The Frog Bigmouth Low Wattage Output section and Classic Power supply

Version 1.1

The Frog Bigmouth low power output section was designed to be added to a Frog Tube Preamp 2.0 or 2.1

preamplifier PCB requires a fairly robust power supply of between 200-250 volts with at least 20ma of current

capacity. The Frog Classic Power supply PCB was designed specifically to power this output section. It provides

enough power to run up to 2.5 watts of output power and requires a 12VAC, 1500ma power source. (NOT DC!)

You can implement this many ways and here are a few examples:

Install the Frog Classic power supply and Bigmouth output section in its own enclosure as a stand-alone

component and then connect to your stand-alone Frog Tube Preamp 2.0 when you are at home

Install the Frog 2.1 add-on board, Bigmouth output section and Classic supply in an enclosure and use as

a Studio or Bedroom amp. The Classic power supply can provide all the power needed for both the 2.1

add-on board, and the Bigmouth output section.

Keep in mind that if you have the Bigmouth output section as a stand-alone component as in the first option

above, you need a strong input signal to make it work. For instance, if you run your guitar into a BOSS distortion

pedal and then out of the pedal directly into the Bigmouth output section, you aren’t going to get much if any

discernable sound. You will need a strong output from whatever is “in front” of the output section to work. The

Frog Tube preamps can do that for you.

The Bigmouth PCB gives you a lot of options to consider despite its small size

Use 4 different tube types: 12AU7, 12BH7A, 6N1P (Russian) or 6n16b (Russian Submini tube). An

optional switchable heater power source allows this option (not for subminiature tubes).

Solder the Russian subminiature tube directly to the board, or use a MillMax socket for a pluggable

option that makes tube replacement much easier (MillMax approach is recommended because once a

subminiature tube is soldered to the PCB, it is real hard to replace later. MillMax installation is

documented in the Frog Tube Preamp 2.1 Add-on PCB documentation.

Optional switchable power output. The switch can select from approximately .75, 1.5 or 2.5 output

watts (do not use with Russian submini tubes, they are only rated up to .9 output watts!) For Russian

Submini tubes, only install R5 (not R2, R3, R4) and you will not need SW2 (switch 2 for switching

between power levels).

Presence control

Output transformer board connections designed for a screw terminal connector which makes it much

easier to build and service

Regulated 6VDC heater power circuit

Presence control

Note: The schematic below looks a little odd, because it has the Vacuum tube diagram parts for both the

standard 12Ax7 style of tube socket diagram and the Russian subminiature socketing diagram. You will only use

one. On the board pictured at the beginning of this document you will see 8 little holes in a circle inside the

bigger circle of holes for a standard preamp tube socket (Noval).

Schematic and Board Diagrams

Note: Center tap of push-pull output transformer goes to pin 2 above on the Output Terminal. See wiring

diagrams

Implementation sample

Frog Preamp 2.1 add-on channel - See implementation documentation for the 2.1 add-on channel on

Product Documentation Page

Frog Bigmouth Output section – implementation documentation in this document

Frog Classic Power Supply – implementation documentation in this document

Frog Preamp 2.0 - See implementation documentation for the 2.0 on Product Documentation Page

o If you want the diode clipping options, you may want to use this implementation option

Note: This document does not include the instructions for building the Frog 2.0 Preamp or Frog 2.1 add-on

PCBs. The documentation for the preamp and the output section/power supply is at the following link:

http://frogpedals.com/index.php/product-documentation/

Notes regarding the Cathode resistor array and switching:

On the schematic you will see R2, R3, R4 and R5. This is the Cathode resistor array.

1. Only use R5 for the Russian Submini tubes. I suggest a 1.5k or 1.8k resistor. Do not install R2, R3, R4. Do

not implement the switching for the cathode resistors. If you implement switching, you will burn the

tube up because it is only rated for about .9 watts. The switching will try to run it at up to 2 watts. Tube

life short, burning smell is on its way!

2. For Cathode switching, use an ON/OFF/ON DPDT switch. I suggest the double pole so if you choose you

can add a current limiting resistor and some LEDs to indicate normal, mid and high power modes. If you

want, a SPDT will work fine, without any LEDS, but just fine.

a. With the switch flipped all the way one way, you will get medium power

b. With the switch flipped all the way the other way, you will get high power

c. With the switch in the center (off) position, you will get normal (low) power.

3. The power levels are achieved by adding resistors in parallel which cuts the over-all resistance. For

instance

a. 1 1.2K resistor is 1.2k ohms of resistance

b. 2 1.2k resistors in parallel equals 600 ohms or half the resistance of the 1.2k resistor

c. 4 1.2k resistors in parallel equals 300 ohms or one quarter of the resistance or the 1.2k resistor

d. Reducing the cathode resistance increases the amount of current flow through the tube which

increases overall power output.

e. My measurements with the 12AU7 or 12BH7A showed about .75 watts with 1.2k ohms, 1.5

watts with 600 ohms and about 2.5 with 300 ohms. These are approximate and your outcomes

may vary a bit.

Bill of Materials

Output transformer – (specs). 3 - 5 watt push-pull with 22.5k primary impedance and 4ohm/8ohm/16ohm

secondary. Determine the requirements of the speaker you choose. 8ohm output is a good middle range

choice. I purchase my output transformers for this at http://musicalpowersupplies.com. The model number is

OT5PP+. Very reasonably priced too!

Frog Bigmouth Output Section

R1 760K

R2, R3, R4, R5 1.2 K For Russian Submini use 1.5k or 1.8k

R6, R7 220k

C1, C2 22nf box cap 100volts plus

C3 47uf 25 volt electrolytic radial

C4 100uf 25 volt electrolytic radial

Socket Noval style or for Russian Submini/Millmax socket

For Russian submini tube, Millmax Mouser: 575-199308

For 12Ax7 type tubes use a ceramic 9 pin PCmount socket

Heater Voltage Regulator

L7806CV (Mouser) Mouser: 511-L7806CV

Heat sink for L7806 Low height wave solid Mouser: 567-274-3AB

Terminal Block– screw type

Mouser 649-220316-G031B01LF Fits pin holes in PCB

I don’t like soldering the output transformer (OPT) to the PCB because the OPT is usually outside the chassis and the PCB is inside. A screw type header is recommended.

SW1 Heater switch On-off-on type SPDT Don’t mount on PCB. Not enough room

SW2 Power switch On-off-on SPDT Mounted off-board

Presence Potentimeter 100k linear Mounted off-board

Master Volume 1 meg Audio taper This isn’t on the schematic, but should/can be implemented between the preamp PCB and the input of the output pcb. See diagram

Rubber grommets Small, maybe 3/8 inch rubber grommets for running output transformer wires through top of enclosure/chassis

To Insulate the wires to prevent grounding out.

¼ inch jack Neutrik Rean – Mouser: 568-NYS229-U For output side of OPT – Best quality I have ever seen.

Classic Power Supply

Power Transformer Mouser part number FP24-250 Triad transformer

Mouser: 553-FP24-250 Note: larger output Triad transformers do not fit this PCB

R1 220 ohm 1 watt

R2 1 meg ½ watt This is in the circuit in order to make sure the Capacitors drain when not in use. Be careful, it still takes some time to drain. Use multi-tester volt ohm meter on DC volts to test. There can be up to 250 volts on these capacitors

R3, R4 1 ohm, 1% 1 Watt If you want to easily be able to test the current draw that the connected devices (preamp, output section, you should install these. Otherwise, these can be replaced with a solid wire. Current draw across a 1 ohm resistor is found by using volt ohm meter. Measure DC volts, which you convert directly to milliamps. So, 12vdc would equal 12millamps (ma) etc…

C1 100uf 350 volt axial electrolytic

Mouser: 647-TVX2V101MDD Use multi-tester volt ohm meter on DC volts to test. There can be up to 250 volts on

Or equivalent

these capacitors

C2 47uf 350 volt axial electrolytic

Mouser: 647-TVX2V470MCD Use multi-tester volt ohm meter on DC volts to test. There can be up to 250 volts on these capacitors

Or equivalent

C3 .1uf film 300 volt minimum

Mouser: 5989-400V.1-F Or equivalent

C4 470uf 25volt electrolytic axial

Mouser: 647-TVX1E471MCD Or equivalent

D1, D2, D3, D4 1n4007 Mouser: 625-1N4007-E3/54 Or equivalent

D5, D6, D7, D8 1n4003 Mouser: 625-1N4001-E3/54 Or equivalent

Cautions

There is dangerous voltage on this project!

Always use a multi-tester volt ohm meter set on DC volts to test B+ Classic Power supply points.

There can be up to 250 volts on these capacitors. This is enough to cause great harm!

Wait a minute after power is removed (12 volt AC wall input transformer power supply), then test for high

voltage. Test the B+ power point on the Classic power supply until it drops below 10 volts