Blackstar HT5-R - Gain and volume knob

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Imelda May
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Joined: Fri Jun 12, 2015 8:15 pm

Fri Jun 12, 2015 8:36 pm

Hi everyone,

I own a Blackstar HT5-R Combo amp, and I'm wondering what the gain and volume knob are connected to in the signal chain of the amp. I know that a regular tube amp with volume (gain) and master volume works as the following more or less. The gain knob is in front of the pre amp stage and functions as a volume control for the pre amp tubes in order to let them clip or not. The master volume is in front of the power tubes to control the output volume and whether the powertubes clip or not. Simplified obviously :p

But I'm not sure if the HT5 works like that same principle? And what happens on the clean channel, does it lower the pre amp volume so only the power tubes start to distort when the volume is cranked up?

Maybe a circuit scheme of the HT5 would help me out, do you guys have any answers on my question?

I also want to change the preamp Sovtek 12ax7 with a lower gain pre amp tube. Any suggestions or experiences? (5751; 12AY7; ...)

Thanks in advance.

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thephantum
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Location: Virginia, United States

Fri Jun 12, 2015 9:51 pm

Imelda May wrote:does it lower the pre amp volume so only the power tubes start to distort when the volume is cranked up?
Bingo. Getting a schematic is almost impossible as B* does not release them to the public...but based on my experience with my HT-5RH and looking at/tracing out the PCB, that's exactly how it works. The gain and volume controls are all on the preamp side. The power amp is essentially maxed out.

I've tried a 5751, a 12AT7 and a 12AY7 in mine. The nice thing was that the gain knob became more granular and it was easier to dial in a gain factor. The amp lost a little overall volume, but that wasn't the biggest issue. The down side was that the amp sounded flat and lifeless. It lost it's edginess, at least to my ears.

You are welcome to try it for yourself, but my recommendation is to stick with a 12AX7. Having said that, get rid of the Sovtek. A TungSol or Mullard will sound much better.

Imelda May
Posts: 3
Joined: Fri Jun 12, 2015 8:15 pm

Fri Jun 12, 2015 10:49 pm

thephantum wrote:The gain and volume controls are all on the preamp side. The power amp is essentially maxed out.
So the gain and volume are both on the preamp stage? Then the gain knob has to be a pedal-like distortion instead of a volume knob to control pre amp distortion. Really? Because when I max out the volume on the drive channel with the gain fairly low, it just breaks up a little ... :p

As far as pre amp tube is concerned, I really don't like the gain structure of this amp. But I've also heard, just like you said, any new 5751 or 12ay7 tube sounds thin, and dull. Only a NOS will sound fairly good. Problem is, it's hard to get my hands on any NOS tubes here in Belgium. Even in the local music store they only sell Electroharmonix and Mesa Boogie tubes :p Not much of a choice.

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thephantum
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Sat Jun 13, 2015 6:36 pm

Most amps use the gain knob in that way. Assuming a two channel amp, on the preamp side:
  • 1) The clean channel has a volume and tone stack
    2) The OD channel has a volume, gain and tone stack - where the gain increases the saturation level of the preamp tube (as opposed to a level/volume increase)
The power amp then has a master volume.

The only differences are that the HT-5 does not have a master volume....it's essentially wide open all the time. There is also some diode clipping happening on the OD channel....it's not pure pre tube saturation. So yes, it's somewhat pedal like (I believe the design is actually based on their HT-Drive pedal).

I tried a NOS 5751...an JAN tube from early 60's made by Phillips. It still sounded thin, at least when compared to a 12AX7.

The gain structure of the OD channel is not too bad, but I prefer the clean side. I'm currently running a NOS TungSol power tube (12BH7a) from 1957. So I crank up the volume and use a boost in the effects loop to help knock that power tube over. The only thing I can't dial in is a metal sound...for that I need to use the OD channel. But I can get a good, heavy, organic sounding overdrive out of the clan channel.

Imelda May
Posts: 3
Joined: Fri Jun 12, 2015 8:15 pm

Sat Jun 13, 2015 7:44 pm

thephantum wrote: I tried a NOS 5751...an JAN tube from early 60's made by Phillips. It still sounded thin, at least when compared to a 12AX7.
Well then I'll probably slip in a Tung Sol 12AX7. Anyone experience?

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thephantum
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Sat Jun 13, 2015 9:40 pm

This is a listing of the various new production tubes I have tried.
  • Shuguang 12AX7-B: Chinese made. Not a bad tube. Cleans aren't as crisp as I like. A bit flat when overdriven
    JJ ECC83-S/12AX7: Slovakian made. Good clean tone. A little fizzy when overdriven
    Electro-Harmonix 12AX7: Decent all around. Crisp sounding, but nothing spectacular.
    Tung-Sol 12AX7: Good crisp clean tones. Nice and gainy in OD with some crunch.
    Mullard 12AX7: Good crisp clean tones. Nice and gainy in OD. Similar to the TungSol, but not as bright.
    Sovtek 12AX7-LPS: Similar to a Mullard, but with less punch and clarity
    Svetlana 12AX7: - Better then the EH, but less gain then the TungSol.
    Genalex Gold Lion B759/12AX7: Very similar to a TungSol. When overdriven has awesome high order harmonics. Best new 12AX7 money can buy. Very pricey though, almost 3x the cost of a TungSol.
IMHO and in terms of "bang for your buck", it's either a TungSol or a Mullard. Both are very good tubes. The Mullard is a little darker, the TungSol is a little brighter.

I will also say this, when it's time to change the power tube...go NOS. To be brutally honest, there are 2 or 3 choices in terms of new 12BH7A's and they all suck. You can pick up "gently used" old stock 12BH7A's on eBay for real cheap that will blow the new tubes away...

Growingblack
Posts: 36
Joined: Fri Jun 13, 2014 4:09 pm

Mon Jul 06, 2015 1:53 pm

I am thinking to follow the suggestions on replacing preamp tube in HT-5R. Great to read your advices, and taking time to answer (Phantum). Once the back is opened, how do I know which is the preamp? As I understand (or not), there are two tubes, one being preamp and one power amp tube? Maybe it will be obvious when opening, but better know.

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thephantum
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Mon Jul 06, 2015 8:04 pm

The power tube sits roughly in the center of the chassis and is taller. The preamp tube is off to the side (closer to the output transformer) and is shorter.

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Classicplayer
Posts: 100
Joined: Sun May 20, 2012 12:00 am
Location: USA

Mon Jul 06, 2015 8:17 pm

Are those tubes label on the board as V1 and V2? The HT1R is like that. It's very small print however.

Classicplayer

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