4 Replies Latest reply on Aug 30, 2016 5:46 PM by Allan Bowers

    Flow Simulation on Audio Guitar Amp

    Allan Bowers

      I am working on a clone of a Vox guitar amp and wanted to use thermal flow simulation to assess the 60's cab design and orientation of the amp chassis.

      In the particular Vox amp the tubes face down (like many amps) inside the amp head. I am not sure how to apply a heat source value for the tubes/filter caps etc in the chassis. I can input a solid or surface value as a temp or in Watts.

      The tube data sheets dont show a working temperature range and capacitors only show a max temp rating.

      The flow simulation should show if the heat convection to the components in the chassis is better or worse depending on its orientation, which is the aim of the study.

      Some people have added fans to their old amps as they have a concern about heat, and some swear that the hotter an amp runs the better it sounds. However there must be an ideal temp range or at least max that tubes and capacitors should run at before their life span is dramatically reduced.

      I can also use the simulation to see if adding a fan will improve the air flow and therefore heat dissipation.

      There is also transfer of heat through the chassis from the tubes to other components on the turret board and I have often wondered if the chassis is better off in an 'upright' config whereby the heat from the tube rises up and out of the head, without passing over the rest of the components as apposed to the 'upside down' config. Although in the amp I want to build the chassis would dissipate heat, however I have seen pics of this amp where it clearly has become so hot it has darkened the chassis around the tube sockets, (see photo below).

      I am unsure what temp to apply to the various components as Heat Sources in the simulation.

      As an e.g. in a typical push-pull amp, the maximum plate dissipation of is 30 watts plus about six watts for the filament and a couple for the screen so I could model a total as 40 watts. The temp runs around 400 degrees F for a tube. Is there a direct relationship between the dissipation in Watts and the Temp of the tube under normal operation?

      An amplifier like this is about 50% efficient (or a little less) should I double the rated output to be somewhat in the ball park of the power (and heat in watts) as input to the amplifier.

      Of course, after a few hours, the transformers get hot along with everything else including the capacitors and resistors in the circuit.

      In my model there is a 300-0-300 V@120mA max and 6.3-0 V/4A secondary mains Tx, and an Output Tx . Again I am not sure what average heat these components would generate.

      The amp will have 1 x EZ81 rectifier tube, 1 x 12AX7, 3 x 12AU7 and 2 x EL84 tubes.

      The DC filter caps are 2 x 32 uF in a can; 2 x 16 uF as a double axial and one single axial 16 uF caps to produce 300V DC filtered B+.

      My schematic also shows the input V at the tubes after the first V drop resistor and the Voltages at the Cathodes of each tube.

      Thus with all that said, do I have enough info to set up a Flow Sim study in Solidworks to assess the best chassis orientation, cab ventilation and need for a fan??

      Thanks

      PS Here is a partially completed model of the amp. Only 3 filter caps shown so far. Most of the other caps are on the turret board and actually soldered above a resistor

       

      VOX AC10 SRT LAYOUT.JPG

      Note heat marks on chassis

      DSCN0160.jpg

      Note small vent slits on front and back of amp head

      1968_Vox_AC10_03250RV_019-1285x600.jpg

      DSCN0146.jpg

      There is no hole behind the cloth. Air can only enter via the thin slit vents

       

      DSCN0148.jpg

        • Re: Flow Simulation on Audio Guitar Amp
          Amit Katz

          When doing an electronics enclosure it's usually best practice to go with volumetric heating (i.e. watts of heat) to model cooling. The real trick is knowing what that value is. There's really only two ways I can think of to find this information out:

          1. Direct from the parts manufacturer. If they are a reputable business they should have a data sheet for this kind of thing, though I'm not necessarily sure it would be available to the general public. You could also try calling them, they might give you the info.

          2. You could find out for yourself, using a controlled experiment. I'm not sure what kind of manufacturing and testing equipment you have access to, this might be too involved.

           

          By the way, your CAD looks pretty detailed. Keep in mind that for this kind of analysis you want to keep your features as simple as possible, eliminating details that don't greatly contribute to heat conduction and convection. You'll want to keep simplified versions of all your components ready for the simulation.

            • Re: Flow Simulation on Audio Guitar Amp
              Allan Bowers

              thanks Amit, the tube specs show Screen where applicable, (continuous and peak) and Plate dissipation in Watts for pre-amp and output tubes. Depending on the tube this can vary from 1-2 watts for the 12au and 12ax7 preamp/reverb/phase inverter and tremolo;and 12 Watts for the el84 output tubes. The EZ81 rectifier tube has no figure for dissipation so a temp maybe. as the tubes are vacuum would i apply the heat source as surface on the glass. i do have some temps from similar amps which can be used for a rough comparison to see if it is in the ball park. what about values for caps, i assume the higher the uf the warmer but specs only show max temp. should i put a watt value on the resistors, most are .5, some 1, a couple 2 and one 10 watts which is first after the rectifier as i assume they gen heat. also mains tx spec does not show value for dissipation