BH5 Head and Combo

No longer in Production but there are still a few of these Amps Hanging around in the Shops.

The Chassis is 1.5mm Thick Steel, Welded Box Construction. Correctly Rated Electronic Components, Very Good Transformers and a Through-Plated, Double Sided PCB. The Circuit Design is Text-Book, Cathode Bias, Single Ended, using a '12AX7' Preamp Valve and an 'EL84' Power Output Valve. Bias Settings are 'Middle-of-the-Road' and the HT Voltage is SPOT-ON. The Amps are Really Well Built. Sadly, they are let down by a rather high level of BUZZ and HUM in the Circuit.

The Stock Amp's Output is a Healthy 5 Watts RMS and the PEAK Power Output is just over 10 Watts.

So before I can start any Hot Rod work, I have to clean up the Buzz and Hum Issues. Once that is done, the Amp is Superb, and Wide Open to some Great Modifications. The Addition of ‘OPR’ and ‘OPR-1+2’ allows the Amp to be used in any environment, From your Bedroom to the Stage.

The Combo Amp (BH5-112) is Fitted with an 'Eminence' Custom Blackheart Speaker. It's 12", Rated at 70 Watts, 16 Ohms. I would suggest Replacing this with a Celestion G12 Vintage 30.

Since their Introduction, I have Modified Hundreds of these little Amps and they have proved to be Solid and Reliable.

Available Modifications

1. 'OPR' - Output Power Reduction. This is the Business and essential for an Amplifier used in the Home. I Have tried ALL of the other systems that claim to Reduce the Volume without changing the Tone - 'OPR' really works.

2. 'OPR-1+2 - - - This Dual Function Switch allows two Distinctly Different Sounds when the Main 'OPR' Control is in Use - 'OPR-1'....Maintains the High Harmonic Content in the Final Sound - 'OPR-2'....Is Much Softer and Cleaner Sounding. Most 'RAT' Amps are Fitted with ‘OPR-1+2’ as Standard.

3. Dual Output Valves Switched- The Amplifier is Fitted with 2 x Different Output Valves, usually an ‘EL84’ and ‘6L6GC -or- ‘6V6GT’. You can Switch between the Valves for a Totally different Amp Sound. Works OK with the Standard Output Transformer but is a LOT Better with an OPT Upgrade.

3a. Dual Output Valve 'MIX' Function - This System is based around one Rotary Control. The Amplifier is Fitted with 2x Different Output Valves. The Rotary Control Allows Selection of Just One Valve or the Other (EL84 -or- 6L6GC) -or- Any 'MIX' Between the Two Valves. Using the 'MIX' Control you can get a Great Bit of 'Brit Crunch' (From the EL84) Mixed in with the 'American Clean' (From the 6L6). This Modification Requires the Output Transformer to be upgraded to a Hammond 125DSE.

3b. Dual Parallel Output Valves - This System uses 2x Output Valves (2x EL84) Running Side-by-Side in Dual Parallel Mode but Retaining the 'Single Ended' - 'Cathode Bias' Configuration. It Gives a Great Power Boost, up to 12 Watts RMS and really Gives the Amp some 'BITE'. The Modification Requires the Output Transformer to be Upgraded to a Hammond 125DSE -or- ESE.

4. 'TRI' Rectifiers - This Option uses a Three Position Switch and a 'GZ34' -or- '5Y3' Valve Rectifier, It Integrates Seamlessly into the BH5H Circuit and offers 3 different types of  Rectification. Position 1= Standard Silicon Diodes - Position 2= V1, Valve Running at Half Power, This gives Maximum 'SAG' - Position 3= V2, Valve running at Full Power, Offers Reduced 'SAG' but maintains the Classic Vintage Valve Rectifier 'Feel'.

5. New Output Transformer (OPT) - This is Perhaps 'THE' most Important Component in any Valve Amp. Changing the OPT to a Better Quality Unit will Increase the Output Power. That is not the Only Benefit, it will offer Better and Tighter ‘Bass’ Response along with Cleaner and Clearer ‘Mids’ and a Much better Defined ‘Top’ end. These Transformers are Expensive but well worth it. The Best is a Hammond 125DSE -or- 125ESE for Superior Headroom.

6. Master Volume Control - If the Preamp Valve 'Bias' is set Correctly, a Master Volume Control will allow - Preamp Overdrive Only -or- Power Amp Overdrive Only -or- Both. Fitted to all ‘RAT’ Amps.

7. Tone Bypass and Variable Boost Footswitch - A Switch on the Front Panel of the Amp enables the 'Tone Bypass'. This Gives the Amp a LOT More Gain and Works in Conjunction with Footswitch. When the Footswitch is connected to the Rear Panel of the Amp, the Tone Bypass Switch is Disabled. The Footswitch has a Built-In Variable Gain Control. You can set just how much Gain you want with the Rotary Control located near the Push-Button On/Off Switch. It Works Great and Gives you that extra 'Kick' when Needed. A Red LED Illuminates when 'Boost' is Selected. There is a Trade-off when using the Variable Boost. The Higher the Boost is set - the Less Effective the Tone Controls Are. In Practice this is not a Problem because at 'Full Boost' the Tone Network is Flat and allows the Mids to Cut Through Nicely.

8. Because of the Hum and Buzz Problems, I have to Remove all of the Rotary Controls from the PCB. The Stock Controls (Pots) are OK Quality but just to be sure I replace them with Bigger '24mm Alpha Parts' and 'Hand Wire' them back into the Amp Chassis using 'Orange Drop' and 'Silver Mica' High Quality Tone Capacitors. I Totally 'Re-Build' the Tone Stack - it can Completely Remove the 'Treble', 'Middle' or 'Bass' from the Signal (if required) - It Now Replicates a 'Fender Twin Reverb' in Operation.

9. Preamp 'Bias' Switch - This Switch Changes the 'Bias' on the Preamp Valve to one of  2 (or 3) Settings. Position 1 (If Fitted) is 'Sharp and Bright', this setting makes use of 'Negative Feedback' (NFB). NFB keeps the Amp 'Clean' by feeding some of the Speaker Output Signal 'Back' to a Previous stage in the Amp circuitry. This creates a 'Cancelling' effect and reduces Distortion and Volume. - Position 2 is 'Clean and Mellow', The Preamp Valve is Biased for Medium Distortion with a Bit more Bass. - Position 3 is the 'Medium/High Gain' Sound, Early Distortion, Great 'Crunch'. The Harder you Drive it the Better it Gets.

10. Line Out and Headphone Sockets - Nothing Special here but these Sockets are Fitted on almost all 'RAT' Amps and Include a Speaker Output 'Dummy Load' - So you can use the 'Headphone Socket' -or- 'Line Out' With or Without a Main Speaker Connected to the Amp.

11. 2 x Speaker Outputs and Impedance Select - The Output Transformer offers individual Speaker Outputs of 4, 8 and 16 Ohms. The Stock Speaker Outputs are Very Confusing and you can easily make a Mistake connecting your Speaker/s. The 'RAT' Setup is Simple and Straightforward to use - Connect a Speaker or Speakers and select the Correct Impedance - Done. Fitted to all ‘RAT’ Amps. The Output is Also protected by an Auto-Switching 'Dummy Load'.

12. There are Several 'Other' Modifications that I do to All of these Amps that are Exclusive to me and both Enhance Performance and Help to Keep the Noise Levels Way Down.

The Background 'Noise' from the ‘BH5’ Amp is now so Low, you may not notice it is Switched on.

'Harmony Central'

Short 'OPR' Sound Clip