I wonder if a 12AX7 could be used in place of the ECC82. Not any deal breaker on my side and it would actually save me money on tubes. I really like the auto bias board. Nice that you could switch between to two modes on that board.
Thanks so much Snapper. We could continue this in PM if necessary so I do not keep derailing this thread.
James
Thanks so much Snapper. We could continue this in PM if necessary so I do not keep derailing this thread.
James
wwwtttwww,
can you give us some feedback on how it sounds and how you set the amp for KT88?
I see you have 5k Raa, what is the voltage you used?
I would guess around 420V at 70mA idle, with around 55 Wrms per channel.
How do you changed other details?
Sockets seem quite close. Is the suggested 4 inch inter-socket respected?
Thanks!
can you give us some feedback on how it sounds and how you set the amp for KT88?
I see you have 5k Raa, what is the voltage you used?
I would guess around 420V at 70mA idle, with around 55 Wrms per channel.
How do you changed other details?
Sockets seem quite close. Is the suggested 4 inch inter-socket respected?
Thanks!
zintolo,
Your guess is very accurate. H+ is 425V, Ia 70mA. But I will increase a little bit higher.
I use an existing chassis and socket layout for the same tube under higher H+ and larger Ia. The distance between the tube is 65mm.
Another mod is the two MJE340 instead of 2N5551 so I can use larger current for driver. The current is 3.5mA.
I don't use H+ on delay. So I modified -Vbias circuit to make sure it gets full -Vbias before the tubes are heated up.
Your guess is very accurate. H+ is 425V, Ia 70mA. But I will increase a little bit higher.
I use an existing chassis and socket layout for the same tube under higher H+ and larger Ia. The distance between the tube is 65mm.
Another mod is the two MJE340 instead of 2N5551 so I can use larger current for driver. The current is 3.5mA.
I don't use H+ on delay. So I modified -Vbias circuit to make sure it gets full -Vbias before the tubes are heated up.
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Hi Dagwood,
mount them on the opposite side of the tubes, insulating pins to safely cross them.
This question aside, most probably during the winter I will try how a cascoded version of the driver will sound. This could be an upgraded version (with proper spacing for KT88 as well) of this project.
But before that, I want to try the PP version of the unset with shunt cascode driver.
mount them on the opposite side of the tubes, insulating pins to safely cross them.
This question aside, most probably during the winter I will try how a cascoded version of the driver will sound. This could be an upgraded version (with proper spacing for KT88 as well) of this project.
But before that, I want to try the PP version of the unset with shunt cascode driver.
In most cases with most tubes it is not needed to switch to MJE340, so before doing it just consider if you exceed the Vce or it is needed to have more current for any reason.
With EL34 or KT77 it is not needed, with KT88 at that voltage (bias should be around -47V), working in AB1, you need around -140V on the negative side and +40 on the positive side, so yes, the top 2N5551 on powerdriver should be substitued with something beefier, but not strictly the MJE340 (I used them because I had them). A TO92 with an higher Vce can be used.
With EL34 or KT77 it is not needed, with KT88 at that voltage (bias should be around -47V), working in AB1, you need around -140V on the negative side and +40 on the positive side, so yes, the top 2N5551 on powerdriver should be substitued with something beefier, but not strictly the MJE340 (I used them because I had them). A TO92 with an higher Vce can be used.
Yes I understand all that, but to suggest that you CAN'T run KT88s without making this modification is simply not true.
Even to suggest that it will somehow create an amp that does not sound good it also not true.
So again, is it better? Maybe so. Is it required, absolutely not. Let's not suggest to people that you can't use KT88s without making this modification. You most certainly can use them. There have been many successful builds using KT88s without making this modification.
gabo
Hi gabo,
Can we say that it is not true that I told what you are saying I did?
Friendly,
Roberto
I've never told you CAN'T, I've told it depends and explained why.
I've never told it will not sound good.
Again I've never stated that, so it is not true that I told this.
Can we say that it is not true that I told what you are saying I did?
Friendly,
Roberto
Sure, no worries. Just wanted to be clear.
This statement from the previous post doesn't explicitly say you can't use the KT88, but it implies that.
But no worries, just don't want to confuse anyone that wants to build the the standard BH and use KT88s.
It's a DIY forum, so modifications and suggestions to improve things are always welcome. Keep up the good work.
gabo
Yes, with KT88s I'd always use another BJT for the top of the CCS for the reasons explained above, but it is not mandatory to do so.
Back on mods, with B+ in the 420-450V range I've experimented two solutions for the PI:
- 220k plate load and CCS set just over 1.4mA
- 270k plate load and CCS set just below 1.3mA
The latter has more gain and keeps the tube out of the grid current area.
It overdrives differently.
Back on mods, with B+ in the 420-450V range I've experimented two solutions for the PI:
- 220k plate load and CCS set just over 1.4mA
- 270k plate load and CCS set just below 1.3mA
The latter has more gain and keeps the tube out of the grid current area.
It overdrives differently.
Thanks for all the input on this guys, I'll do some more homework but will definitely go KT88 or KT90
.
My current BH build is stock EL34 monoblocs with a few of Ian's (gingertube) tweaks. I've used a 9v battery to power the bias LCD so just waiting on the holder to arrive and I'm done
Idh Flat Mount 9V Electric Guitar Bass Battery Case Box Holder Cover Black-buy at a low prices on Joom e-commerce platform
Cheers
.My current BH build is stock EL34 monoblocs with a few of Ian's (gingertube) tweaks. I've used a 9v battery to power the bias LCD so just waiting on the holder to arrive and I'm done
Idh Flat Mount 9V Electric Guitar Bass Battery Case Box Holder Cover Black-buy at a low prices on Joom e-commerce platform
Cheers
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Thank you Francois,
I'm very slowly proceeding in designing the pcb for the hybrid cascoded BH, including the power supply on the pcb as in the first run of the BHEL34 (with bigger caps), and separate positive and negative supply of the powerdrive to easily allow different voltages (in module). This will allow to run big tubes even in AB2 with less than 1.5Vrms at its input.
I will post updates when available.