Marcel's scratch build AC15 based clone

[Marcel]

That Trinity site seems a good site for kits there Simon.

I like in particular the Trinity 18 and the Trinity TC15, and both seem to be at a good competitive price too...

[Marcel]

So everyone is on the same page I'll put it here that I've been using this amp build quite a lot. On guitar builds, on Bass builds, fiddling with effects, and everything else into a 2x12 vented cab loaded with a 16 ohm V30 and a 16 ohm G12H75....and it sounds great !!! It has more overall bottom end than maybe it should which I put down to the home made vented cab and I can adjust out using the bass and treble controls, but otherwise it is so easy to dial in sounds, the nice sounds, the sounds I want to hear.... it's great !!... Just sometimes I wish I had a real AC15 or AC30 to be able to do a worthwhile A/B comparison...

I hadn't noticed until today, but the amp has developed a low level 50Hz hum... not enough hum to stop using it for practice or testing or even gigging, but it would be or is too much for recording at lower volumes. High volumes it's no issue, low volumes it's a real pain... and the hum varies up and down with the master volume control but is not affected by the gain control, so it is getting into the amp via the 2nd pre-amp tube or adjacent circuitry. Yet another item to add to the list for when I decide to take the covers off and wire in the EF86...

[DrNomis_44]

So everyone is on the same page I'll put it here that I've been using this amp build quite a lot. On guitar builds, on Bass builds, fiddling with effects, and everything else into a 2x12 vented cab loaded with a 16 ohm V30 and a 16 ohm G12H75....and it sounds great !!! It has more overall bottom end than maybe it should which I put down to the home made vented cab and I can adjust out using the bass and treble controls, but otherwise it is so easy to dial in sounds, the nice sounds, the sounds I want to hear.... it's great !!... Just sometimes I wish I had a real AC15 or AC30 to be able to do a worthwhile A/B comparison...

I hadn't noticed until today, but the amp has developed a low level 50Hz hum... not enough hum to stop using it for practice or testing or even gigging, but it would be or is too much for recording at lower volumes. High volumes it's no issue, low volumes it's a real pain... and the hum varies up and down with the master volume control but is not affected by the gain control, so it is getting into the amp via the 2nd pre-amp tube or adjacent circuitry. Yet another item to add to the list for when I decide to take the covers off and wire in the EF86...

Could very well be an earth-loop that's causing the hum, I was watching a video on youtube about a guy who was in the process of diagnosing a hum-noise issue in one of his Fender Tweed Bassmans, and he eventually traced the cause to the 6.3V AC heater supply going to the two 6L6 power valves and three 12AX7 preamp valves, his conclusion was that the heater-wiring between the two 6L6 power valves hadn't been done correctly at the Fender Factory as the amp was being manufactured, the Fender Bassman in question was an old early 50's vintage amp.


Incidentally, I wouldn't mind having a go at building a 50's Fender Tweed Bassman from a kit of parts, would be a fun project.

[Marcel]

It could be as you said Doc.

In this amp build the heaters for the 12AX7's are from the grounded centre tap 6.3VAC winding, so if the centre tap isn't exactly dead centre then there will be a imbalance that could cause hum. As this has a solid state rectifier diodes the EL84 heaters are on the otherwise unused winding that would have powered the rectifier tube with two 1W 100 ohm resistors to ground to balance up that sub-circuit. However from initial operational diagnosis and knowing the circuit as well as I do I'm more inclined to think it is a 22uF filter cap beginning to fail and starting its climb up the ESR ladder. When I open it up it will one of the first things I check.

I also have a 2.9H 120mA Fender champ power supply filter coil that I want to incorporate, so that may also go some way to eliminating all hum.

As for the Tweed amp Doc... I've lost count on how many times I've had to stop myself from getting a Tweed kit.. or even a small kit from one of those places Simon recomended... I gotta get my spending under better control before my wife gets fed up and takes over control by hiding my credit card...lol

[Marcel]

FB has some gems occasionally..

From the 'Tube Amp Builders (DIY)" group, a link to a very informative Mullard manual... theory and some practical circuits...

http://basaudio.net/blog/wp-content/...Amplifiers.pdf

[Marcel]

And another one ...

http://www.tubebooks.org/tubedata/RC30.pdf

[DrNomis_44]

It could be as you said Doc.

In this amp build the heaters for the 12AX7's are from the grounded centre tap 6.3VAC winding, so if the centre tap isn't exactly dead centre then there will be a imbalance that could cause hum. As this has a solid state rectifier diodes the EL84 heaters are on the otherwise unused winding that would have powered the rectifier tube with two 1W 100 ohm resistors to ground to balance up that sub-circuit. However from initial operational diagnosis and knowing the circuit as well as I do I'm more inclined to think it is a 22uF filter cap beginning to fail and starting its climb up the ESR ladder. When I open it up it will one of the first things I check.

I also have a 2.9H 120mA Fender champ power supply filter coil that I want to incorporate, so that may also go some way to eliminating all hum.

As for the Tweed amp Doc... I've lost count on how many times I've had to stop myself from getting a Tweed kit.. or even a small kit from one of those places Simon recomended... I gotta get my spending under better control before my wife gets fed up and takes over control by hiding my credit card...lol

Have you thought about converting the 6.3V AC heater supply from AC to DC?, that may reduce the hum level a bit, I think some well-known guitar amp manufacturers use a 6.3V DC heater supply in some of their high gain amps that they manufacture.


Cheers for the links to the two pdfs, I downloaded the Mullard pdf and I'm in the process of downloading the other one, the Mullard pdf has some really interesting projects in it, most of the theory in the Mullard pdf is geared towards HiFi applications, but it still applies to guitar amplification applications as well, definitely worth downloading a copy of each of the pdfs.


Note that the RCA pdf has lots of good valve theory in it, good reading material if anyone is interested in learning about how valves operate.

[Marcel]

No Doc, I haven't. However I have considered putting a +ve DC offset on the entire heater circuit to reduce the potential failure of V2b.

In this and most other designs where V2b is a cathode follower the voltage on the cathode is often up in the high hundreds with 190VDC being typical. The limit specified by most ECC83/12AX7 manufacturers is 200V maximum between heater and cathode, so most amp designs are pushing this potential failure point for nil other reason than simplicity of design and reduced component count. Adding a simple voltage divider to the centre tap of the heater windings and raising the DC level to around 30 or 40 volts DC evens up the "stress" on all the ECC83/12AX7 tubes but most notably on V2b where in most normal designs if an 'arc out' were to occur and apart from possibly irrevocably damaging V2 it will also send a rudely large and potentially lethally damaging spike to the speakers... And given the quality of most tubes today it is a cost that I'm not keen on enduring.

A comment on "Quality" - Tubes have never been 'quality' items. Some people are lucky and their tubes can last a life time, others not so much. so you get good ones and bad ones and the brand has little to do with it. In the last 12 months I've purchased near 2 dozen tubes of various types and have had 2 fall short for different reasons. So on average for me that works out at about a rough guesstimate of a 10% failure rate from new, which is far poorer than what the solid state cousins can account for. It is what it is, but if I can save risking $400 or $600 (or AU$800 for a new pair of Alnico Blue's) worth of speakers by doing what costs me a $1 modification then I'll spend the $1.

[DrNomis_44]

No Doc, I haven't. However I have considered putting a +ve DC offset on the entire heater circuit to reduce the potential failure of V2b.

In this and most other designs where V2b is a cathode follower the voltage on the cathode is often up in the high hundreds with 190VDC being typical. The limit specified by most ECC83/12AX7 manufacturers is 200V maximum between heater and cathode, so most amp designs are pushing this potential failure point for nil other reason than simplicity of design and reduced component count. Adding a simple voltage divider to the centre tap of the heater windings and raising the DC level to around 30 or 40 volts DC evens up the "stress" on all the ECC83/12AX7 tubes but most notably on V2b where in most normal designs if an 'arc out' were to occur and apart from possibly irrevocably damaging V2 it will also send a rudely large and potentially lethally damaging spike to the speakers... And given the quality of most tubes today it is a cost that I'm not keen on enduring.

A comment on "Quality" - Tubes have never been 'quality' items. Some people are lucky and their tubes can last a life time, others not so much. so you get good ones and bad ones and the brand has little to do with it. In the last 12 months I've purchased near 2 dozen tubes of various types and have had 2 fall short for different reasons. So on average for me that works out at about a rough guesstimate of a 10% failure rate from new, which is far poorer than what the solid state cousins can account for. It is what it is, but if I can save risking $400 or $600 (or AU$800 for a new pair of Alnico Blue's) worth of speakers by doing what costs me a $1 modification then I'll spend the $1.

I recently had a few 12AX7 valves fail on me, two of them were a pair of Chinese 12AX7B valves that were the original preamp valves in my 5 Watt Legacy amp, I noticed that they were a bit weak in terms of gain and then they simply stopped working altogether even though their heaters did light up, the getter inside the glass envelopes still looked normal though, and then there were two of the original Marshall ECC83 preamp valves from my Marshall MA100C amp, they were both very weak in gain and then the getter in them started turning white, indicating that gas had gotten into the glass envelopes, the three Genelec Gold Lion ECC83 valves I used as replacements for the original preamp valves seem to be holding up pretty well, I was surprised at the difference in tone that the Gold Lions made compared to the originals, they really brought the Marshall amp to life, will definitely buy more of them from Evatco in future.

[Marcel]

I've not encountered many Genelec Gold Lion's, but the few I have they seem to be quite good in terms of gain and microphonics. Never tried the cheap Chinese tubes though I do have four of them in my stock. I do like the Telefunken tubes though one that I have could almost pretend to be a Shure SM58, and in the dim dark past the old Groove Tube was a popular choice though these days they seem to be out of favour.

These days my typical purchases for myself are JJ's. I figure if they are good enough for the likes of Angus Young then they will do me okay, and they are usually quite well priced.

Worst faults I typically encounter with tubes is low emission and/or low gain... and then heater flash...

FYI - Heater flash - A tube reliability issue - If there is this construction fault with the tubes heaters and when power is first applied to that tube's heaters there can be a bright flash of light from near the base of the tube. Happening only at power on, and for only a few seconds, the exposed section of heater at the base of the tube will light up like a light globe until the remainder of the heater that is surrounded by the cathode comes up to temperature. This 'flash' does not affect the operation of the tube as once warmed up the tube will work as normal, however it is a weak point in the heater that becomes more fragile with age. With repeated power on/off cycles the heater is likely to break at this increasingly fragile flash point and thus usually at a moment of least convenience rendering of the tube to a useless failed/dead state with nil 'Red glowy bits' in that tube and nil noises from the connected speakers for your listening pleasure.

Useless fact - I have an infrared temperature meter which I have used to measure the temperature of all sorts of things including various amplifier parts during operation.... On an average 30C day here at my tropical location and after 1hr of moderate use of this amp build I measured the following - Power transformer @44C, Output transformer @ 42C, ECC83/12AX7 RFI protective aluminium shields @67C, EL84 plates (cathode biased to 11.7W dissipation) @157C.