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PostPosted: 14 Jun 2016, 13:09 
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Joined: 29 May 2011, 12:24
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Location: Netherlands
It must have been about 3 or 4 years ago that I had the intention of building a stereo version of Bruce Heran's OddBlock design with KT77 tubes. I started ordering parts, then my life changed drastically, moved houses and changed jobs and on top of that busy weekends with gigs (musician). Needless to say I never managed to find the time to finish the amp. It's always been on the back of my mind though, I had all the parts I needed, all I needed was some time! Then finally last weekend I had Saturday evening and a whole Sunday off! Wow. Guess what? It's the perfect time to put this amp together.

Since this is my first DIY hifi amplifier I decided to make a prototype first. This temporary chassis is made of zinc. It's easy to drill holes in and find the right places for the components, especially if I have to add some stuff or change position of current components. Later on in the year I intend to focus more on the looks and build a proper chassis.

I had to improvise a bit, I couldn't find proper heatsinks for the LM317's anymore so I had to make two out of one. The fuse holder I intended to use broke. I was happy to find another fuse holder I could temporarily use. I didn't have all the proper screws I originally intended to use so I had to collect some I had laying around. I ran out of solid core heater wire, so I had to mix different types of wire. I guess that's what happens when you have to wait years until you can start to build.

I have some questions I hope some of you more experienced builders can answer:
- My voltages are a bit low, which I think is due to the KT77's bias running quite hot. I have a B+ of 433V while the circuit asks for 450V - 475V. The bias of the KT77's measured across the 1 ohm resistor reads around 63mV per tube (in fact: 63,3mV / 63,3 mV on one side, while the other side reads 57,9mV / 69,2mV. Seems like this set isn't as perfectly matched as it should be! The rheostat is ) Am I right the bias seems hot? Am I correct this output stage is an SRPP design? I don't have much experience with biassing this type of circuit. I would love some input.

- How big should the heatsink be for the LM317s? The heatsinks are getting incredibly hot now after just a couple minutes. Can they be mounted on the chassis so the chassis functions as one big heatsink?

- I didn't use line filter as I didn't have one. The amp is very quiet at normal listening levels but induces a slight hiss when I crank it. Would the line filter help with the hiss?

Anyway, just wanted to thank Bruce Heran for kindly sharing this design. I'm incredibly happy with this amp. The sound is amazing. My setup is a Marantz CD-50 CD player which I modded years ago as described on the Lampizator website (converted to tube CD player). Oehlbach interconnects in to the OddBlock amp and some Magnat speakers of which I upgraded the filter capacitors. My other amp is an Accuphase E-205. AFAIK the Accuphase is quite highly rated as great amplifier, but the OddBlock just blows it away. It sounds like there's band playing live in your room. You can hear all the details in every instrument. Even my girlfriend was amazed! :smoking:

Cheers

Steven


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PostPosted: 15 Jun 2016, 03:01 
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After some research on the internet I can basically answer my own questions: the pair of Kt77's should see about 120mA, so my values are correct. The LM317's lug is connected to Vout so cannot be connected to chassis without a spacer. I'll see if I can buy a line filter today and install it, see what it does!


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PostPosted: 15 Jun 2016, 11:34 
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I now believe I wired the ground loop breaker wrong. Shouldn't mains earth be connected straight to the chassis? And then have the 120 R resistor / 0.1uf cap connected from chassis ground to the ground bus?


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PostPosted: 24 Jun 2016, 21:54 
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Hi, I have been away for a few weeks and will get back to you tomorrow. BTW I like the looks of the build.

Good listening
Bruce

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PostPosted: 25 Jun 2016, 21:00 
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Location: Arizona, USA
Hi Yes the schematic is unclear, but the earth mains ground should go to the chassis.

On the earlier questions.... the tubes will run hot. This is a class A design and the worst dissipation is at idle. The values you indicate are slightly high because of the higher B+ voltage but still within the tube ratings. I would probably recalculate the sense resistor for 60ma. If the tubes do not balance try swapping their positions. Sometimes it matters. Also try swapping from one set to the other. The balance control allows for a small range of what could be called a bias adjust. It sets the idle points. The circuit allows about a 1-3 volt shift between the tubes and the LM317 actually sets a "bias" point for the pair kind of like a cathode resistor would.

For the heat sink...I use extruded aluminum ones that are about 5cm X 6cm X 2cm. Bigger is better. You can fasten them to the chassis if you use the insulation kit for the LM317. It is a wafer and feed through for the mounting bolt. I use heat sink compound when I need to use the insulation kit as it will improve the thermal conduction.

Good listening
Bruce

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PostPosted: 27 Jun 2016, 13:51 
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Hi Bruce, thanks for the reply! When I measure the tubes for actual mA (instead of the 1 ohm resistor method) I can get around 59mA ~ 59.8mA per tube at 418Vdc plate voltage. That sounds better right? I guess my 1 ohm resistor is not exactly 1 ohm!

You mention my B+ is high which surprises me as your schematic calls for 450 - 475Vdc B+? I'm way below that with 423Vdc?

I wanted to let you know I noticed an error on your schematic; it says pin 4 and 5 are the heater for the 6SL7 but this should be pin 7 and 8. The schematic being 4 years old I guess you know about this already but I thought I'd mention it just in case.

Cheers,

Steven


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PostPosted: 27 Jun 2016, 14:20 
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Hi, Oops. I mis read the voltage you had. Yes the correct pins on the 6/12SL7s are 7 and 8. Your 1 ohm resistors were probably within the posted tolerance, but on the high side.

Good listening
Bruce

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PostPosted: 02 Aug 2016, 10:05 
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The amp has worked fantastic for weeks, but yesterday it started blowing fuses. I did the usual troubleshooting and narrowed it down to the point where as soon as the B+ leads of the power transformers gets connected to the rectifier diodes it blows the fuse. Power transformer voltages without anything connected to the power transformer read like they should. 6.5VAC for the heaters and 380VAC for both B+ leads measured to ground. So it seems the transformer is good.

I completely bypassed the standby switch, that didn't help. With just the rectifier diodes connected to the B+ wires and nothing else after the diodes it blows the fuse. I have replaced the diodes three times, that didn't help.

I then completely bypassed the power and standby switching circuitry and used a new fuse holder. So basically it is the AC wiring connected straight to the new fuse holder in to the power transformer primary. Same problem.

What can be the issue here? I don't have another power transformer laying around to test with. But like I said before all the voltages read perfect. I'm stuck!

I've used both 1.6A and 2A slow fuses. The amp has worked perfectly thus far on the 1.6A fuse.


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PostPosted: 02 Aug 2016, 21:40 
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Hi, Humm, first I believe a 1.5 or 2 amp fuse is a bit small. I would probably use a 4 amp fuse as all you want to do is protect in the event of catastrophic failures and tube amps are prone to high initial inrush amounts of power, well over the operational level. If a 4 amp fuse blows then there is certainly a problem. It would likely be a short. Since this is a stereo build it is in some ways it is easier to check out and some ways harder. Pull the output tubes from one channel and see if the fuse holds. If not, put them back and pull the other two. If not again then it is most likely a short in the power filter string. You will have to go through step by step to locate it. Possibly a failed filter cap. Now if one channel holds and one fails then we can trouble shoot that channel. There are a few common possibilities. First try swapping the tubes from the failed channel with the good ones. See if the problem follows the tubes. If so then a tube is bad. If not, then it is likely that the bad channel is drawing too much current. There are only a few ways this can happen. First and most obvious is a failed LM317. They generally short through when failed. To test this you need to find a larger fuse...5-7 amps ought to be enough and set up a meter from the center tap on the balance control for those tubes (in the bad channel) and measure the voltage to the signal or power supply ground. Put tubes only in that channel. Turn the power on..... measure the voltage quickly (up to about 60 seconds should be OK) and then shut it back off to prevent damage elsewhere. If the voltage is low....under about 6 volts and perhaps as low as 0-4 volts then the circuit with the LM317 is shorted. If it is in the 15-30 volt range then it is OK. LM317s are quite durable but will fail if the voltage across them is too high (for a LM317HV about 60 or more volts) or the heat sink is not sufficient and they overheat. There is generally only one way to get over voltage there. That is if one grid is made very positive or one grid becomes un-grounded. This can occur if the coupling cap has failed allowing dc through or if the resistors from the grids to ground either fail (become open) or the solder that fastens them is not good. If you suspect the LM317...you can disconnect it and put a 250 ohm 10 watt resistor between the center tap of the balance control and ground. The amp should function reasonably well (not up to spec, but OK). Measure the voltage across it. It ought to be somewhere in the 12-30 volt range. If it does function and the voltage is in the range then it is conclusive the LM317 has failed. If the voltage is much higher, perhaps as much as 50 volts then the fault is elsewhere.

Another less probable cause of excess current are shorts at the tube socket.

What you can also do is (if the one channel will function) is to measure the resistances to ground from each pin on the output tubes and compare them to the ones on the bad channel. Often it can pinpoint problems.

Based on the fact the amp worked and then failed I strongly suspect one LM317 has failed. The question is whether it was heat related or circuit fault causing over voltage. How large are you heat sinks. Send a photo of the insides and if needed the heat sinks if you can.

First however, try the larger fuse.

Good listening
Bruce

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PostPosted: 03 Aug 2016, 12:17 
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Hi Bruce, thanks for the reply again. I had already been through pulling the tubes etc and probably didn't explain things as clear as I should have, the problem is all this happens with the amp in standby.

The amp doesn't blow fuses with all power transformer leads disconnected, except for the moment I connect the B+ wires to the rectifier with the amp still in standby. So the only thing in the circuit is the 50R/20W resistor and the rectifier diodes. All heater wiring is disconnected, center taps connected or disconnected to earth doesn't change a thing. When I disconnect the resistor (only the rectifier diodes in circuit) it also blows the fuse. Bypassing the resistor or standby switch doesn't help. Different diodes doesn't help. Resistance on the xformer leads read good as well, I can't measure any short circuits on the power transformer with my DMM. When I don't have anything connected it doesn't blow the fuse and all AC voltages read correct on the transformer leads! This is what baffles me.


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