need help with A15MK2

    As I have not had any replies to the 4 emails sent to Klaus and Martin I thought I would post here, can someone offer advise as I cannot get any customer support via email;

    On page 19 of the manual it says to check the resistance of the output transformer.

    When I connect probes from ground to the connections on the 4,8&16 ohm speaker sockets I get 0.7, 0.9, & 1.2 ohms respectively (this is incorrect according to manual).
    I do not know where to measure the primary side (I have tried between ground and the various blue wires and get a resistance that climbs slowly to about 75.7 kohms).

    Am I measuring incorrectly? Is the transformer faulty?

    i want to complet this build but have had aseveral weeks delay waiting for an answer.

    cheers

    Hi p-trax,

    customer support is ment to be primarily via this forum, so good thing your posting here.
    I do not have the A15 but the M15, but I suppose you should measure the primary side between b+ and plate of the output tubes, could well be that all three wires are blue!

    As for the resistace of the secondary side, if you just have a fairly cheap DMM I think the measurements corrispond quite well.

    let us know
    martin

    Hi,

    are you shure the manual says to measure the Rdc of the secondaries?

    Because the resistance is so low that you can't them measure correctly with a standard multimeter. (So it seems ok what you measure on the secondaries).

    It makes more sense to measure the primaries. According to measurements I did there is a Rdc on each half of about 250 ohms and 125 ohms to the g2 tap. These are averaged values. They may differ up to 5%.

    Bernd

    Jaichweiß (Andy Pipkin)

    thanks for the input guys.
    I will measure again.

    BUT, can you help further by describing exactly where the measurement points are please?


    sorry for the remedial questions, this is the second amp i have built and I am not familiar with all the terms (the first was a 1/2watt gilmore jnr, which is much simpler and didn't require any testing).

    Hi,

    Zitat

    BUT, can you help further by describing exactly where the measurement points are please?


    is this not done one page 19 of the manual as you mentionned before?

    Bernd

    Jaichweiß (Andy Pipkin)

    Zitat von p-trax;268246

    thanks for the input guys.
    BUT, can you help further by describing exactly where the measurement points are please?

    I don't have the same amp so im not 100% sure, but since you have the same OT I suppose you should test 6-7 vs. 12 and 13 respectively. The absolute values aren't so important as long as they stay in the range of a few 100 Ohms.

    Bernd may correct me if I'm wrong, the manual isn't to clear abaut that.


    PS did you really stop the build for considerable time waiting for these measurements? :eek: thats self control :o

    So, the manual says;

    "chk the res of the OT after assembly to make sure everything wired correctly. At teh primry side you should measure about 500Ohm, at the secondary side you should measure about 1 Ohm at the 4 Ohm (I missread this as 10 Ohm -DOH) I get 0.7
    about 1.2 at the 8Ohm, i get 0.9 and 1.4 ohm at teh 16ohm, i get 1.2.

    so those all seem ok.

    I measured between the star ground and teh out put sockets for 4, 8, 16 ohms.

    I didn't know where to measure the primary between though.


    with your advice I measured between 6 and 12 and between 7 and 13. both readings where 250 ohm (but 6 and 7 are connected!)

    between 12 and 13 i get approx 500 ohm (

    as for stopping build, I carried on wiring components, but did not want to power up for fear of damaging something.

    so I am at page 30 "checking and testing"

    I have checked all ground leads. gone over teh woring diagram a couple of times.

    now i need to connect a speaker and then power up (I only have 16 ohm speakers, persumably it does not have to be an 8 ohm as stated by manual (as long as plugged into correct output socket)?

    once Ive done this i need to add tubes etc :)

    if you guys reckon my readings are OK for OT I will move on.

    this is my wiring completed. (sorry for dodgy picture, taken with my phone)

    [Blockierte Grafik: http://img.photobucket.com/albums/v369/p-traxJapan1/4245_107804309008_622629008_2552121.jpg]

    Hi

    seen from far, I think it's OK. But I miss the point "measuring all dc voltages" -at cathodes and plates - with speaker connected but no input signal.

    By this, you will measure the operating points of every tube - that's important.

    If you don't feel capable of measuring "under running conditions" - there are a few hundred volts; that's lethal! - let this do an amp technician.

    Bernd

    Jaichweiß (Andy Pipkin)

    Zitat von p-trax;268302

    So, the manual says;


    with your advice I measured between 6 and 12 and between 7 and 13. both readings where 250 ohm (but 6 and 7 are connected!)

    between 12 and 13 i get approx 500 ohm


    These readings are OK, if you look at the schematics you see why 6-7 are connected.

    Of course you can use a 16Ohm speaker (on the correct output) to test your amp. When measuring DC voltage be careful, thats far more dangerous than the 230 V AC mains...

    Do you know the general safety rules?

    Martin

    HI GUys
    so I connected a speaker and power, let the tubes warm for a minute and then flicked the standby. after about 2 seconds i heard a high pitched squeal. so i killed the power (all pots are full off).

    the same for channel 2.

    all my wiring looks correct. but there must be something wrong right?

    Can any one help me?

    Hi,

    maybe you have to swap the anode connections (pin #3) of the power tubes to the output xfrmr, so that the NFB-loop works right, in the moment you have a power oscillator...;D

    Tell us if it helps...or not.

    Bernd

    Jaichweiß (Andy Pipkin)

    Hi Bernd
    I'm not really sure what that means :)

    but I have checked my layout again and it matches the photo of the wiring diagram exactly colour for colour at each tube.

    Zitat von p-trax;268352

    Hi Bernd
    I'm not really sure what that means :)

    but I have checked my layout again and it matches the photo of the wiring diagram exactly colour for colour at each tube.


    what Bernd means is to swap the blue lines on 12 and 13 of the OT. This could help because the negative feedback is an out of phase signal used for attenuation, but if your signal is in phase you get a positive feedback loop. So try this and tell us.

    You should not so much depend on the layout but mainly consider the schematics - they show you how it should work

    Thank you both!!!!

    :)


    that has cured the squeal.


    I have read all the voltages and they are all OK, the high voltages were between 6 and 9% over the schematic.


    By the way i really do appreciate all the help you are giving me, I AM EXTREMELY grateful :)

    one more small problem :)

    when i go to high gain mode on channel 2, i get about a second of sound, then it cuts out. if i switch off and back again i get another second then silence.

    i thiught it might be a bad solder joint, but every timt i reset the switch it works for about 1 second.

    any thoughts?

    Hi,

    for my understanding:

    Channel 2 is the high gain channel?

    You don't mean the "Gain" switch (+/++)?

    Has the "Fat" switch influence on this phenomenon?

    Lasts the sound really for a second or longer or shorter when switching?

    Can you make a record of this?

    Bernd

    Jaichweiß (Andy Pipkin)

    HI Bernd

    yes channel 2 is the high gain channel, the +/++ is what i was refering to :)

    i cannot record it (sorry) and the fat switch didn't seem to make any difference.

    when i flick the + to ++ all is OK until i strum.

    if i lightly tap the strings i get sound, as soon as i strum or pick a string it "cuts out". there is still sound coming out of the amp, but it is so quiet it is barely audible.

    its almost as if too much signal causes the amp to drop the volume right down.

    any ideas?

    Sorry,

    I lost your thread because it's in the wrong board...

    So if the error occurs when you in the High Gain channel switchung to "++"

    you should measure the grid voltage of V3a at pin #7 referred to ground (black lead of your multimeter connected to ground).

    Normally you would see 1 V DC. If you switch and/or strum an the DC voltage changes distinctly there may be an error in wiring or caps.

    For this test you have to mount the leads of your DVM in a way that they will clip safely to the lugs - you must not keep them in your hands. You'll need these for switching and strumming.;)

    And be aware you're measuring only 1 V but the next pin of the tube has some 100s volts! :eek:

    If you don't have the right probes on your DVM for this (the "needles" which are standard are not suited!) - leave it.

    Check all connections of/to the "+/++"-switch, especially the connections and values of C11 (2,2 nF) and C15 (22 nF)... and report again.

    Bernd

    Jaichweiß (Andy Pipkin)

Jetzt mitmachen!

Sie haben noch kein Benutzerkonto auf unserer Seite? Registrieren Sie sich kostenlos und nehmen Sie an unserer Community teil!

Benutzerkonto erstellen Anmelden