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PostPosted: 18 Sep 2020, 08:52 
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Joined: 29 Jul 2020, 12:26
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Hello again, just completed collecting all the components needed, and currently have a couple of questions before I start the build:

1. Can somebody please confirm the colour coding for the primary of the EMO 750 O/P transformer, is it as per the Type "E" circuit diagram, in that Brown goes to pin 7 of the top EL84, moving down to Brown/White for pin 9, then Red for B+, then Blue/White to pin 9 of the bottom EL84, and finally Blue to pin 7? With Yellow being the 8 ohm (and feedback) tap for the secondary?

2. For the shared PSU, is the 50 ohm series resistor (made up of two 100 ohm) correct, or should it be a single 100 ohm?

Many thanks.

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Type E.jpg


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PostPosted: 18 Sep 2020, 10:14 
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Location: Arizona, USA
Hi, Those are the correct markings. The brown pair goes to the tube that get the signal directly from the driver stage. The blue ones go to the tube that has the grid going to ground and depending on the variation you are building it might have some other circuitry as well. The two 100 ohm resistors in parallel are a hidden safety feature. Since most resistors in this size and wattage only have 10-20% tolerance and one will always carry more current. If the output tube stage fails (stuff happens on occasion) the resistor carrying the most current will usually fail and the other one shortly thereafter as it can't take the load by itself. I considered using one 75 ohm resistor and one 150 ohm resistor in that location for better guarantee of operation but it seems that the 100s do fine. Now to be sure, I have only had one pair of amps fail in a way that made the resistors blow. The amps were used in the rain and even though the LM317s failed the amps still made some distorted sound for a few minutes then the resistors failed . A fuse would be difficult to get the proper size and type in the B+ line as the inrush on initial power up tended to blow anything that was not a slow blow type and even then nothing below 1 amp would hold over a period of months. The current is BTW way less than that. So I went for reliability. The actual things protected are the power and output transformers. Any failure enough to blow the resistors has 99.99% of the time already cooked the LM317s and probably one or both tubes in one pair.

Good listening
Bruce

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PostPosted: 19 Sep 2020, 07:51 
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Thanks again Bruce for the clarifications. Will report back when the amp is hopefully up-and-running, perhaps by the end of October.
PS: I have three spare LM317s, and a few more 100 ohm 2 watt resistors!


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PostPosted: 19 Sep 2020, 20:30 
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Hi, A good concept in building anything. Always order a few extra of the components that might fail if you don't get the build quite right and smoke them. Just today I sent on LM317 into oblivion. One KT88 output tube had a bad connection on the grid. This let it float up and in turn the 317 tried to hold the current down.....until it hit about 105 volts. Poof. They can tolerate a lot of abuse and I have had ones sustain nearly that much voltage...but not with more than 200 ma at the same time.

Good listening
Bruce

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PostPosted: 22 Sep 2020, 09:32 
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Hello again, anyone see any problems with my draft layout? The power transformer is the specified Edcor but I had to strip and respray the bell-ends because of rust. The tag boards will be under the top plate, one for the PSU components, and the other two for the amps. Many thanks.
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Draft layout 1.jpg


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PostPosted: 22 Sep 2020, 21:52 
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Hi, It should be fine. Remember to allow for the balance control and a way to get rid of heat from the LM317s. For a Poddwatt you can use an insulator with heat sink compound and fasten it to the chassis. Just not under something like the transformers. If the chassis has good air flow one of the fasten on heat sinks (often look like butterflys) can be used. Dissipation is fairly small at about 1-1.5 watts in most builds. The newest types (versions D and E) with the raised dc on the grids will increase the dissipation of the LM317s to about 2 watts.

Good listening
Bruce

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PostPosted: 23 Sep 2020, 02:40 
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Thanks for your comments Bruce. The LM317s will be going on the aluminium sides of the chassis, and yes I will pull the valves closer to the front so that the balance pot and test points have more room between the EL84s and OPTs. Cheers, Richard.


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PostPosted: 06 Oct 2020, 08:31 
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New version 'D' up and working from first switch-on (after some careful checks) and sounds great with my DIY full-range speakers. Thanks for the design and guidance Bruce.


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PostPosted: 06 Oct 2020, 21:49 
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Hi, 8-) Nice looking build. I have used the perforated metal in some early projects and might give it a whirl again. It makes screwing things to the bottom easy and allows air flow to boot. I still use similar metal sheets on the bottom of many builds. They look nice and keep fingers out of the juice. One source of aluminum ones that are inexpensive is the hardware store. They sell them as "kick" panels for screen doors. Cheap and lots of patterns. Both in silver and gold tone.

Good listening
Bruce

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PostPosted: 19 Oct 2020, 23:57 
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Joined: 27 Sep 2010, 19:47
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Location: AB, Canada
Hi Bruce,
I, (Kokanee) usually browse the diyaudio forum, and have a thread there in the tube/valve area called tube amp identification. I was given a old tube amp with some Hammond iron. The thread is active if you would like to have a read.

Basically looking for a PP project that I can utilize a pair of Hammond 1626 OT's with 6.6kohm primary impedance. Could these be used in one of your EL84 designs. Page 1, Post #7 of the thread has a list of the specs for the 1626. Thanks for the help.

Myles


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