Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Leevi on April 08, 2013, 10:36:09 am

Title: Secret of the big headroom?
Post by: Leevi on April 08, 2013, 10:36:09 am

I have built several SE amps using 12AX7 and EL84.

One of my last projects was based on Bad Cat Mini schematic.

The sound of this amp differs from the others with its big headroom.
The amp is still very loud. What are the essential factors that make this possible?
For more information see the schematic:

http://www.freestompboxes.org/viewtopic.php?f=2&t=1756 (http://www.freestompboxes.org/viewtopic.php?f=2&t=1756)

/Leevi

Title: Re: Secret of the big headroom?
Post by: kagliostro on April 08, 2013, 10:52:13 am

Hi Leevi

which transformers did you used on it ?

have you find a layout or have some photo to share ?

Thanks

K

Title: Re: Secret of the big headroom?
Post by: Leevi on April 08, 2013, 11:57:39 am

Hi Kagliostro

PT: http://www.tube-town.net/ttstore/product_info.php/info/p527_Toroidal-50VA.html (http://www.tube-town.net/ttstore/product_info.php/info/p527_Toroidal-50VA.html)
OT: http://www.hammondmfg.com/jpeg/125DSE_B.jpg (http://www.hammondmfg.com/jpeg/125DSE_B.jpg)

I think I made some small changes in the tone stack.

/Leevi

Title: Re: Secret of the big headroom?
Post by: Leevi on April 08, 2013, 12:12:44 pm

Quote

have some photo to share

/Leevi

Title: Re: Secret of the big headroom?
Post by: kagliostro on April 08, 2013, 01:38:07 pm

Nice Cab and big magnet on that Speaker  :smiley:

Thanks for the PT & OT data

No Amp Porn ?  :angel

K

Title: Re: Secret of the big headroom?
Post by: echuta13 on April 08, 2013, 04:50:23 pm

I would think the tone-stack is dropping gain and giving some headroom there.  The slightly higher uf bypass cap on the output might help a bit.  The power supply is also very stiff.  Other than that, I would think a beefy output transformer (125DSE being  beefy), and the relatively efficient speaker would be factors.

Might be misreading the schematic, but it looks like the 220k resistor (from grid to ground) might be reining in the second triode as well.

Title: Re: Secret of the big headroom?
Post by: DummyLoad on April 08, 2013, 05:21:56 pm

there is no path to ground for DC in the EL84's grid leak ckt. as shown in the schematic.

--pete

Title: Re: Secret of the big headroom?
Post by: stratele52 on April 08, 2013, 06:04:59 pm

IMO it is the circuit desing . Seems very original to me.

It is not the power transformer or output transformer wich are very basic.

Title: Re: Secret of the big headroom?
Post by: echuta13 on April 08, 2013, 06:45:39 pm

The Valve Junior or Champ have what I would consider a pretty basic output transformer rated @ around 50ma and weighing in around .75 lbs.  The DSE is rated for 70ma and weighs in around 2lbs. That's quite a difference IMO.

Title: Re: Secret of the big headroom?
Post by: jazbo8 on April 09, 2013, 12:11:40 am

I don't think there is any secret... it is just a low gain amp, with nicely center-biased gain stages, and a well bypassed screen supply for the EL84. None of the stages breaks up easily thus the perceived big headroom.

Jaz

Title: Re: Secret of the big headroom?
Post by: DummyLoad on April 09, 2013, 12:33:02 am

very nice work leevi!

 :icon_biggrin:

--pete

Title: Re: Secret of the big headroom?
Post by: stratele52 on April 09, 2013, 03:34:02 am

Quote from: echuta13 on April 08, 2013, 06:45:39 pm

The Valve Junior or Champ have what I would consider a pretty basic output transformer rated @ around 50ma and weighing in around .75 lbs.  The DSE is rated for 70ma and weighs in around 2lbs. That's quite a difference IMO.

Ok it is not a basic OT

Title: Re: Secret of the big headroom?
Post by: Leevi on April 09, 2013, 09:19:11 am

Thanks for your responses.
I think there is not any special trick but it's a sum of different solutions also mentioned in this thread.
E.g. the 220k ground resistor and 100k+2.2n cap on the EL84 grid
affect definitively.

/Leevi