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Help with Hybrid Design

Started by SerpentRuss, December 09, 2020, 01:53:20 PM

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Hey guys, new here.  I've been lurking for a few weeks and enjoying looking back through old posts hoping to gather some knowledge.   I started a project a couple of months ago and I have a couple of questions.

There is a seller on eBay that occasionally sells small tube guitar amplifiers housed in small, unique enclosures.   I have been outbid on the couple that I've tried to buy.  I look on eBay infrequently and he seems to offer amps on a rare basis.  I contacted him directly and asked him if he would sell me a kit and he was kind enough to reply with a schematic, but he wasn't interested in selling me a box of parts or supporting his design.  I get the feeling that he makes each amp a bit differently.

The power supply for the amp I'm trying to build features an inexpensive 7-volt Antec Toroid transformer with the 120-volt coils split, making it a small isolation transformer.  The 7-volt side is used for the tube filament.  The output transformer is a cheap 70-volt distribution transformer. The output is supposed to be in a 1/2 watt range.  I have sourced all the parts and have started working.  I think I have about 30 dollars in the parts.

If I was going to insert a tone stack in this circuit in the solid-state pre-amp, where would it go?  I'm thinking it would insert prior to the gate of the third LND150.  Also, the quality of the schematic jpeg is pretty bad.  I've assumed the gaps in some values are missing decimal points.  For example, this would make the cap in series with the volume pot 4.7N.  Does that seem right?


As it happens this seller has an amp up for sale made into a computer power supply.   Interesting build and he included a schematic for this one.  I wonder if he's on this forum.  And no, he's not me.



The problem with tone stacks (assuming a normal Hiz fender type tone circuit)
is that they suck a lot of energy from the signal level (often termed insertion loss)
You can loose as much as 70% of the signal that way and you would need another gain stage otherwise the Amp will be very low on output. xP

A better option might be to add a separate tone circuit in front.
I have a circuit for such situations if you are interested I can post it?

Or maybe just use an EQ type pedal to give you an idea of how well that works.


Phil, I'd like to see it, if you don't mind posting it.


Go here , Schematic is on the 2nd post.

Yes I know it does not have a preamp  :o and some think it strange but this does not need a preamp just plug your guitar into it.
I've lost count of how many of these I have built for players and No one has yet complained.
Just make sure you you use a Log pot for the Bass. I doubt you will ever need more treble but if so just lower the value of C8.  will run on 9V or up to the limit of the opamp and cap voltages used.
I run mine on 18V
Have fun, Phil.


That's interesting.  I really didn't want to build a pedal.   I'm wondering if there will be enough power in the 7 volt AC side to make a 9 volt supply.  I imagine the load is very small for this.


Yes my tone circuit draw little current.
Well you need to research another Fet stage to run a tone section.
Sorry but beyond my scope to design something from scratch like that but you can always just add another stage in front with enough gain to pickup the loss from a tone circuit. I assume it's doable,, outcome unknown??
My thoughts ,, not worth it for such a tiny amp. xP


Rather than screw with this design, since it's my first project, I've decided to build this as drawn as a small combo amp using a switched output jack that will allow me to use another cabinet if I want to experiment.  I will include some build photos as I go.   Wish my luck.


Hang on I just reread the 1st post and as this is your first build,, I'll assume you don't have a lot of teck skill.

Warning!!! RE this;
"The power supply for the amp I'm trying to build features an inexpensive 7-volt Antec Toroid transformer with the 120-volt coils split, making it a small isolation transformer.  The 7-volt side is used for the tube filament."

Unless I've missed something in translation,,,that tells me it is a 120/120 split *Primary* so something sounds wrong :loco

Also the way in which the power Tx is drawn up in that schematic seems strange.
Others here may know more but meantime be careful cause if I'm right as it could kill you. :police: :police:

Normally when a Tx states 120/120 split primary that means it can be wired up to either 120 mains ans 240 mains. It is most likely NOT an isolation Tx :-\
Maybe post a picture of the Tx you wish to use ideally showing the label.


This is my first amp build, but I have about 40 years of experience working with electricity all the way up to medium voltage circuits like 4160-volt motors.   I know how to work safely.

You're right, it's not an isolation transformer, but it's being used as one.   This design has a very low current draw because it is a sub-1-watt amp.  The guy who builds these has used this exact transformer in this way on several small amp builds. One primary set of leads on the transformer gets driven by the mains inducing about 120 volts on the other set.  The 7-volt secondary is dropped to 6.3 volts to run the filament on the tube.  The 6CL6 power amp doesn't require a lot of DC voltage.  I think this supply will put out about 135 volts DC.  The OT is wired using the 1/2 watt tap.


Quote from: phatt on December 28, 2020, 08:14:57 PM

Also the way in which the power Tx is drawn up in that schematic seems strange.

I agree, I would not have drawn it this way.  I would have placed the 117 VAC label under the AC source symbol and I would have put the 7 VAC symbol on the there side of the bottom transformer symbol.  Essentially those two series windings on the left side represent one pair of the 120-volt primary leads.

Here is the web page for the transformer.   Antek makes some inexpensive and interesting toroidal transformers.  I will probably use one of their tub amp models for a Rob Robinette "Deluxe Micro" project.




It should be drawn like antek draws it in the datasheet.

The two 115V windings are on the same side of the transformer, so they are not transformer (inductive) coupled.
Drawing it like they are on opposite sides of the transformer is just wrong.
I think the coupling would be very limited in terms of current, and it is just lucky that it works.


And if one of the secondary 7V windings is actually in series with a 115V primary winding as the drawing implies, well I don't even want to try and think about that.  ;)


Agree with G1, :dbtu:
Well the data sheet tells it as I suspected,, It's Not an *isolated 115v Tx*

It states very clearly it has 2x115v primaries and 2x 7V secondaries.

So unless I've lost it, :loco ,the only way one could get enough HT to run a valve is from one of the primary windings which means it's a death trap waiting to happen as the circuit runs off the primary. ARRRRGH!!!!
OK sure there are tons of old radio circuits that did indeed run direct from mains but most of those old units where bakalite cases and one would never come into contact with the circuit under normal use but this is an electric guitar amplifier and you ARE in direct contact with part of that circuit so if something fails you could end up Dead.
If you want to build it that is your call but I'm writing this to warm other non teck savvy folks that it's a death trap waiting to happen.

Be very careful grabbing ideas off the net some are really bad designs. 8|

To attain HT for a small valve amp there are much SAFER ways,, here is a better option for High voltage, uses back to back Tx's.

The added bonus is that it uses a 12Volt wall wart which keeps you a long way away from dangerous main voltages.
HTHelps, Phil.


We agree that is drawn incorrectly, and I've already stated it's not an isolation transformer, but it does work.  The primary windings and secondary windings are wound around the same toroid core so they are definitely "coupled" inductively.   In most applications, this transformer would have both primary and secondary winding sets paired in parallel.  That said all the winding sets are independent.  I've tested. 

In this design execution, the secondary windings are paralleled since the filament of the valve has the highest current draw.  I estimate that the TOTAL current on the 2nd primary (used as a secondary) winding is about 20 mA.

Here is a picture of one of this guy's builds, kind of sloppy point-to-point wiring, but you can clearly see that one black and red set of primaries are connected to power and the other set are connected to a diode bridge in the upper left-hand part of the photo.  The blue and green 7 volt AC secondaries are twisted together and are wired to the tube socket and dropping resistors. 

I appreciate the other design, but I've already purchased the parts for this amp and started, so I will definitely attempt to complete it.  I will be done with this soon and I will post photos and sound files if it works.