AX84 BBS Recent Posts as of Sat, Aug 01st, 2015 @ 05:19:26 http://ax84.com Amp Parts and KitsDoberman Music Products, LLC offers a variety of parts and kitshttp://shop.dobermanamps.com Help with theory / maths - preamp bias(by tristanc) Hi all, An appeal for help with the theory / maths side of things... I've spent the last couple of months reading (and re-reading) Merlin's and RK's books, along with the P1-eX theory document and this forum, trying to understand what is going on in my amp - the AmpMaker N5X <a href="http://www.ampmaker.com/images/ak01kit/n5xsch2.jpg. " target="newwin1438420770">[link]</a> This is very similar to the P1-eX with a single ECC83 in the preamp but with an EL84 / 6V6 as output. The result is that I've worked through each stage calculating the gain, distortions etc for both the 'typical' values supplied by Barry and real values measured from my own build, using different valves in each position to observe the changes. This has involved a lot of work digitising the charts for the EL84, 6V6 and ECC83, then interpolating for the differing grid voltages and screen values etc. Probably overkill... The 1st issue I came to is getting the 'correct' bias calcs for the 1st preamp stage. The typical values provided for the circuit are: B+ = 235V V_p = 155V V_k = 1.3V R_p = 100k R_k = 1.5k So 80V dropped across R_p, giving a current of I_p = 80/100 = 0.8mA Following pg 6 of the P1-eX document, e.g., this would give V_k as 0.8 * 1.5 = 1.2V, rather than 1.3V given above. If I use the charts (interpolated values for 1.2V and 1.3V grid lines), drawing up at 155V, I get 1mA and 0.88mA respectively. Neither of these is particularly close to the 0.8mA value above. Using my own measured values (using a new JJ and an old Mullard) and the same procedure gives: B+ = 238V V_p = 158V V_k = 1.17V So, I_p = 0.8mA and V_k calculated as 1.2V. This is closer to the measured value of 1.17V. From the charts, the current is I_p = 1.13mA at 158V and grid of 1.17V (interpolated). This is nowhere near the 0.8mA value as calculated. All this brings me to ask what is going on here? Is this a result of all the approximations / interpolations going on? Or due to variations in the resistance values in the valve / components? Thanks for any help! Tristan Sat, 01 Aug 2015 03:16:44 -0400 http://ax84.com/bbs/dm.php?id=491555 RE: Cathode biasing 6L6GC's?(by JKCavitt) If you knock down the screen voltage, you can probably retube w/JJ 6V6S. They are good up to 500 volts on the plates, 450 on screens. That'll knock down the volume and get an earlier breakup if that is what you are looking for.Sat, 01 Aug 2015 03:00:45 -0400 http://ax84.com/bbs/dm.php?id=491554 RE: +9VDC PSU from DC-heater supply (by Stephen Keller) <div class="quoteblock" >On 07/31/2015 @ 14:28, JPSH79 wrote : I was hoping that I could've somehow used the existing winding to do it but maybe I just have to bolt another tranny to the chassis or buy/build an external psu for the pedals. Well.. thanks anyway. </div> Not if you want to elevate the heater winding at the same time. There are several approaches that could work: 1. A separate transformer off the mains (as discussed). 2. An isolation transformer off the heater winding to allow you to lift one of the feeds without affecting the other. 3. A autotransformer off the hv winding to drop from 300 or so volts to 12 or so volts. 4. You could implement an isolated switching power supply off of the mains, but it would need to be well implemented to avoid injecting noise into the amp. 5. You could tap off the B+ and implement a power scaling circuit to drop to a regulated 9 volts. Assuming a 350 volts B+, you'd need a beefy regulator and heat sink to drop 340 volts and still deliver a third of an amp (~100 watts). 6. If you have a cathode-biased power amp, you could tap off the cathodes and rectify that voltage to 9 volts d.c. You'll have to design it to avoid upsetting the power amp bias. That should be too difficult. StphFri, 31 Jul 2015 18:03:43 -0400 http://ax84.com/bbs/dm.php?id=491553 RE: Dragonfly Spare Parts Build(by robrob) The AC30 has a coupling cap between the phase inverter input and the 220k mixing resistor so the resistor can't function as a grid stopper and it shouldn't affect the PI's overdrive tone. Typically a cathodyne phase inverter does benefit from a large grid stopper of around 470k to control blocking and double-frequency distortion.Fri, 31 Jul 2015 17:56:51 -0400 http://ax84.com/bbs/dm.php?id=491552 RE: +9VDC PSU from DC-heater supply (by JPSH79) <div class="quoteblock" >On 07/31/2015 @ 13:29, Stephen Keller wrote : <div class="quoteblock" >On 07/31/2015 @ 13:09, JPSH79 wrote : : <div class="quoteblock" >On 07/31/2015 @ 12:49, Stephen Keller wrote : : : [quote]On 07/31/2015 @ 11:36, JPSH79 wrote : OK. Do you have a better solution or should I just trash the idea? : : : </div> : : : How much current do you want to supply at 9 volts d.c.? : : : : : : Stph : : -- : : : : </div> : : : : Not much. What does it take to power up a few pedals... maybe 200-300mA. : : : : I was thinking of taking the power to the amplifiers footswitch, since it will be on the floor near the pedals after all and distribute the power to the pedals from there with daisychain or something. I could use the same +9V to light up the leds in the footswitch and maybe even drive relays with it if there's spare current available. : : : </div> : A fair bit of current then. : : Assuming there's room in or on the chassis, I think your best bet would be to wedge in a small 12.6 volt transformer and regulate its output to 9 volts d.c. Something like the Hammond 187B12 would do the job (~$9US <a href="http://www.mouser.com/ProductDetail/Hammond-Manufacturing/187B12/?qs=sGAEpiMZZMvwUzoUXIIvyQPvPmwnNFGy%2fUB3BvSXVfc%3d " target="newwin1438420770">[link]</a> from www.mouser.com). : : This eliminates any induced switching noise in the heaters or high voltage circuits and you don't have to worry about conflicts with heater elevation. : : Stph -- [/quote] I was hoping that I could've somehow used the existing winding to do it but maybe I just have to bolt another tranny to the chassis or buy/build an external psu for the pedals. Well.. thanks anyway.Fri, 31 Jul 2015 14:28:13 -0400 http://ax84.com/bbs/dm.php?id=491551 RE: +9VDC PSU from DC-heater supply (by Stephen Keller) <div class="quoteblock" >On 07/31/2015 @ 13:09, JPSH79 wrote : <div class="quoteblock" >On 07/31/2015 @ 12:49, Stephen Keller wrote : : <div class="quoteblock" >On 07/31/2015 @ 11:36, JPSH79 wrote : OK. Do you have a better solution or should I just trash the idea? : : </div> : : How much current do you want to supply at 9 volts d.c.? : : : : Stph : -- : : </div> : : Not much. What does it take to power up a few pedals... maybe 200-300mA. : : I was thinking of taking the power to the amplifiers footswitch, since it will be on the floor near the pedals after all and distribute the power to the pedals from there with daisychain or something. I could use the same +9V to light up the leds in the footswitch and maybe even drive relays with it if there's spare current available. : </div> A fair bit of current then. Assuming there's room in or on the chassis, I think your best bet would be to wedge in a small 12.6 volt transformer and regulate its output to 9 volts d.c. Something like the Hammond 187B12 would do the job (~$9US <a href="http://www.mouser.com/ProductDetail/Hammond-Manufacturing/187B12/?qs=sGAEpiMZZMvwUzoUXIIvyQPvPmwnNFGy%2fUB3BvSXVfc%3d " target="newwin1438420770">[link]</a> from www.mouser.com). This eliminates any induced switching noise in the heaters or high voltage circuits and you don't have to worry about conflicts with heater elevation. StphFri, 31 Jul 2015 13:29:57 -0400 http://ax84.com/bbs/dm.php?id=491550 RE: +9VDC PSU from DC-heater supply (by JPSH79) <div class="quoteblock" >On 07/31/2015 @ 12:49, Stephen Keller wrote : <div class="quoteblock" >On 07/31/2015 @ 11:36, JPSH79 wrote : OK. Do you have a better solution or should I just trash the idea? : </div> : How much current do you want to supply at 9 volts d.c.? : : Stph -- </div> Not much. What does it take to power up a few pedals... maybe 200-300mA. I was thinking of taking the power to the amplifiers footswitch, since it will be on the floor near the pedals after all and distribute the power to the pedals from there with daisychain or something. I could use the same +9V to light up the leds in the footswitch and maybe even drive relays with it if there's spare current available. Fri, 31 Jul 2015 13:09:31 -0400 http://ax84.com/bbs/dm.php?id=491549 RE: +9VDC PSU from DC-heater supply (by Stephen Keller) <div class="quoteblock" >On 07/31/2015 @ 11:36, JPSH79 wrote : OK. Do you have a better solution or should I just trash the idea? </div> How much current do you want to supply at 9 volts d.c.? StphFri, 31 Jul 2015 12:49:15 -0400 http://ax84.com/bbs/dm.php?id=491548 RE: Dragonfly Spare Parts Build(by mcabe) Could also be that the first incarnation of this amp had a pre-PIMV. I increased the voltage and switched to a post-PIMV around the same time. How does your dragonfly sound with the gain dimed?Fri, 31 Jul 2015 12:15:35 -0400 http://ax84.com/bbs/dm.php?id=491547 RE: Dragonfly Spare Parts Build(by mcabe) You are undoubtedly correct. It probably wasn't the 220K resistor that was affecting the tone. I was getting some gnarly (not good sounding) PI distortion when my volume was up past 2 or 3 o'clock. So I installed the 220k resistor with a 1 meg resistor to ground. I am now using a 511k resistor to ground with an additional 120k resistor in parallel on a switch. This allows me to switch between 511k and about 100k, giving some control over the amount of PI relative to preamp distortion. The 220k/100k voltage divider allows me to really overdrive the preamp without the PI crapping out. The 220k/510k gives a nice volume boost and sounds really good for clean tones. Interested to see whether you have a similar experience with the PI. Any reason why increasing the voltage would make the PI sound worse when overdriven?Fri, 31 Jul 2015 12:04:48 -0400 http://ax84.com/bbs/dm.php?id=491546 RE: +9VDC PSU from DC-heater supply (by JPSH79) <div class="quoteblock" >On 07/31/2015 @ 07:38, Paul Fawcett wrote : <div class="quoteblock" >On 07/30/2015 @ 09:50, JPSH79 wrote : : ... I just realized that I could do it just like the bias-supply, couldn't I? : : : : I could just take the AC from the both ends of that 10,5VAC (non-CT) winding through a pair of 100n caps to another rectifier bridge and add another smooting cap and so on...right? : -- : : </div> : : No, this is no good. You can develop a voltage this way, but a cap-coupled supply can provide only a puny amount of current. -- </div> OK. Do you have a better solution or should I just trash the idea? Fri, 31 Jul 2015 11:36:20 -0400 http://ax84.com/bbs/dm.php?id=491545 RE: Clean round holes(by Stephen Keller) I've used wood spade bits a few times as well. Best results seem to depend on holding the piece very securely and not trying to rush the bit. If the bit grabs at all and the piece is not secure, you'll get chatter and a ragged hole. StphFri, 31 Jul 2015 09:23:52 -0400 http://ax84.com/bbs/dm.php?id=491544 RE: Clean round holes(by ampedup) They last quite a while and they are cheap. I like the ones with little barbs on the leading edge. And they make the big holes for tube sockets, just not good for holes under 3/8.Fri, 31 Jul 2015 08:25:05 -0400 http://ax84.com/bbs/dm.php?id=491543 RE: NON-Amp related power supply advice needed(by tedmich) if your PWM cicuit can handle 80mV p-p noise this would be small cheap and cool(er) <a href="http://www.mouser.com/ProductDetail/Mean-Well/PS-25-75/?qs=sGAEpiMZZMsPs3th5F8koJemULuvcPkzgWwXxxRHPOw%3d " target="newwin1438420770">[link]</a> albeit a little bit of a cop out ;)Fri, 31 Jul 2015 08:11:28 -0400 http://ax84.com/bbs/dm.php?id=491542 RE: NON-Amp related power supply advice needed(by tedmich) I see now! I'm not a big fan of NPN followers off linear regulators, why not use a LD1084V and have 5A output at 1.5-28VDC right there? The live tab is a pain but once you isolate it you can hang it on a big heatsink and fine tune the Vout with a trimmer. Of course a switched reg would be more efficient (less HEAT) but a little more complicated, as a cooler easier design I'd keep the Vin only a volt or 2 over the drop out voltage (~1.5v) so the regulator doesn't have to dump so much heat. As an example built a 24VDC fan with a Vicor 300vdc front end (rectified line voltage!) feeding a Vicor DC/DC converter. I wanted a variable speed so I added a LM338 with a pot to the Vicors output (I was young...). Of course the thing got hotter than hell at low speed (low V). I then used the Vicor DCDC trim function (+10%/ -80%) and its been working great and barely warm ever since.Fri, 31 Jul 2015 07:54:48 -0400 http://ax84.com/bbs/dm.php?id=491541 RE: Clean round holes(by GeorgeG) How long does a wood spade bit last on aluminum?Fri, 31 Jul 2015 07:40:56 -0400 http://ax84.com/bbs/dm.php?id=491540 RE: +9VDC PSU from DC-heater supply (by Paul Fawcett) <div class="quoteblock" >On 07/30/2015 @ 09:50, JPSH79 wrote : ... I just realized that I could do it just like the bias-supply, couldn't I? : : I could just take the AC from the both ends of that 10,5VAC (non-CT) winding through a pair of 100n caps to another rectifier bridge and add another smooting cap and so on...right? -- </div> No, this is no good. You can develop a voltage this way, but a cap-coupled supply can provide only a puny amount of current. Fri, 31 Jul 2015 07:38:17 -0400 http://ax84.com/bbs/dm.php?id=491539 RE: Dragonfly Spare Parts Build(by Doug H) The series 220k's on the AC30 were really just mixer resistors to keep the the normal and top boost channel volume controls from interacting with each other. "Theoretically", the 220k should have no audible effect on the sound.Fri, 31 Jul 2015 06:01:38 -0400 http://ax84.com/bbs/dm.php?id=491538 RE: Cathode biasing 6L6GC's?(by steve25) Yup, that's the other thing, the plate voltage is too high for a good cathode biased sound, and even if it was not for get about getting into class with that non gapped small OT!Fri, 31 Jul 2015 05:33:31 -0400 http://ax84.com/bbs/dm.php?id=491537 RE: Audio Cyclopedia,2nd Edition on its way(by PaulP) It's a complete joke. The last three volumes of Proust's "In Search of Lost Time" will be under copyright in the US until 2018. Proust died in 1922. <a href="http://www.slate.com/articles/arts/culturebox/2005/03/in_pursuit_of_proust.html" target="newwin1438420770">[link]</a> Fri, 31 Jul 2015 05:15:40 -0400 http://ax84.com/bbs/dm.php?id=491536 RE: Cathode biasing 6L6GC's?(by PaulP) In my experience, cathode bias + high voltage tends to lead to a lousy overdriven sound. The difference in loudness between 30W and 50W isn't really great either. If you're looking for quieter, the 410 configuration is holding you back. Doubling the number of speakers gets you a +3dB increase which is equivalent to doubling your power amp's wattage. Oop, nevermind, steve25 already covered this. If the schematic I'm looking at is correct, it uses a non-center-tapped B+ winding so you'll have to copy the original power supply instead of using the power supply from a blackface circuit. You can buy power scaling kits; that should take some of the challenge out.Fri, 31 Jul 2015 05:10:34 -0400 http://ax84.com/bbs/dm.php?id=491535 RE: can't find the rectifier. (by steve25) That amps Lafayette part number was 99 K 9101W, the year before with the left side mounted plastic logo it was 99 A 9101W. That amp with the verb and trem must of came out in the latter 67 catalog that I do not have, or in 68. Lafayette put out a new catalog every season back then, the things they sold blew my mind so to speak, power tools , row boats , microscopes , out board motors, even encyclopedia's and more!Fri, 31 Jul 2015 03:35:11 -0400 http://ax84.com/bbs/dm.php?id=491534 RE: Cathode biasing 6L6GC's?(by steve25) Doing the cathode bias thing will not knock the volume down much, but will make the amp sing better. Since you have 4 speakers the cheapest and quick way to drop off half of the amps volume is to rewire and just run two of the four drivers!Fri, 31 Jul 2015 03:28:29 -0400 http://ax84.com/bbs/dm.php?id=491533 RE: +9VDC PSU from DC-heater supply (by JPSH79) Yes I know, but I only have the 10,5VAC winding to use and it gives me that 12VDC under load when rectified and filtered and it will still be quite noisy for pedals. I could use voltage doubler but why bother when I already have a ready made board with 7809 and caps to use. So. Would it work in the way I reasoned in the second message or am I missing something? Fri, 31 Jul 2015 02:00:13 -0400 http://ax84.com/bbs/dm.php?id=491532 Cathode biasing 6L6GC's?(by Alva Goldbook) I have a Fender Hot Rod Deville 410 that I've gutted. I never liked the tone very much. Plus, it is way too loud. So I've been thinking of replacing it with a blackface kind of circuit, and cathode biasing the 6L6GC's. The rectified B+ was 486v on the plates before I gutted it. Can I reduce the overall wattage down to something more reasonable like say 25 watts? I was thinking it might be nice to add power scaling control (just be able to cut down the B+ voltages to reduce volume), but I don't have any experience with power mosfets. Not sure how I should proceed with this. Sat, 01 Aug 2015 03:00:45 -0400 http://ax84.com/bbs/dm.php?id=491531