Hi I'm.new here but have been browsing for a while and thought I would join as I've decided to have a go at a project, I've ordered a mona el34 se amplifier from hifi-exquis in China and decided I want to mod if to a kt88 amp.
The reason I chose to buy this amp and mod it is the fact it's already together and working and I like the look of the chassis I'm new to tubes but have been reading and I've already got a new power transformer and a 4 amp 5v for the rectifier as my new power trans have no 5v tap.
Has anyone done similar?
The reason I chose to buy this amp and mod it is the fact it's already together and working and I like the look of the chassis I'm new to tubes but have been reading and I've already got a new power transformer and a 4 amp 5v for the rectifier as my new power trans have no 5v tap.
Has anyone done similar?
Attachments
Thats a bit of work involved those two tubes have different operating conditions as well as primary inductance though the primary impedance is near similar .
As the output transformers are designed for EL34 and not KT88 which can obtain =100Watts (depending on HT voltage -screen/grid etc voltages ) you will need to limit the KT88,s to suite the existing output transformers specs.
"Plug&Play" doesn't work in cases like this unless you want far from optimum reproduction .
Looking at the long list of differences in operating conditions at different voltages you will first of all need to specify the output transformers operating specs and then build the KT88 to their specs this could result in a lowering of the output wattage for one and there are many more differences.
It can be done but get a full spec. tube replacement manual before you start ,shortened versions in this case are useless.
As the output transformers are designed for EL34 and not KT88 which can obtain =100Watts (depending on HT voltage -screen/grid etc voltages ) you will need to limit the KT88,s to suite the existing output transformers specs.
"Plug&Play" doesn't work in cases like this unless you want far from optimum reproduction .
Looking at the long list of differences in operating conditions at different voltages you will first of all need to specify the output transformers operating specs and then build the KT88 to their specs this could result in a lowering of the output wattage for one and there are many more differences.
It can be done but get a full spec. tube replacement manual before you start ,shortened versions in this case are useless.
Thanks for that, and yes I will be getting some new output transformers that are designed for kt88 at 20w as I don't think there would be any point trying to modify it using the originals as they are going to be low quality/power units.
One thing tho, could I use my new power trans in place of the Chinese one without a re bias and different rectifier or would it be risky, I think the Chinese one is 315-0-315 not sure of the ma and mine is 330-0-330.
One thing tho, could I use my new power trans in place of the Chinese one without a re bias and different rectifier or would it be risky, I think the Chinese one is 315-0-315 not sure of the ma and mine is 330-0-330.
What results are you expecting from this change?
The real limitation in that amp is the small size of the output transformers.
They list 8W per channel. To get that from EI66 SE iron is optimistic to say the least. 🙄
The real limitation in that amp is the small size of the output transformers.
They list 8W per channel. To get that from EI66 SE iron is optimistic to say the least. 🙄
I'm am going to get a set of 20w output transformers designed for the kt88, from what I've read it seems 5k may be the best to go for but I think 3.5k would be okay too.
Why not use a KT77? I believe those would be equivalent to EL34s.
As for 315 vs 330 that is well within 10% which should be within tolerance of any design anyways.
As for 315 vs 330 that is well within 10% which should be within tolerance of any design anyways.
Thanks and that's what I thought, as for using the kt77's I could do but I would still be limited by these output transformers so would still realistically only get 6wpc? Whereas if I go bigger and use kt88 I could get at least double which is better for me and then I could always rebias to use kt77?
I do have a set of eh6ca7 which I will use untill I've got the new output transformers.
I do have a set of eh6ca7 which I will use untill I've got the new output transformers.
315-0-315 ---at what current (it does matter ) ?
In class AB1 using KT88,s you would be under running it so not any real problem .
At 300V as a single ended tube it takes-130mA-- at AB1 at the voltage you are using the current is much less.
As the wattage you want is low have you thought about running it in class A --triode mode (screen connected to anode ) ?
What I meant is the Chinese transformer will run at 315-0-315 and I will change it out to the British one I've got that's 330-0-330 and continue to run it with stock output transformers with el34's until I get some new ones and then change it to run kt88's, sorry if I was confusing.
And I thought 15wpc was around the highest your going to get se?
Sounds like you really want KT88s so if you just want to be able to use different tubes why not KT88 and 6550, then you don't have to go backwards to the EL34 KT77.
That's the plan really bud the amp will come as a el34 amp and I plan to mod it so as to make it kt88/6550 I would of gone kt120 but my power transformer isn't up to it.
Se is class A my equivalent books put output at 27W (at a set voltage ) but read-
KT88 - KT88.pdf
I also think you really want -KT88,s ,bias will need to be adjusted if you swop KT88,s for 6550,s.
Is it Fixed bias or Self bias on the output tubes?
6550 Maximum g1 resistance:
50k for fixed bias, 100k for self bias
KT88 Maximum g1 resistance:
100k for fixed bias, 220k for self bias
EL34 Maximum g1 resistance:
500k for fixed bias, 700k for self bias
The driver stage probably will not be able to drive the output stages 50k or 100k g1 resistor.
Food for thought.
And yes, if you get new output transformers (even if you get them specified for KT88), try them on the EL34 first (easy test, and the results may surprise you).
The driver tube
6550 Maximum g1 resistance:
50k for fixed bias, 100k for self bias
KT88 Maximum g1 resistance:
100k for fixed bias, 220k for self bias
EL34 Maximum g1 resistance:
500k for fixed bias, 700k for self bias
The driver stage probably will not be able to drive the output stages 50k or 100k g1 resistor.
Food for thought.
And yes, if you get new output transformers (even if you get them specified for KT88), try them on the EL34 first (easy test, and the results may surprise you).
The driver tube
Thanks and I haven't got it delivered yet but from what I've seen of other similar amps from China they use a 500k cathode resistor I'm waiting on a schematic and will check what's going on once it arrives, also they state auto bias and from the pics I've seen there is no pots but will have a look when it comes.
Sorry I meant 500 ohm, actually I think it's 300ohm they use but will have to check.
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You were right the first time cathode resistor =525ohms,
all the "K" range are near that value when used for audio power amp applications .
Thanks like I say I'm new but keep reading and trying to learn it's completely different to fixing ss amps as I've done in the past but I will get there.