting- powder coating.
 

reubensm

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I decided to build the first model
Tubes used: 6FQ7-2, EL84 -3, GZ34-1

Sorry for being so curious, how to you plan to wire the EL84s? If its one per channel, how would you get 5+5+8 watts out of it. Would you know if one used 4 x EL84 for a single channel power output, running off say, 300 volts (6.3v heaters), what wattage can one expect?
 

radium7

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Sorry for being so curious, how to you plan to wire the EL84s? If its one per channel, how would you get 5+5+8 watts out of it. Would you know if one used 4 x EL84 for a single channel power output, running off say, 300 volts (6.3v heaters), what wattage can one expect?
EL84 connected in single end pentode mode can easily deliver more than 5W. fix the operating points for 5w for 2 channels and change them to deliver 8w for woofer.

for 4 X El84
May be they are connected in parallel push pull. you may aware of triode, ultra linear, pentode mode of operations and biasing types.
power delivered an tube amp will depends on mode of operation, biasing type, operating points, etc.,
i will give few approximate power output values (for accurate value you have to do some paper work)

4 X EL84 parallel push pull - triode- cathode bias = 14 watts
4 X EL84 parallel push pull - ultra linear - cathode bias = 25 watts
4 X EL84 parallel push pull - pentode - cathode bias = 30 watts

4 X EL84 parallel push pull - triode- fixed bias = 20 watts
4 X EL84 parallel push pull - ultra linear - fixed bias = 35 watts
4 X EL84 parallel push pull - pentode - fixed bias = 45 watts

if you check the datasheets can find out easily.
 

yogibear

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Above would make more sense if accompanied with distortion levels.
 

reubensm

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Heavy power transformer (local custom made) for continuous running

View attachment 33869

View attachment 33871
Another curious query, based on the output transformer - one challenge faced when looking at hot-rodding a mono tube amp with stereo conversion is the capability to replicate the existing output transformer. Can this be done? or is it better to go in for 2 new output transformers. Also on a say, 300volts rail, is it safe to perhaps half the output 4 x EL 84 to 2 x EL84 and add another channel instead with 2 EL84? Both channels are to be driven by EF86, ECC83 combos (Mullard 5-10) Thanks in advance.
 

yogibear

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Another curious query, based on the output transformer - one challenge faced when looking at hot-rodding a mono tube amp with stereo conversion is the capability to replicate the existing output transformer. Can this be done? or is it better to go in for 2 new output transformers. Also on a say, 300volts rail, is it safe to perhaps half the output 4 x EL 84 to 2 x EL84 and add another channel instead with 2 EL84? Both channels are to be driven by EF86, ECC83 combos (Mullard 5-10) Thanks in advance.

Replicate the mono amp and use two new but similar spec OPTs and also the power transformers.
 

radium7

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Another curious query, based on the output transformer - one challenge faced when looking at hot-rodding a mono tube amp with stereo conversion is the capability to replicate the existing output transformer. Can this be done? or is it better to go in for 2 new output transformers. Also on a say, 300volts rail, is it safe to perhaps half the output 4 x EL 84 to 2 x EL84 and add another channel instead with 2 EL84? Both channels are to be driven by EF86, ECC83 combos (Mullard 5-10) Thanks in advance.

for 4 X EL84 the pri imp of OPT will be 3,5 to 4.5K, but for Mullard 5-10 you need 8K with 43% tap for UL loading. so you can not replicate the old one.
Regarding power txr for mullard 5-10 stereo conversion you need 300-0-300V/180mA, 6.3/6A, 6.3/2A (rectifier).

As your monoblock has 4 X El84 it must have 180mA current rating for HT and 6A for LT. you can use the same power TXR if it deliver 300v-0-300v AC secondary voltage.

Try to operate amp in correct voltages specified in the original circuit at every point. if it changes it works but you can not get the correct sound quality of the specific circuit. if it deviates more than 10% it works, but affects the tube and OPt and shorten their life, they fails quickly.

for the same pwr txr the B+ will be different for tube rectifier and diode rectifier(higher).
 

yogibear

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Your build is similar to mine except that mine has much larger OPT for bass channel than the other two channel OPTs.
 

radium7

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Large size OPTs will give good frequency response. I build this amp as a trail and rear project, don't want to invest more in it. All the transformers(power and OPTs) used in this is amplifier wound by myself and working fine.
Regarding the sound quality, It is a perfect match for movie sound no booming bass, with good bass and lively mid. For bass channel i uses philips 8" woofer, not a sub woofer.
This amp is not suitable for hifi system or for listening songs. The sound signature is different from ordinary stereo amp.

Best for movie sound.
 
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