6550's in Blackheart BH100H Hothead

[Mattallica]

I recently came across some information on eurotubes.com about a modification to the Blackheart BH100H hothead that allows it to run 6550 power tubes instead of EL34's. From their description, it's not so much a modification as it is a simple bias adjustment. Something about taking the cathode bias up to 50ma?

Their info left me wondering several things. The hothead is fixed bias in class AB mode and cathode bias in Class A mode. Did they have to change an actual resistor to get the class AB fixed bias into an acceptable range for the 6550's? There's a bias pot for class A cathode inside the amp on the PCB looks like you can get a tiny screwdriver into. I'm not sure what the stock fixed and cathode bias is from the factory, but it's worth noting that it's dialed in for JJ EL34 (not the EL34L's).

Here is a pic of the 6550's running inside of the hothead, boy they sure do look awfully hot!


...and a link to the page on eurotubes site

I'd like to do this to my BH100H amp. Any input on whether this is a safe thing to do? If so, what bias settings would you recommend? Thanks!

[Mattallica]

Re: 6550's in Blackheart BH100H Hothead

I also read somthing about el34s using some type of screen grid resistor at 220, that needs to be changed out to one that reads 100 for the 6550s? im guessing that if i swapped this screen grid resistor and cranked the bias up to around 40-50 from 30-40 then the amp and the 6550s would be shaking hands.

[Mattallica]

Re: 6550's in Blackheart BH100H Hothead

Been emailing with Michael from eurotube on this and apparently all that has to be done is turn the fixed bias trim pot up to 50-52ma to run kt88s or 6550s. Plenty of sweep on the pot to get up to this value, no need to even change the screen grid resistors. I'll be doing this re-tube pretty soon with my hothead, ill make sure to update after doing so. Pretty exciting prospect for hothead owners.

[Hamstein]

Re: 6550's in Blackheart BH100H Hothead

Hi, I just got one of these amps myself, with the notion of tinkering about with it! - so thanks for posting this info.

Have you got around to trying the different tubes in it yet?

I've been rooting through the preamp lately in order to find out which bit does what, as I want to try and increase the bass response from the amp.

Currently thinking about changing the coupling capacitors to 0.047uF for starters.

Also I've been reading about Resonance control mods ( on Marshalls ) and wondering if I can implement something similar on the Hothead.

[grimm26]

Re: 6550's in Blackheart BH100H Hothead

I'm interested in your experience with putting 6550's in there, too. I just read the info at eurotubes about this amp a couple days ago and happened to see a new 'B' stock on ebay for $150 and decided that I couldn't go wrong for that price.

[Hamstein]

Re: 6550's in Blackheart BH100H Hothead

Ok, well I've been experimenting.


Changed the preamp coupling capacitors from 0.022 uF to 0.047.



...and changed the power amp input coupling caps from 0.022 to nice big sprague orange drop 0.1 ' s!



This seemed to warm up the sound quite a lot, and passed more bottom end through the chain.

However, I next added a resonance control pot mod, using 1 meg pot and 4.2 nF capacitance ( 0ne,2.2 nano and one 2.0 nano - it was all I had! )- this brings the bottom end out nicely giving a little bit of boom and thud! No pics of this yet as I was in too much of a hurry to slap it back together.
Next time I'll increase the capacitance on the reso mod to see if I can get a bit more bottom end boost...........

[Axeman67]

Re: 6550's in Blackheart BH100H Hothead

Hi Hamstein ,

how did Your mods turn out ? I also have a BH100H, and would like to
change the tone more towards old JCM800. Also I'd like to have more distortion
on channel 2. Have You had any trials with changing preamp tubes ?

I have tested my BH with Bugera 4x12, an old marshall 4x12 cab and an old
musicman 2x12

bh100 sounds better with bugera cab, but I can't the old jcm800 + marshall 4x12
bottom end even with using marshall4x12 cab with bh100

[Hamstein]

Re: 6550's in Blackheart BH100H Hothead

Hi,
It is quite hard to get good bottom end from the Bh100, the 'resonance' mod definately works well, it is the easiest way to get more bass, and I'd be tempted to raise the value of the capacitance a bit more when I get round to it. Also I think I would hard wire it rather than using a potentiometer, as I just leave it on full all the time!

As I've got mine at the moment I've discovered it sounds awesome used with my Rickenbacker bass and Acoustic model 406 cabinet! Class A mode, full power setting,has the most full sound for this! I like it!

Using Les Paul, through my '70s marshall 4x12 it is a bit brighter than through my 1960bc classic (25w greenbacks) - Both is nice balance. Generally the louder it goes the better the bottom end becomes.

I havent tried swapping out tubes as there is generally enough gain for me.

I'm a bit limited for time to footle with the amp at the moment, but keep nudging me and I'll try out a few more mods in future, simple alterations to the component values in the tone stack should render some more improvement.

[Axeman67]

Re: 6550's in Blackheart BH100H Hothead

Hi

I haven't tried it with a bass

Did You find a schematic for the amp or did You tarce it Yourself ?
I already thought about removing the PCB and tracing it , but decided
not mess with that for the time being , to avoid breaking the thing

So, how is this resonance mod actually done ?

[Hamstein]

Re: 6550's in Blackheart BH100H Hothead

I just traced back the bits of circuit I thought would be applicable, looks like the Hothead is mostly a Marshall style circuit.
The reso mod consists of putting a 1 megohm Log potentiometer and 47 nF capacitor into the negative feedback loop. Easy on the BH, the nfb wire is labelled and easy to find!
When I get time I'll post a pic or two. In the meantime Google 'Marshall Amplifier Resonance mod' and you'll get the idea!

[Axeman67]

Re: 6550's in Blackheart BH100H Hothead

Today I did some A/B testing with JCM800 + 1960Lead cab vs BH100 + Bugera 4x12.
I thought that I should get more distortion on BH100 since it has more Preamp tubes on F'n Loud channel , but only a slight amount. Tone point of view marshall is better,
but BH + Bugera comes close . BH works better with Bugera4x12 slanted than with
Marshall 1960Lead cab.

[Mattallica]

Re: 6550's in Blackheart BH100H Hothead

Hi, I just got one of these amps myself, with the notion of tinkering about with it! - so thanks for posting this info.

Have you got around to trying the different tubes in it yet?

I've been rooting through the preamp lately in order to find out which bit does what, as I want to try and increase the bass response from the amp.

Currently thinking about changing the coupling capacitors to 0.047uF for starters.

Also I've been reading about Resonance control mods ( on Marshalls ) and wondering if I can implement something similar on the Hothead.

I have called Loud Technology about what preamp tubes do what, and here is what he had to say:

V1: Main tube for tone variation...This is the most important one in terms of changing the sound of the amp (it's connected to both of the gain knobs on loud and f'in loud channels)

V2: Tone stack ch1 "Loud" (the bass, mid, and treble knobs)

V3 & V4: Tone stack ch2 "f'in Loud" (the bass, mid, and treble knobs again)

V5: fx send & return, this is also probably important in the way it affects the gain boost that can be used in place of the fx send/return.

V6: Phase Inverter

I hope that helps mate. I haven't yet had the chance to try this tube swap, but I will be sure to post about it when I do. Could be next month, could be next year lol! Cheers!

[Mattallica]

Re: 6550's in Blackheart BH100H Hothead

Hi

I haven't tried it with a bass

Did You find a schematic for the amp or did You tarce it Yourself ?
I already thought about removing the PCB and tracing it , but decided
not mess with that for the time being , to avoid breaking the thing

So, how is this resonance mod actually done ?

I have a schematic for this amp that I obtained from Loud Technologies, pm me with your email address and I will send you the schematics in an attachment.

-Matt

[Mattallica]

Re: 6550's in Blackheart BH100H Hothead

Thanks for posting the info on changing the caps & the resonance mod! Greatly appreciated fellas!

Seeing how similar this amp is to a Marshall Circuit, any chance someone could come up with a "cascaded gain stage" mod ala Randy Rhoads? You know, like the white amp sounds...

Thanks again for the mod info, you guys rock!

And remember, I have the factory schematics for the BH100H if anyone wants/needs them

[JP6L6]

Re: 6550's in Blackheart BH100H Hothead

Hello,

First please excuse my English because I'm a French speaker from Belgium.

I also own a BH100H, for one year. Opening it in order to bias new tubes I observed the filter capacitors are very weak. In a 100W amp a sevice tension of 350V is too light. The heat in the amp and the strong current the capacitors are solicited of make them unstable. The capacity of 2*220 µF could be enough though but I'd prefer twice that value because of the channel II darkness.

A little later I heard the power falling down and when I opened the amp I saw the first filter capacitor was bubbled. It is dangerous for the power tubes and a little less for the whole amp. So my first opinion was right.

Now I'm decided to remove original capacitors and I'll fit two cans of 500µF/500V upon the chassis, next to the tubes, cabled with enough length allowing to take off the PCB again. I'll solder these in parallel on unpolarized little cap's (around one µF) I'll connect to the free place of current capacitors. Little values limit the hysteresis effect electrolitic capacitors produce in cutting highs.

This will maybe interest somebody ?