New build: Magnatone 213 (Tonemaster/Titano/Evil Robot) + mods --- advice wanted!

[klooon]

PSUD2 will help you identify the peak current and rms current in a zener - use an RC first stage filter and make R very low and use as the sensing part for current through the zener.

Look through the zener datasheet. It may have some guiding graphs - the aim should be to keep Tj below 110-120C, even though it may have a max spec of say 175C. Having the zener in free air with a longish lead, will give a high thermal resistance. Mounting on a tag board certainly is much better than soldering the leads in series in free air.

Try and gather the data above and do a thermal calculation as to what Tj may get up to - you will need to estimate the maximum ambient air temp - which could be quite high if under a chassis near to other hot parts, in a head unit, butted against a wall in a heated room.

I have never worked with PSUD2 and I don't know which are the right conditions to use, but is this the idea?


1.5A max and 0.2A RMS..

If this is right, now what to do with it exactly? I must say that this goes a bit over my head.

Practically, would a 5.6V 5 Watt Zener series string with short leads do the job? I see that a lot of people report this option, but isn't it safe to do?

Or is the amplified zener option a better and/or safer way?

[trobbins]

If the amp is the same as someone has done a mod for then you have some confidence. Doing a mod on another amp is then an uncertain outcome, and doing some 'design' is the better way to approach it.

If the design aspect is too much at the moment, then 'try it and see' is valid, especially if you can check the outcome is working ok somehow. The zener failing is one outcome. Trying to measure the temp of the zener is somewhat nebulous, but could be tried.

There is a softstart option in the simulation to use for valve diodes, as valve diodes don't conduct instantaneously when power is applied to the amp. The R1 current is the same current that would flow through the zener. The peak R1 current can be taken from the plot. The rms current will be lower, eg. 0.2A - which is then a good estimate to use with the zener voltage to estimate zener power loss.

PSUD2 is a great tool to use as a learning curve - as it has good help notes that will fill out an understanding of the power supply parts and operation. But like all simulations - shit in gives you shit out - so you do need to double-check that the circuit and values are set up correctly. Some part values are not too influential, but you may need to read the notes and make some transformer resistance measurements on primary and secondary windings, if 31 ohm was not your input value. Also, if running class AB, then the output load current may rise above idle, and that needs to be thought about.

[klooon]

Thanks. Let's try to do some design then! I found the specs of a 5W Zeners (here) and looked at the Tj. Taking a 0.2A RMS value and taking short leads (<3/8" = 1cm), the Tj is in case of 10V Zener 70degC above ambient temperature and in case of a 5.6V Zener 35degC above ambient (T_j,max = T_amb + (15 . P_D) + (20 . P_D) with P_D = 0.2A RMS times either 5.6V or 10V)., which are then both acceptable (?), but 5.6V zeners are more safe.

PSUD2 is nice to play around with. BTW should I take the loaded or unloaded secundary PT voltage? I found the PT resistance on the Hammond datasheet. (ca. 100R over the total 330-0-330, about 50R with regard to the CT). I tried to put the full Magnatone power line, but it seems it doesn't want to take last filter stage with a 47n... and the voltage there drop straight to zero... Am I right?

[trobbins]

For PSUD2 I'd use likely max unloaded PT secondary voltage - as that would take in some mains voltage margin (we sometimes get 250V here with residential solar generation, depending if they have not tapped down the local distribution transformer for a while). The PT resistance also includes the reflected primary winding resistance (that's in the help notes), which may be substantial.

The zener thermal cures relate to clamping the ends of the zener leads at the identified temp (eg. 75C). That infers an 'infinite heatsink at 75C'. A tag board may mean the zener has a 3/8" lead, but there is no big heatsink keeping the tag at say 75C, and so the curves for at least 1" lead length may be more applicable. So your design calcs may need to use something like Rjlead ~ 40-50, and assume Rjlead is Rjambient. So with a 50C amb, and 2W dissipation (10V zener), then Tj is about 130-150C, which I suggest is getting a tad high.

[klooon]

For PSUD2 I'd use likely max unloaded PT secondary voltage - as that would take in some mains voltage margin (we sometimes get 250V here with residential solar generation, depending if they have not tapped down the local distribution transformer for a while). The PT resistance also includes the reflected primary winding resistance (that's in the help notes), which may be substantial.

OK. For my situation it is going the other way around. In case of the 240V primary winding on the Hammond and actually having something around 225-230V here in the Netherlands, here is already a bit of a safety margin built in. If I run the PSUD2 calc. then I get with 350V PT sec. (unloaded figure from the Hammond datasheet and R around 50R or higher), a B+ swinging between 395V and 370V, on average about ~380-385V. With the 230V present it's even lower, around 370 or below.... With increasing PT resistance this is going to be a tad more. Just in the right ball park without the zeners then??

The zener thermal cures relate to clamping the ends of the zener leads at the identified temp (eg. 75C). That infers an 'infinite heatsink at 75C'. A tag board may mean the zener has a 3/8" lead, but there is no big heatsink keeping the tag at say 75C, and so the curves for at least 1" lead length may be more applicable. So your design calcs may need to use something like Rjlead ~ 40-50, and assume Rjlead is Rjambient. So with a 50C amb, and 2W dissipation (10V zener), then Tj is about 130-150C, which I suggest is getting a tad high.

Clear! Then it is much safer to use the 5.6V Zeners. About the long leads... The last zener goes to ground, but in my layout I am using only one chassis ground point which is 'on the other side', close to the input jacks. Is this going to be a problem in terms of heating up the 'grounding leads'?

[trobbins]

It sounds like you could start with no zeners, and check what conditions you idle at. Other subtle details come in to play, such as OT primary resistance voltage drop, which also move values around a bit. Many wouldn't worry about +/- 20-40V on B+, as that's somewhat equivalent to mains varying +/-10 to 15%, and idle dissipation is usually set below 100% of max design centre.

You seem to have a good amount of design and details under the belt so as not to get caught unawares - testing is the next step - then perhaps back to a bit of design/faultfinding if measurements don't align well with expectations.

[klooon]

Thanks! Time to order the components then...

[Geoff Cloney]

Thanks! Time to order the components then...

Hello klooon,

first, thanks for sharing this great amp-design with us! One question, if you dont mind. Which OT are you going to use?

Regards,
Geoff Cloney

[klooon]

Hello klooon,

first, thanks for sharing this great amp-design with us! One question, if you dont mind. Which OT are you going to use?

Regards,
Geoff Cloney

Hi! I will use a Hammond "Deluxe Reverb" OT 1750H with 6,600 Ohm primary and 8 Ohm secundary.
For the PT: Hammond 291BEX with 330-0-330V secundary.
See details in the schematic of post #27

Further, most goodies are in. I am now working on the chassis... a bit more work than I expected...

[ashw182]

Hello,

I am new to amp building, and your schematics have been very helpful. I was wondering, is it possible to only have one input jack, and use a switch to change between the 2 channels?

[überfuzz]

Yes it is possible! As far as I can see the only difference between the two channels is the bass response. If you want more control of the tone on one channel there are other tone stacks you can opt for. Try to find Tone stack Calculator for inspiration, it's a freeware simulating the frequency response of classic tone stacks.

[überfuzz]

I briefed through this thread. Interesting to see what kind of modulation you end up liking the most. I thought of implementing a switch-able modulation but never done it or seen it be done. Seems you given the ground scheme a thought, or two. I never got a significant difference in noise-levels no mater how I grounded (well as long you don't do any obvious stuff as ground looping etc). Would be interesting to see what a star ground would do to a home-brewed amp...

[klooon]

I briefed through this thread. Interesting to see what kind of modulation you end up liking the most. I thought of implementing a switch-able modulation but never done it or seen it be done.

I really like tremolos, so it is a 'must-have' in an amp build for me. I like the one in my Princeton Reverb the best so-far. Very deep and lush. Even a tad better than my Tweed Tremolux (5G9) build. I will keep you posted of the result!

Seems you given the ground scheme a thought, or two. I never got a significant difference in noise-levels no mater how I grounded (well as long you don't do any obvious stuff as ground looping etc). Would be interesting to see what a star ground would do to a home-brewed amp...

So far good results with that method. I haven't made any comparison, unfortunately, as this is a bit tedious. Therefore, this way I play it safe... I think

[klooon]

Finally some progress to report. I have made the chassis from aluminium and have chosen to have them anodised. Please see the picture below:


Next: to find a place where I can engrave them or do some white lettering (not sure which technique is possible...)

And here are some pictures of the cabinets, Fender style... in custom colours...

[klooon]

More progress: eyelet boards are ready. FR4 fire resistant material; size about 390mm x 75 mm. 1/8" eyelets are used.
Here are some pics.